climate change research topics

• Climate Change - A Global Issue
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• A Global Issue
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This site contains links and references to third-party databases, web sites, books and articles. It does not imply the endorsement of the content by the United Nations.

What is climate change?

• Background Climate change is an urgent global challenge with long-term implications for the sustainable development of all countries.

Exploring the Topic

• Climate.gov NOAA (National Oceanic and Atmospheric Administration) maintains this gateway to peer-reviewed information on climate change for various audiences, from the layperson to teachers to scientists to planners and policy makers. Provides access to relevant data sets from a number of agencies, including the National Climatic Data Center and the NOAA Climate Prediction Center.
• Eldis resource guide on climate change (IDS) The Eldis website is maintained by the Institute of Development Studies at the University of Sussex. It facilitates the sharing of information on development issues by aggregating information materials from reputable sources into the resource guide on climate change. It offers tools to create online communities for development practitioners; several such communities exist to discuss specific aspects of climate change. Eldis topic editors compile email newsletters, so-called reporters, including the “Climate Change and Development” reporter.
• IIED - International Institute for Environment and Development Well-established policy research institute that offers an online library of information materials on climate change and related topics, such as energy, biodiversity and forests. Publicizes its research output through email newsletters and on various social media channels.
• IISD - International Institute for Sustainable Development IISD offers a searchable and browsable knowledge base of its publications and video on climate change. IISDs LINKAGES reporting services closely monitor major international climate change meetings, including those of the IPCC and under the UNFCCC. IISD publishes the Earth Negotiations Bulletin, hosts the climate-l electronic mailing list and publicizes its work on twitter and Facebook.

Climate Change 2022: Mitigation of Climate Change

Watch this video by the Intergovernmental Panel on Climate Change to learn more about what is at stake and what actions need to be taken to mitigate the impact of climate change globally.

More videos on other aspects of climate change can be found on the IPCC's YouTube page .

Climate Change FAQs

Check out what questions others have asked about climate change and related issues in  Ask DAG , our FAQ database.

Related Research Guides

UN Research Guides on issues related to environment:

• Climate Change - A Global Issue by Dag Hammarskjöld Library Last Updated Jan 5, 2024 7927 views this year
• UN Documentation: Environment by Dag Hammarskjöld Library Last Updated Dec 29, 2023 6039 views this year

Other resource guides on climate change and related topics:

• Peace Palace Library: Environmental Law Starting point for research in the field of International Environmental Law provided by the Peace Palace Library in The Hague.
• Next: At the United Nations >>
• Last Updated: Jan 5, 2024 5:23 PM
• URL: https://research.un.org/en/climate-change

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The Top 10 Most Interesting Climate Change Research Topics

Finishing your environmental science degree may require you to write about climate change research topics. For example, students pursuing a career as environmental scientists may focus their research on environmental-climate sensitivity or those studying to become conservation scientists will focus on ways to improve the quality of natural resources.

Climate change research paper topics vary from anthropogenic climate to physical risks of abrupt climate change. Papers should focus on a specific climate change research question. Read on to learn more about examples of climate change research topics and questions.

Find your bootcamp match

What makes a strong climate change research topic.

A strong climate change research paper topic should be precise in order for others to understand your research. You must use research methods to find topics that discuss a concern about climate issues. Your broader topic should be of current importance and a well-defined discourse on climate change.

Tips for Choosing a Climate Change Research Topic

• Research what environmental scientists say. Environmental scientists study ecological problems. Their studies include the threat of climate change on environmental issues. Studies completed by these professionals are a good starting point.
• Use original research to review articles for sources. Starting with a general search is a good place to get ideas. However, as you begin to refine your search, use original research papers that have passed through the stage of peer review.
• Discover the current climatic conditions of the research area. The issue of climate change affects each area differently. Gather information on the current climate and historical climate conditions to help bolster your research.
• Consider current issues of climate change. You want your analyses on climate change to be current. Using historical data can help you delve deep into climate change effects. First, however, it needs to back up climate change risks.
• Research the climate model evaluation options. There are different approaches to climate change evaluation. Choosing the right climate model evaluation system will help solidify your research.

What’s the Difference Between a Research Topic and a Research Question?

A research topic is a broad area of study that can encompass several different issues. An example might be the key role of climate change in the United States. While this topic might make for a good paper, it is too broad and must be narrowed to be written effectively.

A research question narrows the topic down to one or two points. The question provides a framework from which to start building your paper. The answers to your research question create the substance of your paper as you report the findings.

How to Create Strong Climate Change Research Questions

To create a strong climate change research question, start settling on the broader topic. Once you decide on a topic, use your research skills and make notes about issues or debates that may make an interesting paper. Then, narrow your ideas down into a niche that you can address with theoretical or practical research.

Top 10 Climate Change Research Paper Topics

1. climate changes effect on agriculture.

Climate change’s effect on agriculture is a topic that has been studied for years. The concern is the major role of climate as it affects the growth of crops, such as the grains that the United States cultivates and trades on the world market. According to the scientific journal Nature , one primary concern is how the high levels of carbon dioxide can affect overall crops .

2. Economic Impact of Climate Change

Climate can have a negative effect on both local and global economies. While the costs may vary greatly, even a slight change could cost the United States a loss in the Global Domestic Product (GDP). For example, rising sea levels may damage the fiber optic infrastructure the world relies on for trade and communication.

3. Solutions for Reducing the Effect of Future Climate Conditions

Solutions for reducing the effect of future climate conditions range from reducing the reliance on fossil fuels to reducing the number of children you have. Some of these solutions to climate change are radical ideas and may not be accepted by the general population.

4. Federal Government Climate Policy

The United States government’s climate policy is extensive. The climate policy is the federal government’s action for climate change and how it hopes to make an impact. It includes adopting the use of electric vehicles instead of gas-powered cars. It also includes the use of alternative energy systems such as wind energy.

5. Understanding of Climate Change

Understanding climate change is a broad climate change research topic. With this, you can introduce different research methods for tracking climate change and showing a focused effect on specific areas, such as the impact on water availability in certain geographic areas.

6. Carbon Emissions Impact of Climate Change

Carbon emissions are a major factor in climate change. Due to the greenhouse effect they cause, the world is seeing a higher number of devastating weather events. An increase in the number and intensity of tsunamis, hurricanes, and tornados are some of the results.

7. Evidence of Climate Change

There is ample evidence of climate change available, thanks to the scientific community. However, some of these implications of climate change are hotly contested by those with poor views about climate scientists. Proof of climate change includes satellite images, ice cores, and retreating glaciers.

8. Cause and Mitigation of Climate Change

The causes of climate change can be either human activities or natural causes. Greenhouse gas emissions are an example of how human activities can alter the world’s climate. However, natural causes such as volcanic and solar activity are also issues. Mitigation plans for these effects may include options for both causes.

9. Health Threats and Climate Change

Climate change can have an adverse effect on human health. The impacts on health from climate change can include extreme heat, air pollution, and increasing allergies. The CDC warns these changes can cause respiratory threats, cardiovascular issues, and heat-related illnesses.

10. Industrial Pollution and the Effects of Climate Change

Just as car emissions can have an adverse effect on the climate, so can industrial pollution. It is one of the leading factors in greenhouse gas effects on average temperature. While the US has played a key role in curtailing industrial pollution, other countries need to follow suit to mitigate the negative impacts it causes.

Other Examples of Climate Change Research Topics & Questions

Climate change research topics.

• The challenge of climate change faced by the United States
• Climate change communication and social movements
• Global adaptation methods to climate change
• How climate change affects migration
• Capacity on climate change and the effect on biodiversity

Climate Change Research Questions

• What are some mitigation and adaptation to climate change options for farmers?
• How do alternative energy sources play a role in climate change?
• Do federal policies on climate change help reduce carbon emissions?
• What impacts of climate change affect the environment?
• Do climate change and social movements mean the end of travel?

Choosing the Right Climate Change Research Topic

Choosing the correct climate change research paper topic takes continuous research and refining. Your topic starts as a general overview of an area of climate change. Then, after extensive research, you can narrow it down to a specific question.

You need to ensure that your research is timely, however. For example, you don’t want to address the effects of climate change on natural resources from 15 or 20 years ago. Instead, you want to focus on views about climate change from resources within the last five years.

Climate Change Research Topics FAQ

A climate change research paper has five parts, beginning with introducing the problem and background before moving into a review of related sources. After reviewing, share methods and procedures, followed by data analysis . Finally, conclude with a summary and recommendations.

A thesis statement presents the topic of your paper to the reader. It also helps you as you begin to organize your paper, much like a mission statement. Therefore, your thesis statement may change during writing as you start to present your arguments.

According to the US Forest Service, climate change issues are related to topics regarding forest management, biodiversity, and species distribution. Climate change is a broad focus that affects many topics.

To write a research paper title, a good strategy is not to write the title right away. Instead, wait until the end after you finish everything else. Then use a short and to-the-point phrase that summarizes your document. Use keywords from the paper and avoid jargon.

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Top 100 Climate Change Topics To Write About

Climate change issues have continued to increase over the years. That’s because human activities like fossil fuel usage, excavation, and greenhouse emissions continue to drastically change the climate negatively. For instance, burning fossil fuels continues to release greenhouse emissions and carbon dioxide in large quantities. And the lower atmosphere of the earth traps these gasses thereby affecting the global climate. To enhance their awareness of the impact of global warming, educators ask learners to write academic papers and essays on different climate change topics.

According to statistics, global warming affects the climate in different ways. However, the earth has experienced a general temperature increase of 0.85 degrees centigrade over the last 100 years. Such statistics show that this increase will eventually pass the acceptable thresholds in the next 10 years or less. And this will have dire consequences on human health and the global climate. As such, writing a paper about a topic on climate change is a great way to educate the masses.

However, some learners have difficulties choosing topics for their papers and essays on climate change. That’s because this is a relatively new subject. Nevertheless, students that are pursuing ecology, political, and biology studies are conversant with this subject. If struggling to decide what to write about, consider this list of topics related to climate change.

Climate Change Topics for Short Essays

Perhaps, your educator has asked you to write a short essay on climate change. Maybe you’re yet to decide what to write about because every topic you think about seems to have been written about. In that case, use this list of climate change topics for inspiration. You can write about one of these topics or develop it to make it more unique.

• How climate change is responsible for the disappearing rainforest
• The effects of global warming on air quality within the urban areas
• Global warming and greenhouse emissions- Possible health risks
• Is climate change responsible for irregular weather patterns?
• How has climate change affected the food chain?
• The negative effects of climate change on human wellbeing
• How global warming affects agriculture
• How climate change works
• Why is climate change dangerous to human health?
• How to minimize global warming effects on human health
• How global warming affects the healthcare
• Effects of climate change of life quality in rural and urban areas
• How warmer temperatures support allergy-related illnesses
• How climate change is a risk to life on earth
• How climate change and natural disasters correlate
• How climate change affects the population of the earth
• How climate change relates to global warming
• How global warming has caused extreme heating in most urban areas
• How wildfires relate to climate change
• How ocean acidification and climate change affect the world’s habitat

These climate change essay topics cover different aspects of human activities and their effects on the earth’s ecosystem. As such, writing a research paper or essay on any of these topics requires extensive research and analysis of information. That’s the only way you can come up with a solid paper that will impress the educator to award you the top grade.

Climate Change Issues that Make for Good Topics

Maybe you want to research issues that relate to climate change. Most people may have not considered such issues but they are worthy of climate change debate topics. In that case, consider these issues when choosing your climate topics for papers and essays.

• Climate change and threat to natural biodiversity are equally important
• Climate change in Miami and Saudi Arabia- How the effects compare
• Climate change as a human activity’s effect on the environment
• Preventing climate change by protecting forests
• Climate change in China- How the country has declined to head to the global call about saving Mother Nature
• Common causes of climate change
• Common effects of climate change
• The definition of climate change
• What is anthropogenic climate change
• Describe climate change
• What drives climate change?
• Renewable energy sources and climate change
• Human and economics induced climate change
• Climate change biology
• Climate change and business
• Science, Spin, and climate change
• Climate change- How global warming affects populations
• Climate change and social concepts
• Extreme weather and climate change- How they relate
• Global warming as a complex issue in climate change

These are great climate change topics for research papers and essays. However, writing about these topics requires extensive research. You should also be ready to spend energy and time finding relevant and latest sources of information before you write about these topics.

Interesting Climate Change Topics for Papers and Essays

Perhaps, you want to write an essay or paper about something interesting. In that case, consider this list of interesting climate change research paper topics.

• Climate change across the globe- What experts say
• Development, climate change, and disaster reduction
• Critical review- Climate change and agriculture
• Schools should include climate change as a subject in geography courses
• Consumption and climate change- How the wind blows in Indiana
• How the United Nations responds to climate change
• Snowpack and climate change
• How climate change threatens global security
• The effects of climate change on coastal areas’ tourism
• How climate change relates to Queensland Australia’s floods
• How climate change affects the tourism and hospitality industry
• Possible strategies for addressing the effects of climate change on urban areas
• How climate change affects indigenous people
• How to avoid the threats of climate change
• How climate change affects coral triangle turtles
• Climate change drivers in the Asian countries
• Economic discourse analysis methodology in climate change
• How climate change affects New Hampshire businesses
• How climate change affects the life of an individual
• The economic cost of the effects of climate change

These are fantastic climate change paper topics to explore. Nevertheless, you must be ready to research your topic extensively before you start writing your academic paper or essay.

Major Topics on Climate Change for Academic Writing

Perhaps, you’re looking for topics related to climate change that you write major papers about. In that case, you should consider these global climate change topics.

• Early science on climate change
• How the world can manage the effects of climate change
• Environmental issues relating to climate change
• Views comparison about the climate change problem
• Asset-based community development and climate change
• Experts’ evaluation of climate change
• How science affects climate change
• How climate change affects the ocean life
• Scotland’s vulnerability to climate change
• How energy conservation can solve the climate change problem
• How climate change affects the world economy
• International collaboration and climate change
• International relations view on climate change
•  How transportation affects climate change
• Climate change and technology
• Climate change policies and human rights
• Climate change from an anthropological perspective
• Climate change as an international security issue
• Role of the United Nations in addressing climate change
• Climate change and pollution

This category has some of the best climate change thesis topics. That’s because most people will be interested in reading papers on such topics due to their global perspectives. Nevertheless, you should prepare to spend a significant amount of time researching and writing about any of these topics on climate change.

Climate Change Topics for Presentation

Perhaps, you want to write papers on topics related to climate change for presentation purposes. In that case, you need topics that most people can resonate with. Here is a list of topics about climate change that will interest most people.

• How can humans stop global warming in the next ten years
• Could humans have stopped global warming a decade ago?
• How has the environment changed over the years and how has this change caused global warming?
• How did the Obama administration try to limit climate change?
• What is the influence of chemical engineering on global warming?
• How is urbanization connected to climate change?
• Theories that explain why some nations ignore climate change
• How global warming affects the rising sea levels
• How anthropogenic and natural climate change differ
• How the war against terrorism differs from the war on climate change
• How atmospheric change influences global climate change
• Negative effects of global climate change on Minnesota
• The greenhouse effect and ozone depletion
• How greenhouse affects the earth’s environment
• How can individuals reduce the emissions of greenhouse gasses
• How climate change will affect humans in their lifetime
• What are the social, physical, and economic effects of climate change
• Problems and solutions to climate change on the Pacific Ocean
• How climate change relates to species’ extinction
• How the phenomenon of denying climate change affects animals

This list prepared by our  research helpers has some of the best essay topics on climate change. Pick one of these ideas, research it, and then compose a winning paper.

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Climate change, energy, environment and sustainability topics research guide

What is climate change.

Climate change refers to long-term shifts in temperatures and weather patterns. The world is now warming faster than at any point in recorded history, which disrupts the usual balance of nature and is a threat to human beings and other forms of life on Earth. This topic guide includes sample keywords and search terms, databases to find sources, and samples of online books.

Example keywords and subtopics

Example keywords or search terms:  

• Climate change
• global warming
• greenhouse effect or greenhouse gas
• climate crisis
• environmental change
• clean energy
• alternative energy or renewable energy
• green energy or renewable energy or clean energy
• Low carbon or carbon neutral
• Carbon offsetting
• sustainability environment or sustainability
• environmental protection
• pollution or contamination
• impact or effect or influence
• cost or price or expense or money or financial
• fossil fuels or coal or oil or gas

Tip: This is a big topic with lots written so you can often focus on one or two subtopics. This will help to find more relevant sources, more quickly and be a better fit for an assignment. 

Possible subtopics ideas:  Pick one or two subtopics and then add those words to your search.

• Health impacts of climate changes (e.g. air pollution, water pollution, etc.)
• impacts on a specific city, state, region or country
• political impacts (e.g. voting, government policy, etc.)
• impact on specific population or culture (e.g. children, elderly, racial or ethic group, country, etc.)
• specific types of renewable or alternative energy (e.g. solar, wind, bio, etc.) 
• example of new technology (e.g. electric cars or electric vehicles or hybrid vehicles
• economic impacts (e.g. business, employment, industry (e.g. oil, coal, etc.)
• weather and impacts (e.g. rising sea levels, flooding, droughts or heat waves, etc.)
• media aspects (e.g. news coverage, advertising, misinformation, movies, music, etc.) 
• Tutorial: Creating an effective search strategy

• Use meaningful keywords to find the best sources
• Apply search strategies like AND and OR to connect keywords
• Tutorial: What is a library database and why should I use one?

• Identify what a library database is
• Recognize the two main types of library databases
• Know why you should use them
• Understand why searching a library database is different than searching the general internet

Databases for finding sources

Article Databases - 

Use articles to find new research, specific information and evidence to support or refute a claim. You can also look at the bibliography or works cited to find additional sources. Some articles give an overview of a specific topic -- sometimes called "review articles" or "meta-analyses" or "systematic review." Databases are like mini-search engines for finding articles (e.g. Business Source Premier database searches business journals, business magazines and business newspapers). Pick a database that searches the subject of articles you want to find. 

• Agricultural & Environmental Science Database Search journals and literature on agriculture, pollution, animals, environment, policy, natural resources, water issues and more. Searches tools like AGRICOLA, Environmental Sciences & Pollution Management (ESPM), and Digests of Environmental Impact Statements (EIS) databases.
• GreenFILE Collection of scholarly, government and general-interest titles. Multidisciplinary by nature, GreenFILE draws on the connections between the environment and agriculture, education, law, health and technology. Topics covered include global climate change, green building, pollution, sustainable agriculture, renewable energy, recycling, and more.
• Ethnic NewsWatch Ethnic NewsWatch is a current resource of full-text newspapers, magazines, and journals of the ethnic and minority press from 1990, providing researchers access to essential, often overlooked perspectives.
• Opposing Viewpoints in Context Find articles on current issues, including viewpoint articles, topic overviews, statistics, primary documents, magazine and newspaper articles.

Sample of online books

Below are a selection of online books and readings on the broad topic. We have more online books, journal articles, and sources in our Libraries Search and article databases.  

• A climate policy revolution : what the science of complexity reveals about saving our planet by Roland Kupers ISBN: 9780674246812 Publication Date: 2020 "In this book, Roland Kupers argues that the climate crisis is well suited to the bottom-up, rapid, and revolutionary change complexity science theorizes; he succinctly makes the case that complexity science promises policy solutions to address climate change."

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• Peer Research Consultants Make an online 30 minute appointment for one-on-one peer assistance with your research. Get help with researching your topic, finding sources, citing sources and more. Peer Research Consultants can also help you get started with faculty-sponsored research.
• Chat 24/7 online with the Libraries Ask us anything! Chat with a librarian, 24 hours a day, 7 days a week with any research or library questions.
• Meet with a librarian Schedule an online consultations for personalized research support primarily for University of Minnesota faculty, instructors, graduate and undergraduate students and staff.

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• Current Topics: An Undergraduate Research Guide
• Climate Change

Current Topics: An Undergraduate Research Guide : Climate Change

• Writing, Citing, & Research Help
• Black Lives Matter Movement
• Hate Crimes
• Fast Fashion
• Health Care
• Sexual Assault/Rape
• Sexual Harassment
• Newspaper Source Plus Newspaper Source Plus includes 1,520 full-text newspapers, providing more than 28 million full-text articles.
• Newspaper Research Guide This guide describes sources for current and historical newspapers available in print, electronically, and on microfilm through the UW-Madison Libraries. These sources are categorized by pages: Current, Historical, Local/Madison, Wisconsin, US, Alternative/Ethnic, and International.

Organizations

• Carbon Migration Initative The Carbon Mitigation Initiative (CMI) is a 20-year partnership between Princeton University and BP with the goal of finding solutions to the carbon and climate problem.
• Climate Change and Wisconsin's Great Lakes From the State of Wisconsin's Department of Natural Resources (DNR)
• Environmental Protection Agency (EPA) - Climate Change
• Kyoto Protocol The Kyoto Protocol is an international agreement linked to the United Nations Framework Convention on Climate Change, which commits its Parties by setting internationally binding emission reduction targets.
• Union of Concerned Scientists Our scientists and engineers develop and implement innovative, practical solutions to some of our planet’s most pressing problems—from combating global warming and developing sustainable ways to feed, power, and transport ourselves, to fighting misinformation and reducing the threat of nuclear war.

About Climate Change

Rising global temperatures have been accompanied by changes in weather and climate. It is usually attributed to an enhanced greenhouse effect, tending to intensify with the increase in atmospheric carbon dioxide. This Research Guide includes sources relevant to the investigation for causes and effects on the environment of the atmospheric greenhouse effect and global climate change.

Try searching these terms using the resources linked on this page: climate change*, greenhouse effect, greenhouse gas*, global climate change, global warming, greenhouse gas mitigation , carbon dioxide mitigation , carbon sequestration , global temperature changes, paleoclimatology , deglaciation , fossil fuel* and climate change*

Overview Resources - Background Information

• Global Climate Change From NASA
• Opposing Viewpoints Resource Center Opposing Viewpoints Resource Center (OVRC) provides viewpoint articles, topic overviews, statistics, primary documents, links to websites, and full-text magazine and newspaper articles related to controversial social issues.
• State of the Climate Fact sheets & briefs from the Pew Center on Global Climate Change

Articles - Scholarly and Popular

• Academic Search Includes scholarly and popular articles on many topics.
• Environmental Sciences and Pollution Management Includes articles on basic science areas of bacteriology, ecology, toxicology, environmental engineering, environmental biotechnology, waste management, and water resources.
• Meteorological & Geoastrophysical Abstracts Includes articles on the fields of meteorology, climatology, physical oceanography, hydrology, glaciology, and atmospheric chemistry and physics
• Web of Science Includes predominately scholarly articles on a wide range of scientific disciplines.

Statistics and Data

• Center for Climate and Energy Solutions (C2ES) An independent, nonpartisan, nonprofit organization working to advance strong policy and action to address our climate and energy challenges.
• National Climatic Data Center NOAA's National Climatic Data Center (NCDC) provids public access to the largest archive of climatic and historical weather data.
• U.S. Global Change Research Program The U.S. Global Change Research Program (USGCRP) was established by Presidential Initiative in 1989 and mandated by Congress in the Global Change Research Act (GCRA) of 1990 to “assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change.”
• U.S. Greenhouse Gas Inventory Report: 1990-2014 EPA develops an annual report called the Inventory of U.S. Greenhouse Gas Emissions and Sinks (Inventory). This report tracks total annual U.S. emissions and removals by source, economic sector, and greenhouse gas going back to 1990. EPA uses national energy data, data on national agricultural activities, and other national statistics to provide a comprehensive accounting of total greenhouse gas emissions for all man-made sources in the United States.
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• Last Updated: Feb 15, 2024 10:29 AM
• URL: https://researchguides.library.wisc.edu/current

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Hot Topics on Climate Change

On June 1, 2017, U.S. President Donald Trump announced he will withdraw the United States from the Paris Climate Agreement. In spite of this announcement, the fact remains that a global climate change agreement under the United Nations was adopted in December 2015 in Paris. Prior to Trump’s presidency, countries—including the United States— had submitted their “intended nationally determined contributions” (INDCs) for the next one-and-a-half decades. These INDCs lower global greenhouse gas emissions compared to existing policies. However, when projected further into the future, the INDCs still suggest a median warming of roughly 2.5 to 3.0°C by 2100. This exceeds the “well-below 2°C” aim of the Paris Agreement, and year-2030 emissions are higher than what energy-economic analyses indicate would minimize overall costs in view of the necessary long-term reductions. Should the United States really depart the Paris Agreement, which can only technically happen on November 4, 2020 (at the earliest), the situation will only get worst.

Many hot topics have marked the year when it comes to climate change. And it is very likely —more than 90 percent probability—using Intergovernmental Panel on Climate Change (IPCC) technical language, that these topics, and many others, will continue to be increasingly hot in the United States and elsewhere during 2017 and beyond.

The Climate in 2016

Climate conditions were not that great in 2016. Last year the National Oceanic and Atmospheric Administration (NOAA) reported that the global surface temperature was record warm in 2015. This presses the record set the year before by 0.16°C, the largest margin ever by which one year has beaten another on the records (NOAA 2016). And climate trends continued to break marks in 2016, according to NASA (2016).

Only in the course of this year will we know for certain, but a preliminary November 2016 WMO report assessed that 2016 will likely be the hottest year on record, with global temperatures reaching even higher marks than the record-breaking temperatures of 2015 (WMO 2016). Global average temperature by the end of 2016 was already running 1.2°C above pre-industrial levels, a number perilously close to the 1.5°C target aim of the Paris climate agreement of December 2015.

On other fronts, while global temperatures warmed, here in the United States the political climate also began to heat up. Exactly a month and a half after the landmark Paris Agreement officially took effect on November 4, 2016—when one hundred nations, accounting for 69 percent of global greenhouse-gas (GHG) emissions, had formally joined the treaty (UNFCCC 2016)—Mr. Donald John Trump was formally elected by the United States Electoral College on December 19, 2016 as the country´s 45th President. 

The hot topic here is that, on various recent occasions, President Trump expressed his skepticism about human-induced climate change. This included a tweet expressing a view that “the concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive,” and various other public manifestations. Trump stated that with his “America First Energy Plan” he would revert all of President Obama´s policies on climate change, which would include cancelling the country’s participation in the Paris Agreement, ending U.S. funding of the United Nations climate change programs, and abandoning the Clean Power Plan—in order to bring back the coal industry. 

Mr. Trump’s leadership choices for the Department of Energy, the Department of Interior and the Environmental Protection Agency—the three most important, energy-policy-related Federal State institutions—have either denied or strongly challenged the science of climate change. In fact, at the same time that many world leaders are creating dedicated policies to support climate change mitigation and supporting renewable energy sources in order to open new economic sectors, some world leaders perceive this movement as a threat to existing, more conservative, economic forces, like the ones associated with the fossil-fuel industry (Nature 2016b). And indeed, on June 1, 2017, when President Trump proclaimed that the United States was quitting the Paris Climate Agreement, he very much pleased some of the forces within his administration that goaded him to do so.

The Paris Agreement: The Starting Point of a Three-Year Process

Under the December 2015 United Nations Framework Convention on Climate Change Paris Agreement, more than 190 nations committed to take ambitious action 1) to hold the increase in global average temperature to well below 2°C above pre-industrial levels, 2) to pursue efforts to limit the increase to 1.5°C, and 3) to achieve net zero emissions in the second half of this century (UNFCCC 2016a). This means that, from emissions of roughly 50 GtCO2eq/yr today, in the second half this century these emissions will not only need to be zeroed completely, but turned negative. 

This will only be possible with massive carbon sequestration, which is the process of removing carbon from the atmosphere and depositing it in a reservoir. The candidate sectors for this process are the land use sector, with the afforestation and reforestation of large areas of the globe, and the power sector, with the use of carbon dioxide removal technologies, such as fossil-fuel-based and biomass-based power plants with carbon capture and sequestration facilities. 

Already earlier, in preparation of the agreement, countries had submitted their “intended nationally determined contributions” (INDCs) for the agreed 2025 to 2030 period, promising to lower global GHG emissions compared to already existing policies. These INDCs outline national plans to address climate change after 2020. They address a range of issues of which targets and actions for mitigating GHG emissions are a core component. 

The Paris Agreement is a general document, with a framework and overarching goals for global climate action. It is the beginning of a longer process. Some of its loose ends were tied up during the 22nd Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change (COP 22) in Marrakech in November of 2016  (UNFCCC 2016b)—which served as the first meeting of the governing body of the Agreement. But ironing out Paris Agreement details will take some time. Countries participating in COP 22 aim to have the process established by 2018, with a review of progress planned for this same year. But the only concrete outcomes of COP 22 were procedural in nature, with parties to the Convention adopting work plans for further discussions.

However, the real result of the Paris Agreement and of COP 22 (and their long-term success) will depend on assessments of whether or not the already committed pledges, and the ones to come, will have the expected effect on reducing aggregate GHG emissions. Success will mean that the world achieved the temperature objective of holding global warming to well below 2°C and is continuing to “pursue efforts“ to limit it to 1.5°C.

Temperature Increase as a Consequence of the INDCs

It should come as no surprise that limiting global warming to any level implies that the total amount of GHG emissions that can ever be emitted into the atmosphere is finite, given the technical and economic limitations of carbon sequestration possibilities to compensate for that. For example, for a higher than 66 percent chance (meaning “likely”) of limiting global warming to below the internationally agreed temperature limit of 2°C, carbon budget estimates range around 590 to1,240 Gt CO2 from 2015 onward (Rogelj et al 2016b). 

According to IPCC language, a statement that an outcome is “likely” means that the probability of this outcome can range from ≥66 percent (fuzzy boundaries implied) to 100 percent probability. This implies that all alternative outcomes are “unlikely” (0 to 33 percent probability). To put this carbon-budged range in perspective, given current annual emissions of about 40 Gt CO2 globally, this means that the world has a budget of no more than 15 to 60 years of CO2 emissions left at the level of today´s emissions to limiting global warming to 2°C. Only the successful deployment of carbon sequestration practices and technologies could extend this time frame. 

More specifically, for keeping warming to below 2°C, some two thirds of the total CO2 budget have already been emitted, with an urgent need for global CO2 emissions to start to decline, so as not to foreclose the possibility of holding warming to below 2°C. The Paris Agreement acknowledges both of these insights and aims, on the one hand, to reach global peaking of GHG emissions as soon as possible and, on the other hand, to achieve “a balance” between anthropogenic emissions and removals of GHGs in the second half of this century (UNFCCC 2016a).

The purpose of this digest is to assess the extent to which the proposed INDCs impact global GHG emissions by 2030, and explore the consistency of these reductions with the “well below 2°C” objective of the Paris Agreement. This analysis draws heavily on a previous published work (Rogelj et al 2016a), in which I was one of the authors, and where we updated and expanded INDC modelling results that were collected in the framework of the 2015 UNEP Emissions Gap Report (UNEP 2015), in which I was also one of the authors.

The number of INDCs considered by the studies we assessed ranged from the initial 118 INDCs submitted by October 1, 2015 to the final 160 INDCs from the different parties submitted by December 12, 2015 (Rogelj et al 2016a). These INDCs cover emissions from Parties to the Convention responsible for roughly 85 to 88 percent to more than 96 percent of global emissions in 2012. Furthermore, we look at projections of global-mean temperature increase over the twenty-first century that would be consistent with the INDCs, and at post-2030 implications of the INDCs for limiting warming to no more than 2°C.

We used four scenario groups to frame the implications of the INDCs for global GHGs in 2030: 1) no-policy baseline scenarios, 2) current-policy scenarios, 3) INDC scenarios, and 3) least-cost 2°C scenarios:

• No-policy baseline scenarios are emissions projections that assume that no new climate policies have been put into place from 2005 onwards. In this analysis, the no-policy baseline scenarios are selected from the scenario database that accompanied the Fifth Assessment Report (AR5) (available at: https://tntcat.iiasa.ac.at/AR5DB/ ) of the Intergovernmental Panel on Climate Change (IPCC) By design, these no-policy baseline scenarios exclude climate policies, but may include other policies that can influence emissions and are implemented for other reasons, like some energy efficiency or energy security policies.
• Current-policy scenarios consider the most recent estimates of global emissions and take into account implemented policies. These scenarios were drawn from three global INDC analyses (see Rogelj et al 2016a for more details).  Not all countries and sectors are covered by these official and independent country-specific data sources. If this is the case, the median estimate of the three global studies for the ‘current-policy baseline’ for that country or sector is assumed.
• INDC scenarios are at the core of this analysis. They project how global GHG emissions would evolve under the INDCs. These projections are based on the eight global INDC analyses (see Rogelj et al 2016a for more details), which in their calculations use official estimates from the countries themselves.
• 2°C scenarios are idealized global scenarios which are consistent with limiting warming to well below 2°C, keeping open the option of strengthening the global temperature target to 1.5°C. These scenarios are based on a subset of scenarios from the IPCC AR5 Scenario Database that meet the following criteria: they have a greater than 66 per cent chance of keeping warming to below 2°C by 2100; until 2020, they assume that the actions countries pledged earlier under the UNFCCC Cancun Accord are fully implemented; and after 2020, they distribute emission reductions across regions, gases and sectors in such a way that the total discounted costs of the necessary global reductions are minimised, often referred to as least-cost or cost-optimal trajectories.

All scenarios are here expressed in terms of billion tons of global annual CO2 equivalent emissions (Gt CO2e/yr), with. CO2 equivalence of other GHGs calculated by means of 100-year global warming potentials (GWP-100) (Rogelj et al 2016a).

INDC Aggregate Emissions Impact

Different countries report their INDCs differently. Some provide ranges instead of a single number of emissions reductions. Many INDCs lack necessary details, including clarity on sectors and gases covered, on the base year or a reference from which reductions would be measured, or accounting practices related to land use and the use of specific market mechanisms. Also, some of the actions listed in INDCs are, implicitly or explicitly, conditional on other factors, like the availability of financial or technological support. The interpretation of all these factors influences the range of possible outcomes. So, conditional and unconditional INDC scenarios have to be distinguished from each other, although some argue that, implicitly, all INDCs are conditional, with “some being more conditional than others.” This is because, even if a country submits an unconditional INDC, later in time facts out of a country’s control may change its future priorities. Even so, we will keep here a distinction between conditional and unconditional INDCs.

Unconditionally, the INDCs are expected to result in global GHG emissions of about 55 (52 to 57; 10 to 90 percent range) billion tons of annual CO2 equivalent emissions (Gt CO2e/yr; see four scenerio groups above and Figure 1 below) in 2030. This is a reduction of around 9 (7 to 13) Gt CO2e/yr by 2030 relative to the median no-policy baseline scenario estimate and around 4 (2 to 8) Gt CO2e/yr relative to the median current-policy scenario estimate. To have these numbers in context, global GHG emissions in 2010 are estimated at about 48 (46 to 50) Gt CO2e/yr (UNEP 2015), and our median no-policy baseline estimate reaches about 65 Gt CO2e/yr by 2030.

A number of countries place conditions on all or part of their INDC. Some included a range of reduction targets in their INDC and attached conditions to the implementation of the more ambitious end. Others indicate that their entire INDC is conditional. Of the INDCs submitted, roughly half came with both conditional and unconditional components, a third was conditional only, and the rest did not make any distinction. 

For a number of countries, the targets included in their INDC submission suggest achieving emission levels above the estimated no-policy baseline or their current-policy scenario. These countries are thus expected to overachieve their INDC climate targets by default.

Uncertainties in the Estimates and Optimal 2°C Pathways

There is a wide range of possible estimates of future emissions under nominally similar scenarios. These differences are a result of a number of factors, including modeling methods, input data, and assumptions regarding country intent. In fact, four confounding factors in this respect can be identified: 1) global and national sectors coverage, 2) uncertainties in projections, 3) land-use emissions, and 4) historical emissions and metrics.

Once the GHG implications of the INDCs by 2030 are quantified, the question that remains is whether these levels are consistent with the Paris Agreement’s aim of holding warming to well below 2°C. The Paris Agreement’s aim of reaching net-zero GHG emissions in the second half of the century goes even further. For some non-CO2 emissions, only limited mitigation options have been identified. Therefore, net-zero CO2 emissions are always achieved before achieving net-zero GHG emissions. The Scenario Database that accompanied the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Chang (IPCC) is used to explore cost-optimal 2°C pathways from 2020 onward (four scenerios).

The comparison of these cost-optimal 2°C scenarios to the INDC projections shows a large discrepancy (Fig. 1). The median cost-optimal path towards keeping warming to below 2°C (starting reductions in 2020) and the emissions currently implied by the unconditional INDCs differ by about 14 (10–16) Gt CO2e/yr in 2030. Even if the conditions that are linked to some INDCs are met, this difference remains of the order of 11 Gt CO2e/yr. As they stand now, the INDCs clearly do not lead the world to a pathway towards limiting warming to well below 2°C.

Implications of INDCs Post 2030

A large share of the potential warming until 2100 is determined not just by the INDCs until 2025 or 2030, but also by what happens afterwards. Different approaches can be followed to extend INDCs into the future, which basically assume that climate action stops, continues, or accelerates. Stopping action is often modelled by assuming that emissions return to a no-climate-policy trajectory after 2030; continuing action by assuming that the level of post-2030 action is similar to pre-2030 action on the basis of a metric of choice; and accelerating action by post-2030 action that goes beyond such a level. Because of the path-dependence and inertia of the global energy system, the INDCs have a critical role in preparing what can come afterwards.

Each approach may lead to different global temperature outcomes, even when starting from the same INDC assessment for 2025 to 2030. As a conservative interpretation of the Paris Agreement, the assumption made here is that climate action continues after 2030 at a level of ambition that is similar to that of the INDCs. The assumption that climate action will continue or accelerate over time is supported by the Agreement’s requirement that the successive nationally determined contribution (NDC) of each country must represent a progression beyond the earlier contributions, and reflect the highest possible ambition of that country.

Under these assumptions of continued climate action, the 2030 unconditional-INDC emission range is roughly consistent with a median warming relative to pre-industrial levels of 2.6 to 3.1°C (median, 2.9°C; full scenario projection uncertainty, 2.2 to 3.5°C; Table 1), with warming continuing its increase afterwards. This is an improvement on the current-policy and no-policy baseline scenarios, whose median projections suggest about 3.2°C and more than 4°C of temperature rise by 2100, respectively.

The successful implementation of all conditional INDCs would decrease the median estimate by an additional 0.2°C, but keeps the outcome far from the targets the Paris Agreement is aiming for, with well-below 2°C and 1.5°C of warming. Moreover, all above-mentioned values represent median projections coming out of emission scenarios, which in themselves are a function of uncertain assumptions with respect to population growth (more growth, more emissions), economic growth (here too, more growth, more emissions) and even rates of technological improvements (more improvements, less emissions).

Because the climate response to GHG emissions remains uncertain, it is also possible that substantially higher temperatures will materialize with compelling likelihoods (Table 1). For example, at the 66th percentile level, warming under the unconditional INDCs is projected to be about 0.3 °C higher (3.2°C, with a range of 2.9 to 3.4°C). Finally, the INDC cases that are discussed here will exceed the available carbon budget for keeping warming to below 2°C by 2030 with 66 percent probability (that is, roughly 750 to 800 Gt CO2e implied emissions under the INDCs during the 2011 to 2030 period compared to the 750 to 1,400 Gt CO2e available).

The question thus arises whether global temperature rise can be kept to well below 2°C with accelerated action after 2030. Global scenarios that aim to keep warming to below 2°C and that achieve this objective from 2030 GHG emissions similar to those from the INDC range have been assessed in detail by recent large-scale model-comparison projects (Clarke et al 2014 and Riahi et al 2015), but show that even with accelerated action after 2030 options to keep warming to well below 2°C from current INDCs are severely limited, particularly if some key mitigation technologies, such as Carbon Capture and Storage (CCS) or CCS with biomass energy (BECCS), for example, do not scale up as anticipated.

Scenarios in which global warming is successfully contained show rapidly declining emissions after 2030, with global CO2 emissions from energy- and industry-related sources reaching net-zero levels between 2060 and 2080. The global economy is thus assumed to fully decarbonize in the time span of three to five decades and from 2030 levels that are higher than today’s. Furthermore, about two-thirds of these scenarios achieve a balance of global GHG emissions between 2080 and 2100. Because some non-CO2 emissions are virtually impossible to eliminate entirely (for example those from specific agricultural or animal agricultural sources), reaching such a balance will involve net-negative CO2 emissions at a global scale to compensate for any residual non-CO2 emissions, limiting global-average temperatures increase over time.

Exploring futures in which a global balance of GHG emissions can be achieved in the second half of this century with technically feasible and societally acceptable technologies represents a major research challenge emerging from the Paris Agreement. This challenge is particularly relevant to policy, because limiting emissions in 2030 does not only increase the chances of attaining the 2°C target, but also reduces the need to rely on unproven, potentially risky or controversial technologies in the future (Clark et al 2014 and Riahi et al 2015).

Final Considerations

The world has made its decision on Climate Change, despite some recent setbacks here and there. As a recent Editorial of the New York Times put it very clearly, “It´s hard to know how Mr. Trump will change climate policy, but it is almost certain that he won’t advance it” (The New York Times 2016). And indeed, if it is true that the United States will leave the Paris Agreement, for sure it will lose the ability to pressure other countries, including the large emerging economies like Brazil, China and India, to do more.

On the global front, as discussed here, actions may still be too slow and/or too weak, but we can be optimistic and say that, in spite of some hurdles on the way, momentum is building. Covering more than 90 percent of the world’s GHG emissions with climate plans in the form of INDCs was a historic achievement. Now that the Paris Agreement came into force, and that the original INDCs are not simply “Intended” anymore (so, they are no longer INDCs but now Nationally Determined Contributions, or NDCs), it will continue with NDCs, subject to strong transparency of individual contributions and a global stock-take, in the light of equity and science, every five years.

However, the optimism accompanying this process has to be carefully balanced against the important challenges that current INDCs imply for post-2030 emissions reductions. Even starting now limiting warming to no more than 2°C relative to preindustrial levels constitutes an enormous societal challenge. While the contributions open a new era for climate policy under the Paris agreement, they also represent both an invitation and call, if not a need, for further action. Furthering deeper reductions in the coming decade, as well as preparing for a global transformation until mid-century are critical. In absence of incrementally stronger policy signals over the coming five years to a decade, the likelihood that our society will be able to meet the challenge of limiting warming to below 2°C with less than even odds will become extremely small.

Therefore, let us put this clear: Should the United States’ new administration, indeed step back from the previous administration commitment, two possibilities could arise. First, other major emitting nations could also follow suit, turning the Paris Agreement an absolutely irrelevant effort of international negotiation, driving the planet towards unknown climate consequences. Second, because the United States is the second largest GHG emitter, with some 15 percent of world´s total emissions, any climate-change global agreement to succeed would probably also require to have the United States on board, something that is now under a question mark. Therefore, the latter in itself is already a problem even if the former does not materialize. Interestingly enough, the very structure of the Paris Agreement, like the Kyoto Protocol, was designed largely to United States specifications, and also an answer to United States’ prayers.

The problem is that, in fact, political upsets could stall coordinated international mitigation action, with long-term consequences, eventually even rendering the 2°C target unachievable (Sanderson et at 2016). Interesting enough, although the governments of the world have requested the IPCC to assess, through a Special Report due in 2018 (IPCC 2016), the impacts of 1.5°C of warming, as well as ways to prevent temperatures from rising higher, many scientists have practically already written off the chances of limiting warming to 1.5 °C (Rogelj et al 2016b and Luderer et al 2016).

As discussed before, the Paris Agreement commits governments to keeping average global surface temperatures to between 1.5°C and 2°C above the preindustrial level, but warming has already passed the 1°C mark (WMO 2016). If the 2°C goal is already seen implausible by some, given a lack of more effective actions and current politics, let alone the even more ambitions 1.5°C target (Nature 2016a), let us hope that the economies of the world will be able to do their homework on time. We cannot travel the last mile with quick fixes, which would be too dependent on extremely risky and uncertain technologies, such as geoengineering, as some have begun to consider (Hubert et al 2016). Unfortunately, the recent move of the current United States Administration with respect to the Paris Agreement is not going to be of much help in that respect.

T his digest has been inspired by from Rogelj et al (2016a), of which Roberto Schaeffer is one of the authors. The author wishes to acknowledge extremely helpful comments from a reviewer of an earlier draft. Any remaining errors are the responsibility of the author alone.

Roberto Schaeffer

Clarke, L. et al. in Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds O. Edenhofer et al.) Ch. 6, 413-510 (Cambridge University Press, 2014). Hubert, AM., Kruger, T. Rayner, S. Code of conduct for geoengineering. Nature 537, 488 (2016). IPCC. Scoping Meeting for the IPCC Special Report on the Impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways. Geneva, Switzerland, 15-16 August. https://www.ipcc.ch/report/sr15/ , accessed on 30 December (2016). Luderer, G., Kriegler, E., Delsa, L., Edelenbosch, O. Y., Emmerling, J., Krey, V., McCollum, D. L., Pachauri, S., Riahi, K., Saveyn, B., Tavoni, M., Vrontisi, Z., van Vuuren, D. P., Arent, D., Arvesen, A., Fujimori, S., Iyer, G. Keppo, I., Kermeli, K., Mima, S., Ó Broin, E., Pietzcker, R. C., Sano, F., Scholz, Y., van Ruijven, B. & Wilson, C. Deep decarbonisation towards 1.5 °C – 2 °C stabilisation. Policy findings from the ADVANCE project (first edition, 2016). NASA. https://www.nasa.gov/feature/goddard/2016/climate-trends-continue-to-bre… , accessed on 20 December (2016). Nature. Climate ambition. Nature 537, 585-586, 29 September (2016a). Nature. Let reason prevail. Nature 538, 289, 20 October (2016b). NOAA. http://www.noaa.gov/climate , accessed on 20 December (2016). Riahi, K. et al. Locked into Copenhagen pledges — Implications of short-term emission targets for the cost and feasibility of long-term climate goals. Technological Forecasting and Social Change 90, Part A, 8-23, doi: http://dx.doi.org/10.1016/j.techfore.2013.09.016 (2015). Rogelj, J., den Elzen, M., Hohne, N., Fransen, T., Fekete, H., Winkler, H., Schaeffer, R., Sha, F., Riahi, K. & Meinshausen, M. Paris Agreement climate proposals need a boost to keep warming well below 2 °C. Nature 534, 631-639, doi:10.1038/nature18307 (2016a). Rogelj, J., Schaeffer, M., Friedlingstein, P., Gillett, N. P., van Vuuren, D. P., Riahi, K., Allen, M. & Knutti, R. Differences between carbon budget estimates unravelled. Nature Climate Change 6, 245-252-, doi: 10.1038/nclimate2868 (2016b). Sanderson, B. M. & Knutti, R. Delays in US mitigation could rulled out Paris targets. Nature Climate Change, advance publication, published online on 26 December, http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3193.html , accessed on 28 December (2016). The New York Times. States Will Lead on Climate Change in the Trump Era. http://www.nytimes.com/2016/12/26/opinion/states-will-lead-on-climate-ch… , accessed on 26 December (2016). UNEP. The Emissions Gap Report 2015. 98 (UNEP, Nairobi, Kenya, 2015). UNFCCC. Adoption of the Paris Agreement. Report No. FCCC/CP/2015/L.9/Rev.1, http://unfccc.int/resource/docs/2015/cop21/eng/109r01.pdf , accessed on 20 December (2016a). UNFCCC. http://unfccc.int/meetings/marrakech_nov_2016/session/9676.php , assessed on 27 December (2016b). WMO. https://public.wmo.int/en/media/press-release/provisional-wmo-statement-… , accessed on 20 December (2016).  

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Research papers: climate change.

By Jonathan Eyer, Casey Wichman

Journal of Environmental Economics and Management 87

Water withdrawals for the energy sector are the largest use of fresh water in the United States. Using an econometric model of monthly plant-level electricity generation levels between 2001 and 2012, we estimate the effect of water scarcity on the US electricity fuel mix. We find that hydroelectric generation decreases substantially in response to drought, although this baseline generation is offset primarily by natural gas, depending on the geographic region. We provide empirical evidence that drought can increase emissions of CO2 and local pollutants. We quantify the social costs of water scarcity to be $330,000 per month for each plant that experiences a one-standard deviation increase in water scarcity (2015 dollars), a relationship that persists under future projections of water scarcity.

By Solomon Hsiang, Paulina Oliva, Reed Walker

Review of Environmental Economics and Policy, Volume 13, Issue 1

Most regulations designed to reduce environmental externalities impose costs on individuals and firms. A large and growing literature examines whether these costs are disproportionately borne by different sectors of the economy and/or across different groups of individuals. However, much less is known about how the environmental benefits created by these policies are distributed, which mirror the differences in environmental damages associated with existing environmental externalities. We review this burgeoning literature and develop a simple general framework for empirical analysis. We apply this framework to findings concerning the distributional impacts of environmental damages from air pollution, deforestation, and climate change and highlight priorities for future research. A recurring challenge to understanding the distributional effects of environmental damages is distinguishing between cases in which populations are exposed to different levels or changes in an environmental good and those in which an incremental change in the environment may have very different implications for some populations. In the latter case, it is often difficult to empirically identify the underlying sources of heterogeneity in marginal damages because damages may stem from nonlinear and/or heterogeneous damage functions. Nevertheless, understanding the determinants of heterogeneity in environmental benefits and damages is crucial for welfare analysis and policy design.

By Gale M. Sinatra

Educational and Developmental Psychologist

The climate crisis is the defining issue of our time. Educational and developmental psychologists can make clear and important contributions to addressing this existential threat. The articles in the Climate Crisis Special Issue take on the issue of climate change from multiple angles, with varied populations, using different research methods and theoretical frameworks. The special issue makes clear the important role psychologists have to play in addressing the climate crisis.

Journal of Educational Psychology

Texts presenting novel numerical data can shift learners’ attitudes and conceptions about controversial science topics. However, little is known about the mechanisms underlying this conceptual change. The purpose of this study was to investigate two potential mechanisms that underlie learning from novel data: numerical estimation skills and epistemic cognition. This research investigated combinations of two treatments—a numerical estimation and epistemic cognition intervention—that were designed to enhance people’s ability to make sense of key numbers about climate change when integrated into an existing intervention. Results indicated that undergraduate students ( N = 516) who engaged with climate change data held fewer misconceptions compared with a group that read an expository text, though their judgments of climate change plausibility were similar. Results also showed that the two modifications to the central intervention did not have statistically significant effects on knowledge or plausibility when compared with the unmodified intervention. However, we found that individuals’ openness to reason with and integrate new evidence significantly moderated the knowledge effects of the intervention when the intervention was supplemented with both modifications. These findings provide emerging evidence that, among those who are open to reason with new evidence, supporting mathematical reasoning skills and reflection on discrepant information can enhance conceptual change in science.

By Doug Lombardi, Gale M.Sinatra, E. Michael Nussbaum

Learning and Instruction, 27

Plausibility is a central but under-examined topic in conceptual change research. Climate change is an important socio-scientific topic; however, many view human-induced climate change as implausible. When learning about climate change, students need to make plausibility judgments but they may not be sufficiently critical or reflective. The purpose of this study was to examine how students’ plausibility judgments and knowledge about human-induced climate change transform during instruction promoting critical evaluation. The results revealed that treatment group participants who engaged in critical evaluation experienced a significant shift in their plausibility judgments toward the scientifically accepted model of human-induced climate change. This shift was accompanied by significant conceptual change postinstruction that was maintained after a six-month delay. A comparison group who experienced a climate change activity that is part of their normal curriculum did not experience statistically significant changes.

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The 2035 report: abundant, affordable offshore wind can accelerate our clean electricity future.

Umed Paliwal, Nikit Abhyankar, Taylor McNair, Jose Dominguez Bennett, David Wooley, Jamie Matos, Ric O’Connell, Amol Phadke. "Abundant, Affordable Offshore Wind Can Accelerate Our Clean Electricity Future" August 1, 2023. 

Plummeting costs and technical performance improvements of offshore wind have dramatically enhanced the prospects for near-term power sector decarbonization. The high resource quality of offshore wind in the United States, coupled with rapidly falling technology costs, makes it possible for offshore wind to provide 10-25% of total electricity generation in the U.S. power system in 2050 without substantially impacting wholesale electricity costs. This report, 2035 Report 3.0, examines the prospect of achieving 90% clean electricity by 2035 and 95% clean electricity by 2050. Three scenarios — Low, Medium, and High Ambition — detail the electricity system impacts of increased offshore wind growth providing 10-25% of total generation.

Global carbon emissions must be halved by 2030 to limit global warming to 1.5 degrees Celsius and avoid the most catastrophic impacts of climate change (UN IPCC, 2023). While the United States continues to make progress on national decarbonization trends, with increases in clean energy produc- tion delivering cuts in power sector emissions, 2022 still saw a slight rise in the nation’s overall greenhouse gas emissions (Rhodium Group, 2023). For the U.S. to achieve net zero emissions, in which the nation emits no more carbon into the atmosphere than can be removed, the U.S. must significant- ly ramp up clean energy production while electrifying other sectors of the economy, such as buildings, transportation, and industry — likely causing U.S. electricity demand to triple by 2050. 

Around the globe, nations have begun to grasp the opportunity on the waters. The global pipeline of offshore wind projects that have been announced or are in pre-construction phases now stands at over 700 GW (GEM, 2023). The European Union will endeavor to build nearly 400 GW of offshore wind by 2050, while China installed 20 GW in the last two years alone (European Commission, 2023; GWEC, 2023). While the Biden Administration has a target to deploy 30 GW of offshore wind by 2030 and 110 GW by 2050, increasing offshore wind ambition beyond these current goals could accelerate the nation’s transition to net zero emissions.

The 2035 Report

Systematic over-crediting in California’s forest carbon offsets program

Grayson Badgley, Jeremy Freeman, Joseph J. Hamman, Barbara Haya, Anna T. Trugman, William R.L. Anderegg, & Danny Cullenward (2021). Global Change Biology, DOI:10.1111/gcb.15943

Carbon offsets are widely used by individuals, corporations, and governments to mitigate their greenhouse gas emissions on the assumption that offsets reflect equivalent climate benefits achieved elsewhere. These climate-equivalence claims depend on offsets providing “additional” climate benefits beyond what would have happened, counterfactually, without the offsets project. Here, we evaluate the design of California’s prominent forest carbon offsets program and demonstrate that its climate-equivalence claims fall far short on the basis of directly observable evidence. By design, California’s program awards large volumes of offset credits to forest projects with carbon stocks that exceed regional averages. This paradigm allows for adverse selection, which could occur if project developers preferentially select forests that are ecologically distinct from unrepresentative regional averages. By digitizing and analyzing comprehensive offset project records alongside detailed forest inventory data, we provide direct evidence that comparing projects against coarse regional carbon averages has led to systematic over-crediting of 30.0 million tCO2e (90% CI: 20.5 to 38.6 million tCO2e) or 29.4% of the credits we analyzed (90% CI: 20.1 to 37.8%). These excess credits are worth an estimated $410 million (90% CI: $280 to $528 million) at recent market prices. Rather than improve forest management to store additional carbon, California’s offsets program creates incentives to generate offset credits that do not reflect real climate benefits.

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Managing uncertainty in carbon offsets: insights from California’s standardized approach

Barbara Haya, Danny Cullenward, Aaron L. Strong, Emily Grubert, Robert Heilmayr, Deborah A. Sivas, & Michael Wara (2020) Climate Policy, DOI: 10.1080/14693062.2020.1781035

Carbon offsets allow greenhouse gas emitters to comply with an emissions cap by paying others outside of the capped sectors to reduce emissions. The first major carbon offset programme, the United Nations’ Clean Development Mechanism (CDM), has been criticized for generating a large number of credits from projects that do not actually reduce emissions. Following the controversial CDM experience, California pioneered a second-generation compliance offset programme that shifts the focus of quality control from assessments of individual projects to the development of offset protocols, which define project type-specific eligibility criteria and methods for estimating emissions reductions. We assess the ability of California’s ‘standardized approach’ to mitigate the risk of over-crediting greenhouse gas reductions by reviewing the development of two California offset protocols – Mine Methane Capture and Rice Cultivation. We examine the regulator’s treatment of three sources of over-crediting under the CDM: non-additional projects, inflated counterfactual baseline scenarios, and perverse incentives that inadvertently increase emissions. We find that the standardized approach offers the ability to reduce, but not eliminate, the risk of over-crediting. This requires careful protocol-scale analysis, conservative methods for estimating reductions, ongoing monitoring of programme outcomes, and restricting participation to project types with manageable levels of uncertainty in emission reductions. However, several of these elements are missing from California’s regime, and even best practices result in significant uncertainty in true emission reductions. Relying on carbon offsets to lower compliance costs risks lessening total emission reductions and increases uncertainty in whether an emissions target has been met.

Key policy insights

- Substantial and ongoing oversight by offset programme administrators is needed to contain uncertainty and avoid over-crediting.

- California’s Mine Methane Capture Protocol may have influenced federal decisions not to regulate methane emissions from coal mines on federally-owned lands.

- Government priorities and methodological choices drive outcomes in carbon pricing policies with large offset programmes, contrary to the common perception that these policies delegate decision-making to private actors.

- Offsets are better understood as a way for regulated emitters to invest in an incentive programme that achieves difficult-to-estimate emission reductions, than as accurately quantified tons of reductions.

Journal article

Working paper version

Coverage by MIT Technology Review

Coverage by KQED

Estimating economic damage from climate change in the United States

Hsiang et al., Science 356, 1362–1369 (2017)

Estimates of climate change damage are central to the design of climate policies. Here, we develop a flexible architecture for computing damages that integrates climate science, econometric analyses, and process models. We use this approach to construct spatially explicit, probabilistic, and empirically derived estimates of economic damage in the United States from climate change. The combined value of market and nonmarket damage across analyzed sectors—agriculture, crime, coastal storms, energy, human mortality, and labor—increases quadratically in global mean temperature, costing roughly 1.2% of gross domestic product per +1°C on average. Importantly, risk is distributed unequally across locations, generating a large transfer of value northward and westward that increases economic inequality. By the late 21st century, the poorest third of counties are projected to experience damages between 2 and 20% of county income (90% chance) under business-as-usual emissions (Representative Concentration Pathway 8.5).

Social and economic impacts of climate

T. A. Carleton and S. M. Hsiang, Science 353, aad9837 (2016).

For centuries, thinkers have considered whether and how climatic conditions—such as temperature, rainfall, and violent storms—influence the nature of societies and the performance of economies. A multidisciplinary renaissance of quantitative empirical research is illuminating important linkages in the coupled climate-human system. We highlight key methodological innovations and results describing effects of climate on health, economics, conflict, migration, and demographics. Because of persistent “adaptation gaps,”current climate conditions continue to play a substantial role in shaping modern society, and future climate changes will likely have additional impact. For example, we compute that temperature depresses current U.S. maize yields by ~48%, warming since 1980 elevated conflict risk in Africa by ~11%, and future warming may slow global economic growth rates by ~0.28 percentage points per year. In general, we estimate that the economic and social burden of current climates tends to be comparable in magnitude to the additional projected impact caused by future anthropogenic climate changes. Overall, findings from this literature point to climate as an important influence on the historical evolution of the global economy, they should inform how we respond to modern climatic conditions, and they can guide how we predict the consequences of future climate changes.

Climate Econometrics

Hsiang, Annual Review of Resource Economics (2016)

Identifying the effect of climate on societies is central to understanding historical economic development, designing modern policies that react to climatic events, and managing future global climate change. Here, I review, synthesize, and interpret recent advances in methods used to measure effects of climate on social and economic outcomes. Because weather variation plays a large role in recent progress, I formalize the relationship between climate and weather from an econometric perspective and discuss the use of these two factors as identifying variation, highlighting trade-offs between key assumptions in different research designs and deriving conditions when weather variation exactly identifies the effects of climate. I then describe recent advances, such as the parameterization of climate variables from a social perspective, use of nonlinear models with spatial and temporal displacement, characterization of uncertainty, measurement of adaptation, cross-study comparison, and use of empirical estimates to project the impact of future climate change. I conclude by discussing remaining methodological challenges.

Potentially Extreme Population Displacement and Concentration in the Tropics Under Non-Extreme Warmi

Hsiang, S., Sobel, A. Potentially Extreme Population Displacement and Concentration in the Tropics Under Non-Extreme Warming.  Sci Rep  6, 25697 (2016)

Evidence increasingly suggests that as climate warms, some plant, animal and human populations may move to preserve their environmental temperature. The distances they must travel to do this depends on how much cooler nearby surfaces temperatures are. Because large-scale atmospheric dynamics constrain surface temperatures to be nearly uniform near the equator, these displacements can grow to extreme distances in the tropics, even under relatively mild warming scenarios. Here we show that in order to preserve their annual mean temperatures, tropical populations would have to travel distances greater than 1000 km over less than a century if global mean temperature rises by 2 °C over the same period. The disproportionately rapid evacuation of the tropics under such a scenario would cause migrants to concentrate in tropical margins and the subtropics, where population densities would increase 300% or more. These results may have critical consequences for ecosystem and human wellbeing in tropical contexts where alternatives to geographic displacement are limited.

Consumers' Willingness to Pay for Renewable and Nuclear Energy: A Comparative Analysis between the US and Japan

“Consumers' Willingness to Pay for Renewable and Nuclear Energy: A Comparative Analysis between the US and Japan” (with Kayo Murakami, Takanori Ida, and Makoto Tanaka), Energy Economics , 50 , July 2015, pp. 178-189.

We investigate through a survey-based choice experiment US and Japanese consumer preferences for two alternative fuels, nuclear and renewable sources, as energy sources that have potential to reduce GHG emissions. The results for the US are similar across the four states sampled concerning consumers’ WTP for the reduction of air emissions: people are willing to pay approximately $0.30 per month for a 1% decrease in GHG emissions. Second, the average consumer expresses a negative preference for increases in nuclear power in the fuel mix in both countries. Japanese consumers have a stronger aversion to nuclear energy than US consumers. Third, US and Japanese consumers will pay more for emissions reduction with the use of renewable sources. Finally, we have shown that results like those found in this study can be useful in helping to set parameters for renewable energy policies like FIT rates and RPS stringency.

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Heat waves: a hot topic in climate change research

Werner marx.

1 Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

Robin Haunschild

Lutz bornmann.

2 Science Policy and Strategy Department, Max Planck Society, Administrative Headquarters, Hofgartenstr. 8, 80539 Munich, Germany

Associated Data

Not applicable.

Research on heat waves (periods of excessively hot weather, which may be accompanied by high humidity) is a newly emerging research topic within the field of climate change research with high relevance for the whole of society. In this study, we analyzed the rapidly growing scientific literature dealing with heat waves. No summarizing overview has been published on this literature hitherto. We developed a suitable search query to retrieve the relevant literature covered by the Web of Science (WoS) as complete as possible and to exclude irrelevant literature ( n  = 8,011 papers). The time evolution of the publications shows that research dealing with heat waves is a highly dynamic research topic, doubling within about 5 years. An analysis of the thematic content reveals the most severe heat wave events within the recent decades (1995 and 2003), the cities and countries/regions affected (USA, Europe, and Australia), and the ecological and medical impacts (drought, urban heat islands, excess hospital admissions, and mortality). An alarming finding is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels measured as “wet-bulb temperature” (WBT). Risk estimation and future strategies for adaptation to hot weather are major political issues. We identified 104 citation classics, which include fundamental early works of research on heat waves and more recent works (which are characterized by a relatively strong connection to climate change).

Introduction

As a consequence of the well-documented phenomenon of global warming, climate change has become a major research field in the natural and medical sciences, and more recently also in the social and political sciences. The scientific community has contributed extensively to a comprehensive understanding of the earth’s climate system, providing various data and projections on the future climate as well as on the effects and risks of anticipated global warming (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). During recent decades, climate change has also become a major political, economic, and environmental issue and a central theme in political and public debates.

One consequence of global warming is the increase of extreme weather events such as heat waves, droughts, floods, cyclones, and wildfires. Some severe heat waves occurring within the last few decades made heat waves a hot topic in climate change research, with “hot” having a dual meaning: high temperature and high scientific activity. “More intense, more frequent, and longer lasting heat waves in the twenty-first century” is the title of a highly cited paper published 2004 in Science (Meehl and Tebaldi 2004 ). This title summarizes in short what most climate researchers anticipate for the future. But what are heat waves (formerly also referred to as “heatwaves”)? In general, a heat wave is a period of excessively hot weather, which may be accompanied by high humidity. Since heat waves vary according to region, there is no universal definition, but only definitions relative to the usual weather in the area and relative to normal temperatures for the season. The World Meteorological Organization (WMO) defines a heat wave as 5 or more consecutive days of prolonged heat in which the daily maximum temperature is higher than the average maximum temperature by 5 °C (9 °F) or more ( https://www.britannica.com/science/heat-wave-meteorology ).

Europe, for example, has suffered from a series of intense heat waves since the beginning of the twenty-first century. According to the World Health Organization (WHO) and various national reports, the extreme 2003 heat wave caused about 70,000 excess deaths, primarily in France and Italy. The 2010 heat wave in Russia caused extensive crop loss, numerous wildfires, and about 55,000 excess deaths (many in the city of Moscow). Heat waves typically occur when high pressure systems become stationary and the winds on their rear side continuously pump hot and humid air northeastward, resulting in extreme weather conditions. The more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations and without human-made climate change (IPCC 2014; CSSR 2017; NCA4 2018). Scientists discuss a weakening of the polar jet stream caused by global warming as a possible reason for an increasing probability for the occurrence of stationary weather, resulting in heavy rain falls or heat waves (Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). This jet stream is one of the most important factors for the weather in the middle latitude regions of North America, Europe, and Asia.

Until the end of the twentieth century, heat waves were predominantly seen as a recurrent meteorological fact with major attention to drought, being almost independent from human activities and unpredictable like earthquakes. However, since about 1950, distinct changes in extreme climate and weather events have been increasingly observed. Meanwhile, climate change research has revealed that these changes are clearly linked to the human influence on the content of greenhouse gases in the earth’s atmosphere. Climate-related extremes, such as heat waves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability to climate change as a result of global warming.

In recent years, research on heat waves has been established as an emerging research topic within the large field of current climate change research. Bibliometric analyses are very suitable in order to have a systematic and quantitative overview of the literature that can be assigned to an emerging topic such as research dealing with heat waves (e.g., Haunschild et al. 2016 ). No summarizing overview on the entire body of heat wave literature has been published until now. However, a bibliometric analysis of research on urban heat islands as a more specific topic in connection with heat waves has been performed (Huang and Lu 2018 ).

In this study, we analyzed the publications dealing with heat waves using appropriate bibliometric methods and tools. First, we determined the amount and time evolution of the scientific literature dealing with heat waves. The countries contributing the most papers are presented. Second, we analyzed the thematic content of the publications via keywords assigned by the WoS. Third, we identified the most important (influential) publications (and also the historical roots). We identified 104 citation classics, which include fundamental early works and more recent works with a stronger connection to climate change.

Heat waves as a research topic

The status of the current knowledge on climate change is summarized in the Synthesis Report of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2014, https://www.ipcc.ch/report/ar5/syr/ ). This panel is the United Nations body for assessing the science related to climate change. The Synthesis Report is based on the reports of the three IPCC Working Groups , including relevant Special Reports . In its Summary for Policymakers , it provides an integrated view of climate change as the final part of the Fifth Assessment Report (IPCC 2014, https://www.ipcc.ch/site/assets/uploads/2018/02/AR5_SYR_FINAL_SPM.pdf ).

In the chapter Extreme Events , it is stated that “changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions … It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-twentieth century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations” (p. 7–8). Under Projected Changes , the document summarizes as follows: “Surface temperature is projected to rise over the twenty-first century under all assessed emission scenarios. It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions” (p. 10).

With regard to the USA, the Climate Science Special Report of the U.S. Global Change Research Program (CSSR 2017, https://science2017.globalchange.gov/ ) mentions quite similar observations and states unambiguously in its Fourth National Climate Assessment (Volume I) report ( https://science2017.globalchange.gov/downloads/CSSR2017_FullReport.pdf ) under Observed Changes in Extremes that “the frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). The frequency and intensity of extreme heat and heavy precipitation events are increasing in most continental regions of the world (very high confidence). These trends are consistent with expected physical responses to a warming climate [p. 19]. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence) [p. 20]. There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). Recent droughts and associated heat waves have reached record intensity in some regions of the United States … (very high confidence) [p. 21]. Confidence in attribution findings of anthropogenic influence is greatest for extreme events that are related to an aspect of temperature” (p. 123).

Among the key findings in the chapter on temperature changes in the USA, the report states that “there have been marked changes in temperature extremes across the contiguous United States. The frequency of cold waves has decreased since the early 1900s, and the frequency of heat waves has increased since the mid-1960s (very high confidence). Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures. The temperatures of extremely cold days and extremely warm days are both expected to increase. Cold waves are projected to become less intense while heat waves will become more intense (very high confidence) [p. 185]. Most of this methodology as applied to extreme weather and climate event attribution, has evolved since the European heat wave study of Stott et al.” (p. 128).

Heat waves are also discussed in the Fourth National Climate Assessment (Volume II) report (NCA4 2018, https://nca2018.globalchange.gov/ ). The Report-in-Brief ( https://nca2018.globalchange.gov/downloads/NCA4_Report-in-Brief.pdf ) for example states: “More frequent and severe heat waves and other extreme events in many parts of the United States are expected [p. 38]. Heat waves and heavy rainfalls are expected to increase in frequency and intensity [p. 93]. The season length of heat waves in many U.S. cities has increased by over 40 days since the 1960s [p. 30]. Cities across the Southeast are experiencing more and longer summer heat waves [p. 123]. Exposure to hotter temperatures and heat waves already leads to heat-associated deaths in Arizona and California. Mortality risk during a heat wave is amplified on days with high levels of ground-level ozone or particulate air pollution” (p. 150).

In summary, climate change research expects more frequent and more severe heat wave events as a consequence of global warming. It is likely that the more frequent and longer lasting heat waves will significantly increase excess mortality, particularly in urban regions with high air pollution. Therefore, research around heat waves will become increasingly important and is much more than a temporary research fashion.

Methodology

Dataset used.

This analysis is based on the relevant literature retrieved from the following databases accessible under the Web of Science (WoS) of Clarivate Analytics: Web of Science Core Collection: Citation Indexes, Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), Conference Proceedings Citation Index—Science (CPCI-S), Conference Proceedings Citation Index—Social Science & Humanities (CPCI-SSH), Book Citation Index—Science (BKCI-S), Book Citation Index—Social Sciences & Humanities (BKCI-SSH), Emerging Sources Citation Index (ESCI).

We applied the search query given in Appendix 1 to cover the relevant literature as completely as possible and to exclude irrelevant literature. We practiced an iterative query optimization by identifying and excluding the WoS subject categories with most of the non-relevant papers. For example, heat waves are also mentioned in the field of materials science but have nothing to do with climate and weather phenomena. Unfortunately, WoS obviously assigned some heat wave papers related to climate to materials science-related subject categories. Therefore, these subject categories were not excluded. By excluding the other non-relevant subject categories, 597 out of 8,568 papers have been removed, resulting in a preliminary publication set of 7,971 papers (#2 of the search query). But this is no safe method, since the excluded categories may well include some relevant papers. Therefore, we have combined these 597 papers with search terms related to climate or weather and retrieved 62 relevant papers in addition, which we added to our preliminary paper subset, eventually receiving 8,033 publications (#3 to #5 of the search query).

Commonly, publication sets for bibliometric analyses are limited to articles, reviews, and conference proceedings as the most relevant document types and are restricted to complete publication years. In this study, however, we have included all relevant WoS document types for a better literature coverage of the research topic analyzed. For example, conference meetings and early access papers may well be interesting for the content analysis of the literature under study. Such literature often anticipates important results, which are published later as regular articles. Furthermore, we have included the literature until the date of search for considering the recent rapid growth of the field. Our search retrieved a final publication set of 8,011 papers indexed in WoS until the date of search (July 1, 2021) and dealing with heat waves (#6 of the search query). We have combined this publication set with climate change-related search terms from a well-proven search query (Haunschild et al. 2016 ) resulting in 4,588 papers dealing with heat waves in connection with climate change or global warming (# 11 of the search query). Also, we have selected a subset of 2,373 papers dealing with heat waves and mortality (#13 of the search query). The complete WoS search query is given in Appendix 1.

The final publication set of 8,011 papers dealing with heat waves still contains some non-relevant papers primarily published during the first half of the twentieth century, such as some Nature papers within the WoS category Multidisciplinary Sciences . Since these papers are assigned only to this broad subject category and have no abstracts and no keywords included, they cannot be excluded using the WoS search and refinement functions. We do not expect any bias through these papers, because their keywords do not appear in our maps. Also, they normally contain very few (if any) cited references, which could bias/impact our reference analysis.

We used the VOSviewer software (Van Eck and Waltman 2010 ) to map co-authorship with regard to the countries of authors (88 countries considered) of the papers dealing with heat waves ( www.vosviewer.com ). The map of the cooperating countries presented is based on the number of joint publications. The distance between two nodes is proportionate to the number of co-authored papers. Hence, largely cooperating countries are positioned closer to each other. The size of the nodes is proportionate to the number of papers published by authors of the specific countries.

The method that we used for revealing the thematic content of the publication set retrieved from the WoS is based on the analysis of keywords. For better standardization, we chose the keywords allocated by the database producer (keywords plus) rather than the author keywords. We also used the VOSviewer for mapping the thematic content of the 104 key papers selected by reference analysis. This map is also based on keywords plus.

The term maps (keywords plus) are based on co-occurrence for positioning the nodes on the maps. The distance between two nodes is proportionate to the co-occurrence of the terms. The size of the nodes is proportionate to the number of papers with a specific keyword. The nodes on the map are assigned by VOSviewer to clusters based on a specific cluster algorithm (the clusters are highlighted in different colors). These clusters identify closely related (frequently co-occurring) nodes, where each node is assigned to only one cluster.

Reference Publication Year Spectroscopy

A bibliometric method called “Reference Publication Year Spectroscopy” (RPYS, Marx et al. 2014 ) in combination with the tool CRExplorer ( http://www.crexplorer.net , Thor et al. 2016a , b ) has proven useful for exploring the cited references within a specific publication set, in order to detect the most important publications of the relevant research field (and also the historical roots). In recent years, several studies have been published, in which the RPYS method was basically described and applied (Marx et al. 2014 ; Marx and Bornmann 2016 ; Comins and Hussey 2015 ). In previous studies, Marx et al. have analyzed the roots of research on global warming (Marx et al. 2017a ), the emergence of climate change research in combination with viticulture (Marx et al. 2017b ), and tea production (Marx et al. 2017c ) from a quantitative (bibliometric) perspective. In this study, we determined which references have been most frequently cited by the papers dealing with heat waves.

RPYS is based on the assumption that peers produce a useful database by their publications, in particular by the references cited therein. This database can be analyzed statistically with regard to the works most important for their specific research field. Whereas individual scientists judge their research field more or less subjectively, the overall community can deliver a more objective picture (based on the principle of “the wisdom of the crowds”). The peers effectively “vote” via their cited references on which works turned out to be most important for their research field (Bornmann and Marx 2013 ). RPYS implies a normalization of citation counts (here: reference counts) with regard to the research area and the time of publication, which both impact the probability to be cited frequently. Basically, the citing and cited papers analyzed were published in the same research field and the reference counts are compared with each other only within the same publication year.

RPYS relies on the following observation: the analysis of the publication years of the references cited by all the papers in a specific research topic shows that publication years are not equally represented. Some years occur particularly frequently among the cited references. Such years appear as distinct peaks in the distribution of the reference publication years (i.e., the RPYS spectrogram). The pronounced peaks are frequently based on a few references that are more frequently cited than other references published in the same year. The frequently cited references are—as a rule—of specific significance to the research topic in question (here: heat waves) and the earlier references among them represent its origins and intellectual roots (Marx et al. 2014 ).

The RPYS changes the perspective of citation analysis from a times cited to a cited reference analysis (Marx and Bornmann 2016 ). RPYS does not identify the most highly cited papers of the publication set being studied (as is usually done by bibliometric analyses in research evaluation). RPYS aims to mirror the knowledge base of research (here: on heat waves).

With time, the body of scientific literature of many research fields is growing rapidly, particularly in climate change research (Haunschild et al. 2016 ). The growth rate of highly dynamic research topics such as research related to heat waves is even larger. As a consequence, the number of potentially citable papers is growing substantially. Toward the present, the peaks of individual publications lie over a broad continuum of newer publications and are less numerous and less pronounced. Due to the many publications cited in the more recent years, the proportion of individual highly cited publications in specific reference publication years falls steadily. Therefore, the distinct peaks in an RPYS spectrogram reveal only the most highly cited papers, in particular the earlier references comprising the historical roots. Further inspection and establishing a more entire and representative list of highly cited works requires consulting the reference table provided by the CRExplorer. The most important references within a specific reference publication year can be identified by sorting the cited references according to the reference publication year (RPY) and subsequently according to the number of cited references (N_CR) in a particular publication year.

The selection of important references in RPYS requires the consideration of two opposing trends: (1) the strongly growing number of references per reference publication year and (2) the fall off near present due to the fact that the newest papers had not sufficient time to accumulate higher citation counts. Therefore, we decided to set different limits for the minimum number of cited references for different periods of reference publication years (1950–1999: N_CR ≥ 50, 2000–2014: N_CR ≥ 150, 2015–2020: N_CR ≥ 100). This is somewhat arbitrary, but is helpful in order to adapt and limit the number of cited references to be presented and discussed.

In order to apply RPYS, all cited references ( n  = 408,247) of 216,932 unique reference variants have been imported from the papers of our publication set on heat waves ( n  = 8,011). The cited reference publication years range from 1473 to 2021. We removed all references (297 different cited reference variants) with reference publication years prior to 1900. Due to the very low output of heat wave-related papers published before 1990, no relevant literature published already in the nineteenth century can be expected. Also, global warming was no issue before 1900 since the Little Ice Age (a medieval cold period) lasted until the nineteenth century. The references were sorted according to RPY and N_CR for further inspection.

The CRExplorer offers the possibility to cluster and merge variants of the same cited reference (Thor et al. 2016a , b ). We clustered and merged the associated reference variants in our dataset (which are mainly caused by misspelled references) using the corresponding CRExplorer module, clustering the reference variants via volume and page numbers and subsequently merging aggregated 374 cited references (for more information on using the CRExplorer see “guide and datasets” at www.crexplorer.net ).

After clustering and merging, we applied a further cutback: to focus the RPYS on the most pronounced peaks, we removed all references ( n  = 212,324) with reference counts below 10 (resulting in a final number of 3,937 cited references) for the detection of the most frequently cited works. A minimum reference count of 10 has proved to be reasonable, in particular for early references (Marx et al. 2014 ). The cited reference publication years now range from 1932 to 2020.

In this study, we have considered all relevant WoS document types for a preferably comprehensive coverage of the literature of the research topic analyzed. The vast majority of the papers of our publication set, however, have been assigned to the document types “article” ( n  = 6.738, 84.1%), “proceedings paper” ( n  = 485, 6.1%), and “review” ( n  = 395 papers, 4.9%). Note that some papers belong to more than one document type.

Time evolution of literature

In Fig.  1 , the time evolution between 1990 and 2020 of the publications dealing with heat waves is shown (there are only 109 pre-1990 publications dealing with heat waves and covered by the WoS).

Time evolution of the overall number of heat wave publications, of heat wave publications in connection with climate change, and of heat wave publications in connection with mortality, each between 1990 and 2020. For comparison, the overall number of publications (scaled down) in the field of climate change research and the total number of publications covered by the WoS database (scaled down, too) are included

According to Fig.  1 , research dealing with heat waves is a highly dynamic research topic, currently doubling within about 5 years. The number of papers published per year shows a strong increase: since around 2000, the publication output increased by a factor of more than thirty, whereas in the same period, the overall number of papers covered by the WoS increased only by a factor of around three. Also, the portion of heat wave papers dealing with climate change increased substantially: from 16.1 in the period 1990–1999 to 25.7% in 2000, reaching 66.9% in 2020. The distinct decrease of the overall number of papers covered by the WoS between 2019 and 2020 might be a result of the Covid-19 pandemic.

With regard to the various impacts of heat waves, excess mortality is one of the most frequently analyzed and discussed issues in the scientific literature (see below). Whereas the subject specific literature on heat waves increased from 2000 to 2020 by a factor of 33.6, literature on heat waves dealing with mortality increased from 2000 to 2020 by a factor of 51.5. The dynamics of the research topic dealing with heat waves is mirrored by the WoS Citation Report , which shows the time evolution of the overall citation impact of the papers of the publication set (not presented). The citation report curve shows no notable citation impact before 2005, corresponding to the increase of the publication rate since about 2003 as shown in Fig.  1 .

Countries of authors

In Table ​ Table1, 1 , the number of papers assigned to the countries of authors with more than 100 publications dealing with heat waves is presented, showing the national part of research activities on this research topic. For comparative purposes, the percentage of overall papers in WoS of each country is shown. As a comparison with the overall WoS, we only considered WoS papers published between 2000 and 2020, because the heat wave literature started to grow substantially around 2000.

Top countries of authors with more than 100 papers dealing with heat waves up to the date of the search

The country-specific percentages from Table ​ Table1 1 are visualized in Fig.  2 . Selected countries are labeled. Countries with a higher relative percentage of more than two percentage points in heat wave research than in WoS overall output are marked blue (blue circle). Countries with a relative percentage at least twice as high in heat wave research than in overall WoS output are marked green (green cross), whereas countries with a relative percentage at most half as much in heat wave research than in overall WoS output are marked with a yellow cross. Only Japan has a much lower output in heat wave research than in WoS overall output, as indicated by the red circle and yellow cross. Most countries are clustered around the bisecting line and are marked gray (gray circle). China and the USA are outside of the plot region. Both countries are rather close to the bisecting line. Some European countries show a much larger activity in heat wave research than in overall WoS output. Australia shows the largest difference and ratio in output percentages as shown by the blue circle and green cross.

Publication percentages of countries in Table ​ Table1. 1 . Countries with large deviations between heat wave output and overall WoS output are labeled. Countries with an absolute percentage of more than two percentage points higher (lower) in heat wave research than in overall WoS output are marked blue (red). Countries with a relative percentage at least twice as high (at most half as much) in heat wave research than in overall WoS output are marked green (yellow)

The results mainly follow the expectations of such bibliometric analyses, with one distinct exception: Australia increasingly suffers from extreme heat waves and is comparatively active in heat wave research—compared with its proportion of scientific papers in general. The growth factor of the Australian publication output since 2010 is 8.5, compared to 5.3 for the USA and 3.3 for Germany.

Figure  3 shows the co-authorship network with regard to the countries of authors of the papers dealing with heat waves using the VOSviewer software.

Co-authorship overlay map with regard to the countries of authors and their average publication years from the 8,011 papers dealing with heat waves. The minimum number of co-authored publications of a country is 5; papers with more than 25 contributing countries are neglected; of the 135 countries, 89 meet the threshold, and 88 out of 89 countries are connected and are considered (one country, Armenia, that is disconnected from the network has been removed). The co-authorship network of a single country can be depicted by clicking on the corresponding node in the interactive map. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/3ywkwv8t

According to Fig.  3 and in accordance with Table ​ Table1, 1 , the USA is most productive in heat wave research. This is not unexpected, because the US publication output is at the top for most research fields. However, this aside, the USA has been heavily affected by heat wave events and is leading with regard to the emergence of the topic. Australia appears as another major player and is strongly connected with the US publications within the co-authorship network and thus appears as a large node near the US node in the map. Next, the leading European countries England, France, Germany, Italy, and Spain appear.

The overlay version of the map includes the time evolution of the research activity in the form of coloring of the nodes. The map shows the mean publication year of the publications for each specific author country. As a consequence, the time span of the mean publication years ranges only from 2014 to 2018. Nevertheless, the early activity in France and the USA and the comparatively recent activity in Australia and China, with the European countries in between, become clearly visible.

Topics of the heat wave literature

Figure  4 shows the keywords (keywords plus) map for revealing the thematic content of our publication set using the VOSviewer software. This analysis is based on the complete publication set ( n  = 8,011). The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 keywords met the threshold. For each of the 718 keywords, the total strength of the co-occurrence links with other keywords was calculated. The keywords with the greatest total link strength were selected for presentation in the map.

Co-occurrence network map of the keywords plus from the 8,011 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 10; of the 10,964 keywords, 718 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/enrdbw

According to Fig.  4 , the major keywords are the following: climate change, temperature, mortality, impact, heat waves (searched), and variability. The colored clusters identify closely related (frequently co-occurring) nodes. The keywords marked red roughly originate from fundamental climate change research focused on the hydrological cycle (particularly on drought), the keywords of the green cluster are around heat waves and moisture or precipitation, the keywords marked blue result from research concerning impacts of heat waves on health, the keywords marked yellow are focused on the various other impacts of heat waves, and the keywords of the magenta cluster are around adaptation and vulnerability in connection with heat waves.

The clustering by the VOSviewer algorithm provides basic categorizations, but many related keywords also appear in different clusters. For example, severe heat wave events are marked in different colors. For a better overview of the thematic content of the publications dealing with heat waves, we have assigned the keywords of Fig.  4 (with a minimum number of occurrences of 50) to ten subject categories (each arranged in the order of occurrence):

• Countries/regions: United-States, Europe, France, China, Pacific, Australia, London, England
• Cities: cities, city, US cities, Chicago, communities
• Events: 2003 heat-wave, 1995 heat-wave
• Impacts: impact, impacts, air-pollution, drought, soil-moisture, exposure, heat-island, urban, islands, photosynthesis, pollution, heat-island, air-quality, environment, precipitation extremes, biodiversity, emissions
• Politics: risk, responses, vulnerability, adaptation, management, mitigation, risk-factors, scenarios
• Biology: vegetation, forest, diversity, stomatal conductance
• Medicine: mortality, health, stress, deaths, morbidity, hospital admissions, public-health, thermal comfort, population, heat, sensitivity, human health, disease, excess mortality, heat-stress, heat-related mortality, comfort, behavior, death, stroke
• Climate research: climate change, temperature, climate, model, simulation, energy, projections, simulations, cmip5, ozone, el-nino, parametrization, elevated CO 2 , models, climate variability, carbon, carbon-dioxide
• Meteorology: heat waves, variability, precipitation, summer, heat-wave, weather, ambient-temperature, waves, extremes, wave, cold, water, rainfall, circulation, heat, air-temperature, extreme heat, climate extremes, heatwaves, temperature extremes, temperatures, temperature variability, high-temperature, ocean, extreme temperatures, atmospheric circulation, interannual variability, sea-surface temperature, oscillation, surface temperature, surface
• Broader terms (multi-meaning): trends, events, patterns, growth, performance, time-series, indexes, system, dynamics, association, index, tolerance, productivity, ensemble, resilience, increase, quality, prediction, frequency, particulate matter, future, framework, 20 th -century, time, reanalysis, systems

Although allocated by the database provider, the keywords are not coherent. For example, the same keyword may appear as singular or plural, and complex keywords are written with and without hyphens.

In order to compare the thematic content of the complete publication set with the earlier literature on heat waves, we have analyzed the pre-2000 publications ( n  = 297) separately. Figure  5 shows the keywords (keywords plus) map for revealing the thematic content of the pre-2000 papers.

Co-occurrence network map of the keywords plus from the 297 pre-2000 papers dealing with heat waves for a rough analysis of the thematic content. The minimum number of occurrences of keywords is 1; of the 389 keywords, 277 keywords are connected, and all items are shown. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/u2zzr399

The major nodes in Fig.  5 are heat waves (searched), temperature, United States, and mortality, with climate change appearing only as a smaller node here. Obviously, the connection between heat waves and climate change was not yet pronounced, which can also be seen from Fig.  1 . Compared with Fig.  4 , the thematic content of the clusters is less clear and the clusters presented in Fig.  5 can hardly be assigned to specific research areas. For a better overview of the thematic content of the early publications dealing with heat waves, we have assigned the connected keywords of Fig.  5 to seven subject categories:

• Countries/regions: United-States, Great-Plains
• Cities: St-Louis, Athens, Chicago
• Events: 1980 heat-wave, 1995 heat-wave
• Impacts: impacts, responses, drought, precipitation, comfort, sultriness
• Climate research: climate, climate change, model, temperature, variability
• Medicine: cardiovascular deaths, mortality, air pollution
• Meteorology: atmospheric flow, weather, heat, humidity index

Important publications

Figures  6 – 8 show the results of the RPYS analysis performed with the CRExplorer and present the distribution of the number of cited references across the reference publication years. Figure  6 shows the RPYS spectrogram of the full range of reference publication years since 1925. Figure  7 presents the spectrogram for the reference publication year period 1950–2000 for better resolving the historical roots. Figure  8 shows the spectrogram for the period 2000–2020, comprising the cited references from the bulk of the publication set analyzed.

Annual distribution of cited references throughout the time period 1925–2020, which have been cited in heat wave-related papers (published between 1964 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 1950–2000, which have been cited in heat wave-related papers (published between 1972 and 2020). Only references with a minimum reference count of 10 are considered

Annual distribution of cited references throughout the time period 2000–2020, which have been cited in heat wave-related papers (published between 2000 and 2020). Only references with a minimum reference count of 10 are considered

The gray bars (Fig.  6 ) and red lines (Figs. ​ (Figs.7 7 – 8 ) in the graphs visualize the number of cited references per reference publication year. In order to identify those publication years with significantly more cited references than other years, the (absolute) deviation of the number of cited references in each year from the median of the number of cited references in the two previous, the current, and the two following years (t − 2; t − 1; t; t + 1; t + 2) is also visualized (blue lines). This deviation from the 5-year median provides a curve smoother than the one in terms of absolute numbers. We inspected both curves for the identification of the peak papers.

Which papers are most important for the scientific community performing research on heat waves? We use the number of cited references (N_CR) as a measure of the citation impact within the topic-specific literature of our publication set. N_CR should not be confused with the overall number of citations of the papers as given by the WoS citation counts (times cited). These citation counts are based on all citing papers covered by the complete database (rather than a topic-specific publication set) and are usually much higher.

Applying the selection criteria mentioned above (minimum number of cited references between 50 and 150 in three different periods), 104 references have been selected as key papers (important papers most frequently referenced within the research topic analyzed) and are presented in Table ​ Table2 2 in Appendix 2. The peak papers corresponding to reference publication years below about 2000 can be seen as the historical roots of the research topic analyzed. Since around 2000, the number of references with the same publication year becomes increasingly numerous, usually with more than one highly referenced (cited) paper at the top. Although there are comparatively fewer distinct peaks visible in the RPYS spectrogram of Fig.  8 , the most frequently referenced papers can easily be identified via the CRE reference listing. Depending on the specific skills and needs (i.e., the expert knowledge and the intended depth of the analysis), the number of top-referenced papers considered key papers can be defined individually.

Listing of the key papers ( n  = 104) revealed by RPYS via CRE ( RPY reference publication year, N_CR number of cited references, Title title of the cited reference, DOI allows easily to retrieve the full paper via WoS or Internet)

*N_TOP10 > 9; the N_TOP10 indicator is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications.

Table ​ Table2 2 lists the first authors and titles of the 104 key papers selected, their number of cited references (N_CR), and the DOIs for easy access. Some N_CR values are marked by an asterisk, indicating a high value of the N_TOP10 indicator implemented in the CRExplorer. The N_TOP10 indicator value is the number of reference publication years in which a focal cited reference belongs to the 10% most referenced publications. In the case of about half of the cited references in Table ​ Table2 2 ( n  = 58), the N_TOP10 value exceeded a value of 9. The three highest values in our dataset are 24, 21, and 20.

Out of the 104 key papers from Table ​ Table2, 2 , 101 have a DOI of which we found 101 papers in the WoS. Three papers have no DOI but could be retrieved from WoS. The altogether 104 papers were exported and their keywords (keywords plus) were displayed in Fig.  9 for revealing the thematic content of the key papers from the RPYS analysis at a glance.

Co-occurrence network map of the keywords plus of the 104 key papers dealing with heat waves selected applying RPYS via CRE software and listed in Table ​ Table2. 2 . The minimum number of occurrences of keywords is 2; of the 310 keywords, 91 meet the threshold. Readers interested in an in-depth analysis can use VOSviewer interactively and zoom into the map via the following URL: https://tinyurl.com/4vwpc4t2

Overall, the keywords mapped in Fig.  9 are rather similar to the keywords presented in Fig.  4 . Besides climate change, temperature, weather, and air-pollution, the keywords deaths, health, mortality, and United-States appear as the most pronounced terms.

The key papers presented in Table ​ Table2 2 can be categorized as follows: (1) papers dealing with specific heat wave events, (2) the impact of heat waves on human health, (3) heat wave-related excess mortality and implications for prevention, (4) the interaction between air pollution and high temperature, (5) circulation pattern and the meteorological basis, (6) future perspectives and risks, and (7) climate models, indicators, and statistics.

Today, the hypothesis of a human-induced climate change is no longer abstract but has become a clear fact, at least for the vast majority of the scientific community (IPCC 2014; CSSR 2017; NCA4 2018; and the multitude of references cited therein). The consequences of a warmer climate are already obvious. The rapidly growing knowledge regarding the earth’s climate system has revealed the connection between global warming and extreme weather events. Heat waves impact people directly and tangibly and many people are pushing for political actions. Research on heat waves came up with the occurrence of some severe events in the second half of the twentieth century and was much stimulated by the more numerous, more intense, and longer lasting heat waves that have occurred since the beginning of the twenty-first century.

As already mentioned in Sect.  1 , the more intense and more frequently occurring heat waves cannot be explained solely by natural climate variations but only with human-made climate change. As a consequence, research on heat waves has become embedded into meteorology and climate change research and has aimed to understand the specific connection with global warming. Scientists discuss a weakening of the polar jet stream as a possible reason for an increasing probability for the occurrence of heat waves (e.g., Broennimann et al. 2009 ; Coumou et al. 2015 ; Mann 2019 ). Climate models are used for projections of temperature and rainfall variability in the future, based on various scenarios of greenhouse gas emissions. As a result, the corresponding keywords appear in the maps of Figs. ​ Figs.4 4 and ​ and9. 9 . Also, the application of statistics plays a major role in the papers of our publication set; some of the most highly referenced (early) papers in Table ​ Table2 2 primarily deal with statistical methods. These methods provide the basis for research on heat waves.

Our analysis shows that research on heat waves has become extremely important in the medical area, since severe heat waves have caused significant excess mortality (e.g., Kalkstein and Davis 1989 ; Fouillet et al. 2006 ; Anderson and Bell 2009 , 2011 ). The most alarming is that the limit for survivability may be reached at the end of the twenty-first century in many regions of the world due to the fatal combination of rising temperatures and humidity levels (e.g., Pal and Eltahir 2016 ; Im et al. 2017 ; Kang and Eltahir 2018 ). The combination of heat and humidity is measured as the “wet-bulb temperature” (WBT), which is the lowest temperature that can be reached under current ambient conditions by the evaporation of water. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature and is different at lower humidity levels. For example, an ambient temperature of 46 °C and a relative humidity of 50% correspond to 35 °C WBT, which is the upper limit that can kill even healthy people within hours. By now, the limit of survivability has almost been reached in some places. However, if global warming is not seriously tackled, deadly heat waves are anticipated for many regions that have contributed little to climate change.

According to high-resolution climate change simulations, North China and South Asia are particularly at risk, because the annual monsoon brings hot and humid air to these regions (Im et al. 2017 ; Kang and Eltahir 2018 ). The fertile plain of North China has experienced vast expansion of irrigated agriculture, which enhances the intensity of heat waves. South Asia, a region inhabited by about one-fifth of the global human population, is likely to approach the critical threshold by the late twenty-first century, if greenhouse gas emissions are not lowered significantly. In particular, the densely populated agricultural regions of the Ganges and Indus river basins are likely to be affected by extreme future heat waves. Also, the Arabic-speaking desert countries of the Gulf Region in the Middle East and the French-speaking parts of Africa are expected to suffer from heat waves beyond the limit of human survival. But to date, only 12 papers have been published on heat waves in connection with wet-bulb temperature (#15 of the search query); no paper was published before 2016. Some papers report excess hospital admissions during heat wave events (e.g., Semenza et al. 1999 ; Knowlton et al. 2009 ), with the danger of a temporary capacity overload of local medical systems in the future. Presumably, this will be an increasingly important issue in the future, when more and larger urban areas are affected by heat waves beyond the limit of human survival indicated by wet-bulb temperatures above 35° C.

The importance of heat waves for the medical area is underlined by the large portion of papers discussing excess hospital admissions and excess mortality during intense heat wave events, particularly in urban areas with a high population density. As was the case during the boom phase of the Covid-19 pandemic, local medical health care systems may become overstressed by long-lasting heat wave events and thus adaptation strategies are presented and discussed. Finally, the analysis of the keywords in this study reveals the connection of heat wave events with air pollution in urban regions. There seems to be evidence of an interaction between air pollution and high temperatures in the causation of excess mortality (e.g., Katsouyanni et al. 1993 ). Two more recent papers discuss the global risk of deadly heat (Mora et al. 2017 ) and the dramatically increasing chance of extremely hot summers since the 2003 European heat wave (Christidis et al. 2015 ).

Another important topic of the heat wave papers is related to the consequences for agriculture and forestry. Reduced precipitation and soil moisture result in crop failure and put food supplies at risk. Unfortunately, large regions of the world that contribute least to the emission of greenhouse gases are affected most by drought, poor harvests, and hunger. Some more recent papers discuss the increasing probability of marine heat waves (Oliver et al. 2018 ) and the consequences for the marine ecosystem (Smale et al. 2019 ).

The results of this study should be interpreted in terms of its limitations:

• We tried to include in our bibliometric analyses all relevant heat wave papers covered by the database. Our long-standing experience in professional information retrieval has shown, however, that it is sheer impossible to get complete and clean results by search queries against the backdrop of the search functions provided by literature databases like WoS or others. Also, the transition from relevant to non-relevant literature is blurred and is a question of the specific needs. In this study, we used bibliometric methods that are relatively robust with regard to the completeness and precision of the publication sets analyzed. For example, it is an advantage of RPYS that a comparatively small portion of relevant publications (i.e., an incomplete publication set) contains a large amount of the relevant literature as cited references. The number of cited references is indeed lowered as a consequence of an incomplete publication set. However, this does not significantly affect the results, since the reference counts are only used as a relative measure within specific publication years.

Two other limitations of this study refer to the RPYS of the heat wave paper set:

• There are numerous rather highly cited references retrieved by RPYS via CRExplorer but not considered in the listing of Table ​ Table2 2 due to the selection criteria applied. Many of these non-selected papers have N_CR values just below the limits that we have set. Therefore, papers not included in our listing are not per se qualified as much less important or even unimportant.
• In the interpretation of cited references counts, one should have in mind that they rely on the “popularity” of a publication being cited in subsequent research. The counts measure impact but not scientific importance or accuracy (Tahamtan and Bornmann 2019 ). Note that there are many reasons why authors cite publications (Tahamtan and Bornmann 2018 ), thus introducing a lot of “noise” in the data (this is why RPYS focuses on the cited reference peaks).

Our suggestions for future empirical analysis refer to the impact of the scientific heat wave discourse on social networks and funding of basic research on heat waves around topics driven by political pressure. Whereas this paper focuses on the scientific discourse around heat waves, it would be interesting if future studies were to address the policy relevance of the heat waves research.

Appendix 1 1)

WoS search query (date of search: July 1, 2021)

Table ​ Table2 2

Author contribution

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Werner Marx, Robin Haunschild, and Lutz Bornmann. The first draft of the manuscript was written by Werner Marx and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Open Access funding enabled and organized by Projekt DEAL.

Data availability

Code availability, declarations.

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Change history

The original version of this paper was updated to add the missing compact agreement Open Access funding note.

Contributor Information

Werner Marx, Email: [email protected] .

Robin Haunschild, Email: [email protected] .

Lutz Bornmann, Email: [email protected] , Email: ed.gpm.vg@nnamnrob .

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Heat and desiccation tolerances predict bee abundance under climate change

by Mary Beth King, University of New Mexico

Recent research by the University of New Mexico alumnus Melanie Kazenel and colleagues predicts climate change will reshape bee communities in the southwest United States, with some thriving and others declining. The research, titled "Heat and desiccation tolerances predict bee abundance under climate change," was recently published in Nature .

Kazenel completed her Ph.D. in Biology at UNM in 2022 and is currently a visiting assistant professor of Biology at Earlham College in Indiana.

"I study how native bees are responding to climate change ," Kazenel said. "Bees are the most important pollinators of many wild plants and agricultural crops, meaning that they are crucial to sustaining natural ecosystems and the human food supply. But there is increasing evidence that bees are threatened globally.

"Factors known to threaten bees include habitat loss , pesticides, disease, and climate change. However, climate change has been less-studied relative to other factors that threaten bees.

"Our research sought to fill this important knowledge gap. In particular, in the southwestern US, the climate is becoming warmer and drier over time, as well as more variable from year to year. We aimed to understand how these changes impact bee communities in our region."

The research was conducted at the Sevilleta National Wildlife Refuge (SNWR) near Socorro, N.M., as part of the Sevilleta Long-Term Ecological Research Program (SEV-LTER).

"This region is home to extraordinarily high bee diversity. We have recorded about 340 species of wild, native bees at the SNWR alone," Kazenel said.

The research began in 2002, when UNM Adjunct Assistant Professor of Biology Karen W. Wright established a long-term bee monitoring study at the SEV-LTER. This study, which is still ongoing, is possibly the world's longest continuous bee monitoring effort.

"Each month from March through October, when bees are active, we open traps in the field at the SNWR to collect bees, which we then identify to species. This gives us an incredible record of how bee abundance and diversity have changed from month-to-month and year-to-year over time. We have been able to pair our long-term bee data with data from on-site weather stations to understand how changes in bee abundance and diversity correspond with climate," Kazenel said.

To complement the long-term bee monitoring data, the researchers collected additional data on bee body size and physiology, taking body size measurements on preserved bee specimens collected in the long-term study. To collect physiology data, they netted bees in the field and placed them in an environmental chamber that allowed them to measure thermal (heat) tolerance and desiccation tolerance (tolerance to dry conditions).

"These data allowed us to determine whether body size and physiology relate to bee population trends," Kazenel explained. "For instance, are large-bodied or small-bodied bees more likely to be in decline? Are bee species with low tolerance of heat and desiccation more likely to be in decline? These data can help us understand which bee species are most threatened by climate change, and why."

The research revealed strong evidence that climate change is reshaping bee communities in the southwestern U.S.

"Climate was a strong predictor of bee population dynamics in our study: for 71% of populations, aridity, or the level of drought, predicted abundance. Some bees—those with high tolerance of heat and desiccation in our experiments—appear resilient to climate change and may even increase in abundance in the future. But our models forecast declines for 46% of species and predict more homogeneous future bee communities."

This is a concerning result, Kazenel said, as bee diversity supports diverse, healthy plant communities.

"We also found that larger-bodied bees dominated communities under the arid conditions that will increase in the future. Change in the body size composition of bee communities could alter pollination of plants, because the size of a bee and a flower need to 'match' for effective pollination to occur. Our study therefore provides important new evidence of how climate change is directly affecting bee communities and the plants they pollinate," she said.

"Our work paints a mixed picture about the future of bees in New Mexico," Kazenel summarized. "On one hand, some bee species appear well-adapted to persist under climate change, which gives us reason for hope. But on the other hand, we predict declines for nearly half of species, which is reason for great concern.

"Our work thus broadly speaks to the importance of swift action to curtail greenhouse gas emissions and mitigate future climate change to conserve the biodiversity that sustains our planet."

Co-authors on the study are Wright and Terry Griswold, research entomologist at the USDA-ARS Pollinating Insects Research Unit at Utah State University, both expert bee taxonomists who identified all bees collected in the study. "An enormous task because of the high species diversity in our region," Kazenel noted. Wright received her Ph.D. in Biology from UNM and now works for the Washington State Department of Agriculture.

The team also includes UNM Distinguished Professor of Biology Jennifer Rudgers and Professor Kenneth Whitney, associate chair of the UNM Department of Biology, who supervised the work, along with numerous field and lab assistants.

"This research has truly been a team effort," Kazenel noted.

Kazenel and other researchers at the SEV-LTER are continuing to study climate change effects on bees. In particular, they are working to document which plants are pollinated by each bee species in the dataset, allowing them to understand in more detail how changes in bee abundance under climate change will influence particular plant species. They are also exploring how changes in bee and plant phenology—the timing of life cycle events—could shift under climate change.

Journal information: Nature

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Two years to save the planet, says UN climate chief, and other nature and climate stories you need to read this week

Top nature and climate news: Two years to save the planet, says UN climate chief, and more. Image:  REUTERS/Susannah Sayler

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• This weekly round-up contains key nature and climate news from the past week.
• Top nature and climate stories: Two years to save the planet, says UN climate chief; Forever chemicals in groundwater exceed safe levels, global study finds; Australia's Great Barrier Reef experiencing one of its worst ever recorded bleaching events.

1. UN climate chief issues stark warning

The next two years are "essential in saving the planet" warns Simon Stiell , Executive Secretary of the United Nations Framework Convention on Climate Change.

Governments, business leaders and development banks must take immediate action to combat climate change or risk far worse climate impacts, he said.

Stiell called for an increase in climate finance through debt relief, cheaper funds for poorer countries, new streams of international finance and World Bank and International Monetary Fund reforms.

"Every day, finance ministers, CEOs, investors, and climate bankers and development bankers, direct trillions of dollars. It's time to shift those dollars," he said.

The climate crisis is moving into uncharted territory, Gavin Schmidt, director of NASA's Goddard Institute for Space Studies told the BBC .

March 2024 was the hottest ever recorded, reaching 0.73°C above the 1991-2020 March average and exceeding the previous March 2016 high by 0.10°C , according to data from the Copernicus Climate Change Service.

2. Chemicals in groundwater exceed safe levels, study finds

Levels of PFAS or "forever chemicals" in some of the world's groundwater sources exceed safe limits, according to a new study.

Researchers analyzed 45,000 data points from different world regions over a 20-year time span.

The first-of-its-kind study published in the Nature Geoscience journal, found levels of per- and poly-fluoroalkyl substances (PFAS) – to use their scientific name – in global source water samples exceeded the limits advised by the US Environmental Protection Agency and Health Canada.

These human-made chemicals are commonly used in industry and found in household products, including non-stick frying pans, cosmetics, insecticides and food packaging.

Once ingested or in the environment they don't degrade, hence the label forever chemicals.

Research has linked PFAS chemicals to environmental harm and human health issues like cancer, but more analysis is needed to realise the extent of the threat they pose.

“We already knew that PFAS is pervasive in the environment, but I was surprised to find out the large fraction of source waters that are above drinking water advisory recommendations,” said Denis O’Carroll, senior author of the research and engineering professor at the University of New South Wales. “We’re talking above 5%, and it goes over 50% in some cases,” he said.

However, Professor O'Carroll notes the study analyzed source water bodies and not tap drinking water, which goes through a treatment process designed to filter bacteria and chemicals, so should be safe to drink.

3. News in brief: Other top nature and climate stories this week

Australia's Great Barrier Reef is experiencing one of its worst-ever recorded bleaching events, according to marine biologists. Aerial surveys show the 2,300 kilometre reef system is suffering its fifth mass bleaching within the past eight years , The Guardian reports.

Our ocean covers 70% of the world’s surface and accounts for 80% of the planet’s biodiversity. We can't have a healthy future without a healthy ocean - but it's more vulnerable than ever because of climate change and pollution.

Tackling the grave threats to our ocean means working with leaders across sectors, from business to government to academia.

The World Economic Forum, in collaboration with the World Resources Institute, convenes the Friends of Ocean Action , a coalition of leaders working together to protect the seas. From a programme with the Indonesian government to cut plastic waste entering the sea to a global plan to track illegal fishing, the Friends are pushing for new solutions.

Climate change is an inextricable part of the threat to our oceans, with rising temperatures and acidification disrupting fragile ecosystems. The Forum runs a number of initiatives to support the shift to a low-carbon economy , including hosting the Alliance of CEO Climate Leaders, who have cut emissions in their companies by 9%.

Is your organization interested in working with the World Economic Forum? Find out more here .

A rare, blind, palm-sized mole that lives almost entirely underground has been photographed in a remote area of the Australian outback , the BBC reports. Sightings of the elusive creatures occur on average only a few times each decade.

The Al Massira reservoir in Morocco is drying up due to six consecutive years of drought and climate change , satellite images show. This vital waterway is the country's second-largest dam, which supplies water to a number of major cities.

The 2024 European Tree of The Year contest has been won by a Polish beech tree rooted in a park in the University of Wroclaw's botanical gardens. Known as Heart of the Garden, it's the third tree from Poland to win the award.

AI and big data are helping conservationists analyze the full annual lifecycle of bird species across continents , harvesting "unlimited" data to help prioritize bird habitats and combat the impact of biodiversity loss.

Extreme flooding has hit Russia's Orenburg region and parts of Kazakhstan , as the Ural river, which rises in the Ural mountains and flows through Kazakhstan to the Caspian Sea, breached embankment dams.

4. More on the nature and climate crisis on Agenda

As environmental concerns climb higher up the agenda, global businesses are well-placed to make financial commitments to conserve forests and help mitigate the climate crisis. Here's how.

"It's now cheaper to save the world that destroy it" says Akshat Rathi, the author of Climate Capitalism and senior climate reporter for Bloomberg. Here he speaks to Radio Davos about how the profit motive is moving the dial .

Is climate inaction a violation of human rights? A recent ruling by the European Court of Human Rights found that Switzerland failed to act "in time and in an appropriate way" to safeguard people from the climate crisis.

Related topics:

Newly sequenced genome reveals coffee's prehistoric origin story -- and its future under climate change

Study charts family history of arabica, world's most popular coffee species, through earth's heating and cooling periods over last millennia.

The key to growing coffee plants that can better resist climate change in the decades to come may lie in the ancient past.

Researchers co-led by the University at Buffalo have created what they say is the highest quality reference genome to date of the world's most popular coffee species, Arabica, unearthing secrets about its lineage that span millennia and continents.

Their findings, published today in Nature Genetics , suggest that Coffea arabica developed more than 600,000 years ago in the forests of Ethiopia via natural mating between two other coffee species. Arabica's population waxed and waned throughout Earth's heating and cooling periods over thousands of years, the study found, before eventually being cultivated in Ethiopia and Yemen, and then spread over the globe.

"We've used genomic information in plants alive today to go back in time and paint the most accurate picture possible of Arabica's long history, as well as determine how modern cultivated varieties are related to each other," says the study's co-corresponding author, Victor Albert, PhD, Empire Innovation Professor in the UB Department of Biological Sciences, within the College of Arts and Sciences.

Coffee giants like Starbucks and Tim Hortons exclusively use beans from Arabica plants to brew the millions of cups of coffee they serve everyday, yet, in part due to a low genetic diversity stemming from a history of inbreeding and small population size, Arabica is susceptible to many pests and diseases and can only be cultivated in a few places in the world where pathogen threats are lower and climate conditions are more favorable.

"A detailed understanding of the origins and breeding history of contemporary varieties are crucial to developing new Arabica cultivars better adapted to climate change," Albert says.

From their new reference genome, accomplished using cutting-edge DNA sequencing technology and advanced data science, the team was able to sequence 39 Arabica varieties and even an 18th century specimen used by Swedish naturalist Carl Linnaeus to name the species.

The reference genome is now available in a publicly available digital database.

"While other public references for Arabica coffee do exist, the quality of our team's work is extremely high," says one of the study's co-leaders, Patrick Descombes, senior expert in genomics at Nestlé Research. "We used state-of-the-art genomics approaches -- including long- and short-read high throughput DNA sequencing -- to create the most advanced, complete and continuous Arabica reference genome to date."

Humanity's favorite coffee evolved without people's help

Arabica is the source of approximately 60% of the world's total coffee products, with its seeds helping millions start their day or stay up late. However, the initial crossbreeding that created it was done without any intervention from humans.

Arabica formed as a natural hybridization between Coffea canephora and Coffea eugenioides , whereupon it received two sets of chromosomes from each parent. Scientists have had a hard time pinpointing exactly when -- and where -- this allopolyploidization event took place, with estimates ranging everywhere from 10,000 to 1 million years ago.

To find evidence for the original event, UB researchers and their partners ran their various Arabica genomes through a computational modeling program to look for signatures of the species' foundation.

The models show three population bottlenecks during Arabica's history, with the oldest happening some 29,000 generations -- or 610,000 years -- ago. This suggests Arabica formed sometime before that, anywhere from 610,000 to 1 million years ago, researchers say.

"In other words, the crossbreeding that created Arabica wasn't something that humans did," Albert says. "It's pretty clear that this polyploidy event predated modern humans and the cultivation of coffee."

Coffee plants have long been thought to have developed in Ethiopia, but varieties that the team collected around the Great Rift Valley, which stretches from Southeast Africa to Asia, displayed a clear geographic split. The wild varieties studied all originated from the western side, while the cultivated varieties all originated from the eastern side closest to the Bab al-Mandab strait that separates Africa and Yemen.

That would align with evidence that coffee cultivation may have started principally in Yemen, around the 15th century. Indian monk Baba Budan is believed to have smuggled the fabled "seven seeds" out of Yemen around 1600, establishing Indian Arabica cultivars and setting the stage for coffee's global reach today.

"It looks like Yemeni coffee diversity may be the founder of all of the current major varieties," Descombes says. "Coffee is not a crop that has been heavily crossbred, such as maize or wheat, to create new varieties. People mainly chose a variety they liked and then grew it. So the varieties we have today have probably been around for a long time."

How climate impacted Arabica's population

East Africa's geoclimatic history is well documented due to research on human origins, so researchers could contrast climate events with how the wild and cultivated Arabica populations fluctuated over time.

Modeling shows a long period of low population size between 20-100,000 years ago, which roughly coincides with an extended drought and cooler climate believed to have hit the region between 40-70,000 years ago. The population then increased during the African humid period, around 6-15,000 years ago, when growth conditions were likely more beneficial.

During this same time, around 30,000 years ago, the wild varieties and the varieties that would eventually become cultivated by humans split from each other.

"They still occasionally bred with each other, but likely stopped around the end of the African humid period and the widening of the strait due to rising sea levels around 8,000 to 9,000 years ago," says Jarkko Salojärvi, assistant professor at Nanyang Technological University in Singapore and another co-corresponding author of work.

Low genetic diversity threatens Arabica

Cultivated Arabica is estimated to have an effective population size of only 10,000 to 50,000 individuals. Its low genetic diversity means it could be completely decimated, like the monoculture Cavendish banana, by pathogens, such as coffee leaf rust, which causes $1-2 billion in losses annually.

The reference genome was able to shed more light on how one line of Arabica varieties obtained strong resistance to the disease.

The Timor variety formed in Southeast Asia as a spontaneous hybrid between Arabica and one of its parents, Coffea canephora . Also known as Robusta and used primarily for instant coffee, this species is more resistant to disease than Arabica .

"Thus, when Robusta hybridized itself back into Arabica on Timor, it brought some of its pathogen defense genes along with it," says Albert, who also co-led sequencing of the Robusta genome in 2014. Albert and collaborators' current work also presents a highly improved version of the Robusta genome, as well as new sequence of Arabica's other progenitor species, Coffea eugenioides .

While breeders have tried replicating this crossbreeding to boost pathogen defense, the new Arabica reference genome allowed the present researchers to pinpoint a novel region harboring members of the RPP8 resistance gene family as well as a general regulator of resistance genes, CPR1 .

"These results suggest a novel target locus for potentially improving pathogen resistance in Arabica," Salojärvi says.

The genome provided other new findings as well, like which wild varieties are closest to modern, cultivated Arabica coffee. They also found that the Typica variety, an early Dutch cultivar originating from either India or Sri Lanka, is likely the parent of the Bourbon variety, principally cultivated by the French.

"Our work has not been unlike reconstructing the family tree of a very important family," Albert says.

Nestlé Research funded the majority of the research. The large international team was co-led by Albert, whose work was supported by the National Science Foundation, and contributions from many other organizations. Other UB contributors include Trevor Krabbenhoft, PhD, and Zhen Wang, PhD, both assistant professors of biological sciences; PhD student Steven Fleck; PhD graduate Minakshi Mukherjee; and former research scientist Tianying Lan -- all from the Department of Biological Sciences.

• Food and Agriculture
• Evolutionary Biology
• Origin of Life
• Early Climate
• Heirloom plant
• Permian-Triassic extinction event

Story Source:

Materials provided by University at Buffalo . Original written by Tom Dinki. Note: Content may be edited for style and length.

Journal Reference :

• Jarkko Salojärvi, Aditi Rambani, Zhe Yu, Romain Guyot, Susan Strickler, Maud Lepelley, Cui Wang, Sitaram Rajaraman, Pasi Rastas, Chunfang Zheng, Daniella Santos Muñoz, João Meidanis, Alexandre Rossi Paschoal, Yves Bawin, Trevor J. Krabbenhoft, Zhen Qin Wang, Steven J. Fleck, Rudy Aussel, Laurence Bellanger, Aline Charpagne, Coralie Fournier, Mohamed Kassam, Gregory Lefebvre, Sylviane Métairon, Déborah Moine, Michel Rigoreau, Jens Stolte, Perla Hamon, Emmanuel Couturon, Christine Tranchant-Dubreuil, Minakshi Mukherjee, Tianying Lan, Jan Engelhardt, Peter Stadler, Samara Mireza Correia De Lemos, Suzana Ivamoto Suzuki, Ucu Sumirat, Ching Man Wai, Nicolas Dauchot, Simon Orozco-Arias, Andrea Garavito, Catherine Kiwuka, Pascal Musoli, Anne Nalukenge, Erwan Guichoux, Havinga Reinout, Martin Smit, Lorenzo Carretero-Paulet, Oliveiro Guerreiro Filho, Masako Toma Braghini, Lilian Padilha, Gustavo Hiroshi Sera, Tom Ruttink, Robert Henry, Pierre Marraccini, Yves Van de Peer, Alan Andrade, Douglas Domingues, Giovanni Giuliano, Lukas Mueller, Luiz Filipe Pereira, Stephane Plaisance, Valerie Poncet, Stephane Rombauts, David Sankoff, Victor A. Albert, Dominique Crouzillat, Alexandre de Kochko, Patrick Descombes. The genome and population genomics of allopolyploid Coffea arabica reveal the diversification history of modern coffee cultivars . Nature Genetics , 2024; 56 (4): 721 DOI: 10.1038/s41588-024-01695-w

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How climate change could be driving ‘killer’ cold outbreaks in oceans

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Published Apr 16, 2024 6:28 AM PDT | Updated Apr 16, 2024 6:28 AM PDT

Researchers electronically tagged bull sharks with a transmitting locator device, which recorded the depth and temperature of the part of the ocean they swam to. (Photo: Gerard Soury/The Image Bank RF/Getty Images/FILE via CNN Newsource)

(CNN) —  It’s not just ocean heat that’s affecting marine life – new research shows extremely cold events are welling up and causing mass mortalities. And the same planet-warming pollution that’s driving the climate crisis is likely to blame for these “killer events” on the other end of the temperature spectrum.

The world’s oceans have been plagued by  unprecedented heat  over the past year, fueling concerns for marine life. Billions of  crabs disappeared  in the northern Pacific; sea lions and dolphins are  washing up sick ; iconic coral reefs are undergoing  mass bleaching .

But even as ocean temperatures climb, extremely cold upwelling events — when strong winds and ocean currents bring pockets of cold water up to the surface, replacing the warm water that was there — are also becoming more frequent and intense, threatening sea life, according to the  study  published Monday in the journal Nature Climate Change.

“Climate change is actually really complex,” said Nicolas Lubitz, lead author of the study and a researcher at James Cook University in Queensland, Australia. “It’s not just warming of the globe, but it’s really changing the way our oceans function.”

When Lubitz heard reports of marine animals like sharks, manta rays and squids washing up dead in the southeast coast of South Africa in March 2021, he started investigating. More than 260 marine animals from 81 different species died in that one extreme event.

He said seasonal upwelling events are common in that area, with water temperature dropping quickly. But Lubitz said the March 2021 die-off was “quite an extreme event, because we had rather warm water before it happened.”

“And then the winds changed, and the currents started changing slightly, which is a seasonal thing,” he added. “Then all of a sudden, the temperature within 24 hours dropped by 11 degrees.”

The researchers analyzed killer upwelling events in the Indian Ocean’s Agulhas Current and the East Australian Current, using 41 years of sea surface temperature data and 33 years of wind records to see how deadly cold ocean extremes can be.

“We’re seeing changes in how often the upwelling occurs, how intense it is, which might impact the fishing communities in these areas,” he said. “It’s really an economic thing as well as the biodiversity thing.”

According to the study, the lethality of a cold event is likely linked to how fast the temperature drops. If the cold event lasts for multiple days, which has been occurring more frequently, research shows that marine animals including turtles and many fish species could suffer from hypothermia and physiological malfunction or ultimately die.

A 2018 study found that young Bull Sharks have expanded their habitat into Pamlico Sound, North Carolina, due to warmer and saltier ocean water.

For a different study, Lubitz already had bull sharks electronically tagged with a transmitting locator device, which also recorded the depth and temperature of the part of the ocean they swim to.

“That was really the key in this study in that we could see when the sharks migrate,” he said. “We could see how the temperature profiles change, and how the sharks were swimming shallower when they were in upwelling areas because they were trying to avoid the colder water from the depths.”

The findings provide a “very reasonable explanation” to the many unexpected marine mortality events people have been seeing around the world, said Ajit Subramaniam, research professor with Columbia University’s Climate School’s Lamont-Doherty Earth Observatory.

“It’s one of those unexpected findings and it’s not something we talk about a lot,” Subramaniam, who was not involved with the study, told CNN. “And therefore, this is a timely thing to remind us that the climate crisis works in both ways.”

The-CNN-Wire™ & © 2024 Cable News Network, Inc., a Warner Bros. Discovery Company. All rights reserved.

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• NEWS FEATURE
• 10 April 2024

The rise of eco-anxiety: scientists wake up to the mental-health toll of climate change

• Helen Pearson

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The Bahamas is vulnerable to storms and hurricanes. Extreme weather can exacerbate mental-health illnesses. Credit: Zak Bennett/AFP via Getty

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Every year for six years, Laureen Wamaitha hoped that her fields in Kenya would flourish. Every year, she’d see drought wither the crops and then floods wash them away. The cycle of optimism and loss left her constantly anxious, and she blamed climate change. “You get to a situation where you have panic attacks because you’re always worried about something,” she says.

Medical student Vashti-Eve Burrows, meanwhile, saw powerful hurricane Dorian rage through the Bahamas in 2019 and now she is fearful about the future of the country, an island archipelago that is vulnerable to sea-level rise and storms. “Will there even be a Bahamas in maybe 20 to 30 years?” she says.

Wamaitha and Burrows are part of a growing chorus of people speaking up about the impacts of climate change on mental health. Climate change is exacerbating mental disorders, which already affect almost one billion people and are among the world’s biggest causes of ill health. A global survey in 2021 found that more than half of people aged 16–25 felt sad, anxious or powerless, or had other negative emotions about climate change 1 . Altogether, hundreds of millions of people might be experiencing some type of negative psychological response to the climate crisis.

What happens when climate change and the mental-health crisis collide?

Scientists say the topic has been sorely neglected, but is leaping up the research agenda. “I’ve seen an explosion of research in the last five years for sure. That’s been very exciting,” says Alison Hwong, a psychiatrist and mental-health researcher at the University of California, San Francisco. The growing severity of heat, hurricanes and other impacts mean “it’s impossible to ignore”, she says.

Researchers want to unpick the many pathways by which climate change affects mental health, from trauma caused by hurricanes, floods, droughts and fires to ‘eco-anxiety ’— a chronic fear of environmental doom. Studies on methods that can help people prevent or manage these problems are also needed, although some work suggests that climate action and activism might help.

A seam of climate injustice is exposed by the research. Young people are likely to experience the greatest mental burden from climate change that older generations have caused. Groups of people that already experience poverty, illness or inequalities are most at risk of deteriorating mental health. “Climate change exacerbates already existing economic situations, where it’s the poorer people who are feeling even worse,” says Jennifer Uchendu, a researcher, climate activist and founder of SustyVibes, an environmental group based in Lagos, Nigeria.

Mental-health toll

The fact that climate change affects people’s mental health is not surprising: what’s new is the attention the issue is attracting — and the myriad ways that scientists are documenting its varied and sometimes shocking effects.

It is well known that extreme weather events and disasters can have an immediate traumatic impact — as well as “a long tail of mental-health conditions like post-traumatic stress disorder, anxiety, depression, substance abuse,” says Emma Lawrance, who studies mental health at Imperial College London. Also taking a mental-health toll in vulnerable countries are less sudden — but nonetheless devastating — disruptions caused by global warming’s impacts, such as forced migration, loss of livelihoods, food insecurity and community breakdown.

Research on how climate-change impacts, such as drought, affect mental health is growing. Credit: Simone Boccaccio/SOPA Images/LightRocket via Getty

There is evidence that directly experiencing higher temperatures can worsen mental health. A 2018 study of suicide data from the United States and Mexico over two or more decades showed that suicide rates rose by 0.7% in the United States and 2.1% in Mexico, with a 1 °C increase in average monthly temperature 2 . The researchers projected an extra 9,000–40,000 suicides by 2050 in the two countries if no action was taken against climate change. Other work has shown that higher temperatures are linked to poor sleep — which can in turn contribute to mental distress 3 .

Studies also suggest that people with existing mental illness are at greater risk of dying during extreme heat 4 , but “understanding why that is and what we can do to stop it is really unexplored”, Lawrance says. One potential explanation is that some psychiatric drugs can interfere with the body’s response to heat 5 .

Eco-anxiety goes global

Another striking field of research examines how the awareness of climate change and its impacts can lead to concern or distress, a phenomenon sometimes called eco-anxiety, eco-distress, climate grief or solastalgia (distress linked to environmental change). In a 2018 survey, 72% of people aged 18–34 said that negative environmental news stories affected their emotional well-being, such as by causing anxiety, racing thoughts or sleep problems (see go.nature.com/3vbbt7p ). A 2020 survey 6 in the United Kingdom found that young people aged 16–24 reported more distress from climate change than from COVID-19.

A few years ago, such ‘eco-emotions’ were sometimes dismissed as fretting of the ‘worried-well’ in high-income countries, Lawrance says. But research that shows the global reach of these feelings is challenging that view. The 2021 survey 1 was the biggest so far on climate anxiety and included 10,000 children and young people in 10 countries. More than 45% of respondents said that worry about climate change had a negative impact on eating, working, sleeping or other aspects of their daily lives. Reports of climate change affecting people’s ability to function were highest in the Philippines, India and Nigeria and lowest in the United States and United Kingdom — contradicting the idea that eco-anxiety is just a rich-country problem (see ‘Climate anxiety around the world’).

Source: Ref. 1

For some, eco-anxiety might be linked to first-hand experience of climate-related devastation. The fact that young people in the Philippines reported some of the highest levels of worry was no surprise to John Jamir Benzon Aruta, an environmental psychologist at De La Salle University in Manila. In 2013, he saw first-hand the devastation and trauma caused in the Philippines by Typhoon Haiyan — one of the most powerful tropical cyclones ever recorded. “You see houses, communities devastated. You also see corpses all over the place,” he says. “Just witnessing the aftermath made me feel traumatized.”

But the 2021 survey documented widespread distress that went beyond those who were immediately affected by extreme climate events. Around 75% of respondents said that climate change made them think the future is frightening and 56% said that it made them think that humanity is doomed. People who felt their government was failing to act on climate issues were more likely to feel eco-distress.

Are we all doomed? How to cope with the daunting uncertainties of climate change

Climate change isn’t the first existential crisis that humanity has faced. But researchers point out that it is different from some other threats: it is happening now rather than being a future risk, such as a nuclear war ; it’s affecting the entire globe at once; and many people feel angry that they have to bear the brunt of climate change that other people have caused.

Feelings of eco-anxiety are not necessarily a sign of dysfunction. “If you are under immediate threat, it is a realistic, rational, healthy survival instinct to react by being anxious or to experience fear,” says Elizabeth Marks, a clinical psychologist at the University of Bath, UK, and one of the survey’s lead authors. It could even be harmful to think of these feelings as a disorder. “If we think of it as a diagnosable condition, that risks placing the blame on the individual as having an unhealthy response,” she says. That said, some people might become so impaired by their eco-distress that they would benefit from psychological help.

Social media is being used to monitor negative feelings linked to climate change. In 2023, Kelton Minor, a research scientist at Columbia University’s Data Science Institute in New York and Nick Obradovich, a climate mental-health researcher at the Laureate Institute for Brain Research in Tulsa, Oklahoma, reported an analysis of more than eight billion posts on Twitter (now known as X) that appeared between 2015 and 2022 from people who had opted to share their geolocation data. (The analysis was part of a wider report on health and climate change 7 .) The researchers analysed the tweets for positive words (such as good, well, new and love) and negative ones (bad, wrong, hate and hurt), and linked them to climate data from the tweeters’ locations. Perhaps unsurprisingly, the team found that heatwaves and extreme rainfall increased negative feelings and decreased positive ones compared with control days without extreme weather in the same place and time of year. They also found that these negative reactions became worse over the years (see ‘Eco-anxiety on social media’).

Source: Ref. 7

Beyond the Western view

The full effects of climate change on mental health are hard to measure. A combination of factors, including the stigma around mental health and lack of access to health-care services, mean that many people with mental-health concerns go undiagnosed. When Wamaitha talked to her family in Kenya about how worried she was, they’d say: “It’s not a big deal, it’s part of life,” she says. Anxiety and depression are barely recognized as disorders in her region, she says. Mental-health services are scarce and older people just “think that you’re very sensitive” because they survived droughts in the past. In the 2021 survey, nearly 40% of young people worldwide said their concerns about climate change had been ignored or dismissed.

Climate change is also a health crisis — these 3 graphics explain why

Researchers are particularly worried that countries and regions that experience the harshest effects of climate change are where the least climate mental-health research has been done. In her studies, Uchendu found that most research was Western-centric. “Not a lot of people were talking about these issues in Africa,” she says. In 2022, she started the Eco-anxiety in Africa Project, which, in collaboration with the University of Nottingham, UK, has documented the emotional turmoil that heat and erratic weather has created for people living in five African cities.

Another question researchers have is how context and culture affect climate anxiety. Some studies have shown that “connection to country” — through cultural practices such as hunting and gathering food — is important to the mental health and well-being of some Aboriginal Australians and Torres Strait Islander communities 8 , says Michelle Dickson, who studies the mental health of Indigenous Australians at the University of Sydney, Australia. But rising sea levels, drought and bushfires threaten those practices. Tools used in health-care settings “rarely take into account the important cultural values that underpin Indigenous mental health”, says Dickson, who is a Darkinjung/Ngarigo Australian Aboriginal.

Dickson is now co-leading a project to empower communities to design their own climate action plans — allowing researchers to test whether doing this could improve people’s mental health.

Heatwaves — such as one that hit New Delhi in 2022 — can worsen mental disorders and are linked to increased negative feelings. Credit: Kabir Jhangiani/Pacific Press/LightRocket via Getty

Overcoming eco-distress

Addressing climate-fuelled mental-health conditions will be a colossal task when mental-health care globally is already poor: only around 3% of people with depression receive adequate treatment in low- and lower-middle-income countries, and 23% in high-income countries 9 . Lawrance says that many communities are finding their own ways to cope, but that the effectiveness of these efforts is rarely studied and shared. “There’s a massive gap around evaluation,” she says.

Some evidence suggests that taking action to combat climate change can help people to manage eco-anxiety . “There does seem to be an argument for supporting people to take collective action,” says Marks, such as joining campaign groups with like-minded people. It’s also important to “recognize that I feel this way because I care”, she says. “These climate emotions are actually something to be honoured and allowed, not pushed away.” Marks also suggests that some people who are feeling eco-distress limit the amount of time they spend ‘doom-scrolling’ through climate news.

Extreme heat harms health — what is the human body’s limit?

Researchers are starting to take collective action themselves. Last month, the Connecting Climate Minds project, one of the most ambitious research efforts in the field of climate-related mental health 10 , released a series of regional ‘research and action’ priorities, including, for example, to understand how climate change compounds the stress of wars, violence and disease epidemics in sub-Saharan Africa . The project includes researchers, policymakers and people with first-hand experience of climate change. Uchendu says that in one of the meetings, someone joining remotely was standing in flood water in their room. “It was mind-blowing,” she says.

Wamaitha, who along with Burrows is one of many people who shared their experiences with Connecting Climate Minds, has turned some of her concerns into action. Last year, after trying and failing to grow drought-resistant crops, she quit farming and is now working at a non-governmental organization in Bura, Kenya, that is focused on poverty relief. She is earning enough to study for a master’s degree in public health, and she raises awareness of global health on the social-networking site LinkedIn. But she is anxious about the future and worries about whether to have children. “I don’t think I am in a good environment to be able to bring kids into this particular place,” she says. “That is the saddest thing when I think about it.”

Burrows, who is studying medicine at the University of the West Indies in Saint Augustine, Trinidad and Tobago, says she chooses to be positive and does small things to help the environment, such as walking instead of driving. She says that she prays that wealthy countries and companies “will really, truly understand what is happening and not just say smooth words to try to pacify us in the moment”. They should act to “help the smaller countries and the world at large”, she says.

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