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Extreme weather events such as life-threatening heat waves and record-breaking downpours are part of the natural climate system, but some kinds of extreme weather events have become more common in recent decades. Scientific advances are allowing researchers to determine the extent to which climate change contributed to some extreme weather events. This is the science of extreme event attribution.

How scientists estimate the role of climate change in specific weather events

  • The most prevalent event-attribution technique involves calculating the “fraction of attributable risk” (FAR) for a specific weather event. This statistical approach has been long used by epidemiologists for public health studies (for example, “What is the risk of cancer that is attributable to cigarette smoking?”). In combination with well-established climate models, FAR allows scientists to assess the probability of an event happening in two different scenarios: a world without our history of greenhouse gas emissions related to human activities, and our world as it is now.
  • Many of these studies have been compiled by the American Meteorological Society (AMS) in a series of reports called “Explaining Extreme Events of [year] from a Climate Perspective”.
  • To minimize bias, studies are selected for inclusion in these reports solely based on the rigor of their designs. AMS publishes the results of all studies it has determined to be acceptably rigorous—whether they show a connection to climate change or not.

What factors complicate the study of climate attribution?

  • Weather is inherently variable, so it takes an enormous amount of data over long periods of time to say with certainty that a specific extreme event was not simply a normal part of this variability.
  • Human activities complicate the process of attributing extreme events to climate change.
    • For example, the frequency and severity of wildfires are affected not just by climate-change-related factors such as earlier spring and drought, but also by land management practices that affect the amount of available fuel.
    • Similarly, extensive surface paving in cities and suburbs has increased runoff and added to climate-related flooding.
  • More data and improved computer models will increase the certainty with which scientists can determine the connections between extreme weather events and climate change. Still, some kinds of weather events are easier to model than others, and different studies will offer different degrees of certainty.

Is it fair to say climate change has “caused” any specific extreme weather events?

  • Many factors contribute to every weather event; there is no “single cause” for any weather event.
  • Scientists can, however, use climate attribution methods to determine the probability that a past weather event would have occurred if not for climate change. For example, scientists estimate that the record-breaking high temperature recorded in western Europe in May of 2020 was made 40 times more likely by human-caused climate change.
  • In some instances, climate attribution methods have indicated that the odds of some past extreme events occurring would have been essentially zero if not for the “last straw” of climate change.
    • Climate change can arguably be said to have caused these extreme events—that is, made otherwise nominal events extreme—in that it enabled them to achieve an intensity that was otherwise effectively implausible.

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  1. Attribution of Extreme Weather Events in the Context of Climate Change (2016) is a comprehensive look at the topic from the National Academies of Sciences, Engineering, and Medicine.
  2. Explaining Extreme Events of 2020 from A Climate Perspective (2021) is the most recent in a series of annual reports produced as a special supplement to the Bulletin of the American Meteorological Society, and includes links to the previous nine reports.
  3. Climate Science Special Report (2017) is an authoritative assessment of the science of climate change, with a focus on the United States. Produced by scientists representing federal agencies, national laboratories, universities, and the private sector, with administrative oversight by the U.S. Commerce Department’s National Oceanic and Atmospheric Administration, it represents the first of two volumes of the Fourth National Climate Assessment, mandated by the Global Change Research Act of 1990. It includes a chapter on Detection and Attribution of Climate Change. The fifth national climate assessment is scheduled for release in 2023.
  4. The U.S. National Climate Assessment’s fourth and latest report on climate impacts (2018) includes a discussion of natural variation and attribution research in Chapter 2.
  5. AR6 Climate Change 2021: The Physical Science Basis is a report produced by the Intergovernmental Panel on Climate Change. It includes a chapter on how human influence is affecting the climate (Chapter 3) and a chapter on weather and extreme climate events that outlines trends and projections (Chapter 11). Climate Change 2022: Impacts, Adaptation and Vulnerability, another IPCC report, evaluates the effects of climate change on ecosystems, biodiversity, and human communities.
  6. The science of attributing extreme weather events and its potential contribution to assessing loss and damage associated with climate change impacts, produced by researchers at the Environmental Change Institute at the University of Oxford’s School of Geography and the Environment, is a brief summary of attribution science and its value, including some details about how attribution modeling works.
  7. Extreme event attribution: the climate versus weather blame game, updated in 2021, is a useful fact sheet produced by Climate.gov, the climate-change information site of the U.S. Commerce Department’s National Oceanic and Atmospheric Administration (NOAA).
  8. The National Center for Atmospheric Research (NCAR) is a federally funded research and development center sponsored by the National Science Foundation. Its website offers numerous evidence-based resources on atmospheric and related sciences, including climate modeling.
  9. The World Weather Attribution Initiative is a collaboration with scientific partners from the University of Oxford Environmental Change Institute (ECI), the Royal Netherlands Meteorological Institute (KNMI), the Laboratoire des Sciences du Climate et de l’Environment, NOAA, Princeton University, NCAR, and the Red Cross and Red Crescent Climate Centre. The Initiative is developing tools and methodologies to perform real-time assessments of whether and to what extent human-induced climate change played a role in the magnitude and frequency of extreme weather events.
  10. climateprediction.net is a nonprofit, collaborative effort led by climate scientists, computing experts, and others, that performs climate modelling experiments using the home computers of thousands of volunteers to answer questions about how climate change is affecting the world now and may affect the world in the future.
  11. Extreme weather is of deep concern to the insurance industry, which has produced a number of documents describing those concerns and strategies for lowering risk. Among them: the National Association of Insurance Commissioners’ Assessment of and Insights from NAIC Climate Risk Disclosure Data, and Climate Change Risk Assessment for the Insurance Industry, by the Geneva Association, an insurance industry think tank.

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