You are reading Part 1 of 3 in this series. What are Quick Facts?
Extreme weather events such as life-threatening heat waves and record-breaking downpours are part of the natural climate system, but by definition they are relatively infrequent—even rare. In recent decades, however, some kinds of extreme weather events have become more common. Science has linked some of these general increases to climate change, but it was challenging for researchers to clarify the influence of climate change on specific extreme weather events. Advances in computer processing power over the last decade and improved methods for sorting out the many factors that contribute to weather are allowing scientists not only to determine the extent to which climate change contributed to some extreme weather events but also to say with confidence that certain extreme weather events would not or could not have occurred but for climate change. This is the science of extreme event attribution.
The difference between “climate” and “weather”
- Weather is what’s happening in the atmosphere over a short period of time—hours, days, or weeks. Climate is a description of atmospheric behavior over longer periods, typically years, decades, or longer.
- In the words of Dr. J. Marshall Shepherd, past president of the American Meteorological Society, “Weather is your mood and climate is your personality.”
What constitutes an “extreme” weather event
- “Extreme weather” is a relative term meaning especially intense or very severe weather compared to what normally occurs over a baseline period of time, such as the average for the 20th century.
- Because there is not a widely accepted technical definition, every analysis must define “extreme”.
Why scientists are studying extreme weather attribution
- Countless scientific papers, as well as assessments of those papers by the U.S. National Academies of Sciences, Engineering, and Medicine (NASEM), the U.S. National Climate Assessment, the Intergovernmental Panel on Climate Change (IPCC), and others have concluded that the Earth’s climate system is warming and that most or all of the warming in recent decades has been caused by human activities—primarily the burning of fossil fuels.
- Knowing whether and how this climate change is contributing to extreme weather (a field sometimes called “extreme weather attribution”) can help decision-makers:
- predict future prevalence and severity of extreme weather events;
- determine how to balance efforts to limit climate change (“mitigation”) versus efforts on preparedness and infrastructure resilience (adaptation);
- identify whether certain efforts to address climate change could save more money than they cost.
How to describe climate attribution findings
- For some kinds of weather events, in some places, there is a clear connection between climate change and weather extremes. When documenting these connections, however, it is important to define terms precisely.
- For example, an “increase” in a kind of extreme weather can mean “happens more frequently” or “is more extreme when it happens” (in intensity or duration) or both. Careful wording can prevent confusion.
- It’s also important to distinguish between the influence of climate change on certain classes of extreme weather, and climate change’s contribution to a specific extreme weather event.
How climate change is affecting extreme weather now
- Climate scientists are confident that climate change is already causing an increase in the frequency and/or intensity of the following types of extreme weather or weather-related events (listed here from highest to lowest confidence):
- multi-day temperature extremes, primarily heat waves
- drought and heavy rains
- tropical storms, including hurricanes and typhoons
- wildfires and perhaps extreme convective storms (e.g., afternoon thunderstorms).
- Wildfires are considered “weather-related,” since they are not meteorological events themselves but are linked to weather extremes such as drought and lightning storms.
- Also not meteorological events per se but among the most certain weather-related impacts of climate change: coastal flooding due to climate-change-linked sea level rise.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.
- 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.
- 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.
- 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.