Extreme event attribution

• 1. From Skepticism To Confidence

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  In 1990, the Intergovernmental Panel on Climate Change issued its First Assessment Report. That document stated it was virtually impossible to demonstrate a cause-effect relationship with high confidence from studies of a single variable. Scientists believed global climate trends could be attributed to human influence, but individual weather events remained beyond reach. The IPCC noted that confidence in attribution increases as climate predictions are borne out by observed data. They referred to the fingerprint method that identifies indicators unique to human-caused global warming. By 2001, the Synthesis Report reported detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years. Most studies found that over the last 50 years, the estimated rate and magnitude of warming due to increasing greenhouse gases alone were comparable with or larger than the observed warming. A breakthrough paper published in 2004 changed everything. Researchers compared simulations accounting for climate change with scenarios where human-caused global warming did not exist. Fraction attributable risk was first applied in a 2004 analysis of the 2003 European heatwave. This provided a direct link between anthropogenic climate change and an individual extreme climate event. The American Meteorological Society described attribution science in a 2011 State of the Climate publication which concluded that human-caused climate change had a role in five of six extreme weather events studied. Greater computing power of the 2000s allowed weather to be simulated over and over again. Conceptual breakthroughs in the early to mid 2010s enabled attribution science. In 2016, the American Meteorological Society stated the science has now advanced to the point that we can detect the effects of climate change on some events with high confidence.

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• 2. Simulating Two Worlds

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  German climatologist Friederike Otto explained that attribution scientists use climate models to simulate what is possible under current climate conditions. They compute the likelihood of this particular event occurring given the forces affecting today's climate. Then the preindustrial world is simulated by removing anthropogenic warming forces from the climate models. Alternatively, researchers perform statistical modeling on observations of the late 19th and early 20th century. Differences in intensity or frequency between the two simulations are deemed to be caused by climate change. Atmospheric scientist Xubin Zeng said attribution studies generally proceed in four steps. First, they measure the magnitude and frequency of a given event based on observed data. Second, they run computer models to compare with and verify observation data. Third, they run the same models on a baseline having no climate change. Finally, they use statistics to compare and analyze the differences between the second and third steps. Event attribution approaches can be generally divided into two classes. A first approach uses observational records to determine the change in probability or magnitude of events over time. A second approach uses model simulations to compare events in two worlds. The first world contains human-caused greenhouse gas emissions. The second world exists without such emissions. Scientists attribute differences to human influence. Climate Central explained that attribution studies calculate whether and by how much climate change affected the intensity, frequency or impact of extremes. These extremes include wildfires, droughts, rainfall, and typhoons. Results from multiple attribution studies may differ from each other. This happens based on differing definitions of what constitutes an extreme event. It also depends on the period of the historical record being analyzed. Spatial resolution of the climate models creates further variation. Model outputs differ based on the event feature being analyzed. Peak wind versus average wind creates different results. Rainfall amount versus duration produces distinct outcomes.

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• 3. Heatwaves And Human Cost

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  Climate Central's November 2024 review covered more than 600 climate attribution studies. These studies addressed almost 750 extreme weather events and trends. The review found that climate change made 74% of events more likely or severe. About 9% were made less likely or severe. Heatwaves are more accurately modeled than other extreme events because long-term temperature data is more reliable than other events' datasets. Rainfall is relatively straightforward, but droughts, snowstorms, tropical storms and wildfires are more complicated. Improved understanding of relevant drivers improves heatwave prediction and projection. Atmospheric dynamics, atmospheric and soil moisture, and surface cover all play a role in these predictions. Climate change was found to affect the intensity and frequency of extreme weather events differently. For example, the 2010 Russia heat wave was made far more likely but not more intense. Thus, some attribution studies simply ask different questions than other attribution studies. Extreme event attribution was applied to 213 historical heatwaves from 2000, 2023. Carbon majors, fossil fuel and cement producers, enabled 16, 53 heatwaves that would have been virtually impossible in a preindustrial climate. Between 2004 and 2024, scientists intensified the ten deadliest extreme weather events during that time period. This contributed to the deaths of at least 570,000 people. A 2022 review of rainfall events in North Atlantic basin attributed $500 billion in damages combined. Days with extreme heat varied significantly across regions between May 2023 and May 2024. Suriname experienced 182 days with extreme heat compared to 24 days without global warming. Ecuador saw 180 versus 10 days. Guyana recorded 174 versus 33 days. El Salvador had 163 versus 15 days. Panama measured 149 versus 12 days.

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• 4. Courts And Policy Shifts

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  Attribution science has been cited in climate change litigation against companies for causing harm. Governments face lawsuits for not addressing climate change effectively. Extreme event attribution was applied to 213 historical heatwaves from 2000, 2023 to attribute events to specific actors. Carbon majors enabled 16, 53 heatwaves that would have been virtually impossible in a preindustrial climate. In late 2025, Copernicus Climate Change Service announced formation of a constantly operating attribution office. This office aims to provide two assessments a month. Each assessment arrives within a week of an extreme weather event. The assessments can be used to help set government policy. They assist insurance company risk assessments. They inform climate litigation strategies. The Sabin Center for Climate Change Law at Columbia University maintains searchable databases. These databases include categories like climate change attribution and impact attribution. Carbon Brief contains an interactive map including more than 600 studies covering almost 750 extreme weather events and trends. The American Meteorological Society publishes annual special collections titled Explaining Extreme Events of [year] from a Climate Perspective. Attribution science gives people a more concrete description of what climate change means to their environment. It provides scientific evidence to the public discourse. Communities use local-scale perspectives to anticipate future extremes at their specific location. Businesses rely on targeted adaptation based on expected increases in specific types of extreme events.

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• 5. Hurricanes And Heat Extremes

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  October 2012 brought Hurricane Sandy which flooded an area larger than previously recorded. August 2016 saw Louisiana floods that were 40% more likely due to human influence. February 2017 experienced a US winter heat wave three times more likely because of global warming. August 2017 Hurricane Harvey was three times more likely and 15% more intense. The 2017-2018 Tasman Sea marine heatwave had overall intensity described as virtually impossible without anthropogenic forcing. The 2019, 20 Australian bushfire season was at least 30% more likely. Heat extremes during that period were more likely by at least a factor of 2. A 2020 Siberia heatwave occurred over 500 times as likely as similar events in 1900. Some estimates placed this probability between 600 times and 99,000 times. The 2021 Western North America heat wave made the event 150 times more likely. The 2022 United Kingdom heatwave had a return period of 100 years for 2-day average temperatures. Single-day maximum temperature reached 1 in 1000 years for one region of the UK. This made the overall event at least 10 times more likely. In 2024, 44% of the loss in Florida from Category 4 Hurricane Helene is attributable to climate change. Another 45% of the loss in Florida from Category 5 Hurricane Milton stems from human influence. Probability ratios varied across regions between 2023 and 2024. Marshall Islands and Micronesia showed a ratio of 35. Indonesia and Philippines recorded ratios of 24 and 29 respectively. Central America measured 13. Spain and Portugal saw 8. Southern South America registered 7.

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