Extreme event attribution, also called attribution science, is a method used to examine how much different causes contribute to a specific event. It also shows how certain scientists are about their findings. This science is most often used to study how human-caused climate change affects the frequency, strength, length, and effects of extreme weather events. The goal of attribution science is to find out how much these events are connected to human-caused global warming, rather than being caused only by natural weather patterns or random climate changes.

History

Before statistical methods were developed to study how human activities affect climate, scientists believed that overall climate changes could be linked to human influence, but they thought it was not possible to connect specific weather events to human causes. The Intergovernmental Panel on Climate Change (IPCC) first discussed attribution in its 1990 report, stating that proving a cause-and-effect relationship with high confidence from studying one factor alone was very difficult. However, the IPCC noted that confidence in linking human activities to climate changes grew when predictions matched real-world observations. This method, called the "fingerprint method," looks for signs unique to human-caused global warming. In its 2001 report, the IPCC said that studies found clear evidence of human influence in climate changes over the past 35 to 50 years. It also reported that most studies showed warming caused by rising greenhouse gases alone was about the same as or greater than the warming observed in the last 50 years.

In 2004, scientists used a method called Fraction Attributable Risk (FAR) to study the 2003 European heatwave, showing a direct connection between human-caused climate change and that extreme weather event. In 2011, the American Meteorological Society described attribution science in a climate report, stating that human-caused climate change played a role in five out of six extreme weather events studied. In the 2000s, more powerful computers allowed scientists to simulate weather events repeatedly. Major scientific advances in the early to mid-2010s helped develop attribution science further. In 2016, the American Meteorological Society said that the science had improved enough to confidently detect climate change effects on some weather events.

Purpose

Climate Central explained that attribution studies determine if and how much climate change influenced the strength, how often, or the effects of extreme events, such as wildfires, droughts, heavy rain, and typhoons. Friederike Otto, a German climatologist, said that attribution science tries to answer the question, "Did climate change play a role" in specific extreme events "within the news time frame – so within two weeks of the event." Results from these studies help scientists and journalists state facts like, "This weather event was at least n times more likely because of human-caused climate change" or "This heatwave was m degrees hotter than it would have been in a world without global warming" or "This event would not have happened without climate change."

Another purpose of attribution science is to evaluate how extreme events affect human weaknesses and society's ability to prepare. It helps people understand more clearly how climate change impacts their environment, provides scientific proof for public discussions, and allows for focused planning based on expected increases in certain extreme events. Individuals, communities, and businesses can use information about their specific area to predict future extreme events in their location.

Methods

In a study published in 2004, scientists compared computer models that included climate change with models that did not include warming caused by humans. The study focused on how much climate change might have influenced the likelihood of certain events, rather than whether climate change directly caused them. This is not a simple yes or no question.

Extreme events can be described in several ways. These include how common an event is compared to past events, how far it is from normal conditions, the chance of it happening in a certain time period (such as "1% chance each year"), or how often it is expected to occur (such as a "100-year drought").

Scientists use two main methods to study event attribution. The first method looks at real-world data to see how the chance or strength of events has changed over time. The second method uses computer models to compare two scenarios: one that includes human-caused greenhouse gas emissions and one that does not. Differences between these scenarios are linked to human influence on climate.

German climatologist Friederike Otto explained that scientists use climate models to simulate events under today's climate conditions and calculate how likely such events are. They then simulate the climate of the preindustrial era by removing human-caused warming from models or analyzing data from the late 19th and early 20th centuries. Scientists compare the intensity or frequency of events in these two simulations to determine if climate change played a role. These methods have been reviewed by experts, allowing new studies to be published quickly.

Similarly, atmospheric scientist Xubin Zeng outlined four steps for event attribution studies. First, scientists measure the size and frequency of an event using real data. Second, they use computer models to compare and check this data. Third, they run the same models using a climate without human-caused warming. Finally, they use statistics to compare results from the second and third steps.

Results from different studies may vary because of how scientists define extreme events, the time period studied, and the level of detail in climate models. Computer models can also produce different results depending on which event feature is analyzed, such as peak wind speed versus average wind speed, or total rainfall versus how long it lasts.

Findings

In November 2024, Climate Central reviewed more than 600 climate studies that examined nearly 750 extreme weather events and trends. The review showed that climate change made 74% of these events more likely or more severe. About 9% of the events were made less likely or less severe.

Climate change affects how often and how strongly extreme weather events happen in different ways. For example, the 2010 heat wave in Russia became much more likely because of climate change, but it did not become stronger. This means some studies focus on different questions than others.

Better understanding of factors like how air moves, how much water is in the air and soil, and what covers the Earth’s surface helps scientists predict and plan for heat waves. Heat waves are easier to model than other extreme events because long-term temperature records are more accurate than records for other types of events. Rainfall is also easier to study, but droughts, snowstorms, tropical storms, and wildfires are more difficult to predict because their data is less clear.

Applications and influence

Attribution science has been used as evidence in legal cases about climate change. These cases involve companies accused of causing climate change and governments criticized for not taking action to address it.

Extreme event attribution (EEA) was used to study 213 heatwaves that happened between 2000 and 2023. This method helps determine which actors are responsible for these events. Research found that "carbon majors," which are companies that produce fossil fuels and cement, contributed to 16–53 heatwaves. These heatwaves would have been almost impossible to occur in a climate before the industrial era.

In late 2025, the Copernicus Climate Change Service announced the creation of a permanent office focused on attribution science. This office will provide two assessments each month, with each assessment completed within a week of an extreme weather event. These assessments can help governments create policies, support insurance companies in evaluating risks, and aid in climate-related legal cases.

Examples

Various groups study how human activities affect weather and climate events. The Sabin Center for Climate Change Law at Columbia University has searchable databases that include categories such as climate change attribution, extreme event attribution, impact attribution, source attribution, and attribution in legal cases. Carbon Brief has an interactive map showing over 600 studies related to nearly 750 extreme weather events and patterns. The American Meteorological Society releases yearly collections of studies titled "Explaining Extreme Events of [year] from a Climate Perspective."

Examples include:

• 2003 European heatwave: Human influence likely doubled the risk of a heatwave reaching this level.
• 2004–2024: Human activities intensified the ten deadliest extreme weather events during that time, leading to at least 570,000 deaths.
• October 2012 – Hurricane Sandy: Flooding covered an area 27 square miles (about 70 square kilometers) larger than it would have without human influence.
• August 2016 – Louisiana floods: Human activities made this event 40% more likely.
• February 2017 – US winter heat wave: Human influence made this event three times more likely.
• August 2017 – Hurricane Harvey: Human activities made this event three times more likely and 15% more intense.
• 2017–2018 Tasman Sea marine heatwave: The extreme heat in the ocean was "virtually impossible" without human-caused changes.
• 1950–2020: Human-caused heat extremes reduced the global population of tropical birds by 25–38%.
• 2019–2020 Australian bushfires: Human activities made this event at least 30% more likely, and extreme heat was at least twice as likely.
• 2020 Siberia heatwave: This event was over 500 times more likely than similar events in 1900, with estimates ranging from 600 to 99,000 times more likely.
• 2021 Western North America heatwave: Human influence made this event 150 times more likely.
• 2022 United Kingdom heatwave: A 2-day average temperature record that would normally occur once every 100 years became at least 10 times more likely. A single-day temperature record that would normally occur once every 1,000 years became more likely.
• 2022: Heat extremes caused tens of thousands of deaths worldwide. Heavy rainfall events in the North Atlantic caused $500 billion in damages.
• May 2023 through May 2024: Extreme heat days increased significantly in Suriname (182 vs. 24 days), Ecuador (180 vs. 10 days), Guyana (174 vs. 33 days), El Salvador (163 vs. 15 days), and Panama (149 vs. 12 days) due to human-caused climate change.
• 2023 and 2024 (various heat events): Probability ratios (PRs) showed heat events were 35 times more likely in the Marshall Islands and Micronesia, 24–29 times more likely in Indonesia and the Philippines, 13 times more likely in Central America, 8 times more likely in Spain and Portugal, and 7 times more likely in southern South America.
• 2000–2023: Fossil fuel and cement producers contributed to 16–53 of 213 historical heatwaves that would have been nearly impossible in a preindustrial climate.
• 2024: Climate change caused 44% of the damage from Hurricane Helene (Category 4) and 45% of the damage from Hurricane Milton (Category 5) in Florida.
• 2026: Climate change made the 2026 Chilean wildfires three times more likely, with a 2.5 times increase in likelihood in Patagonia.