How much did climate change add to one heatwave or storm?

Scientists answer by asking a different question: not whether climate change caused one heatwave or storm, but how much it changed the odds or the strength of the event. Extreme event attribution is the science of identifying and quantifying the role human-caused climate change plays in the frequency, intensity, duration and impacts of individual extreme weather events.

That was long considered impossible. Before statistical methods existed, the consensus was that global climate trends could be attributed to human influence, but attribution of individual weather events was not thought possible. A 2004 paper, regarded as a breakthrough, compared simulations with and without climate change. The breakthrough was not to ask whether climate change caused the event, but how much it might have affected the risk of it occurring at all. It is not a yes or no question.

In practice researchers use model simulations to compare a factual world, with human greenhouse gas emissions, and a counterfactual world without them. Atmospheric scientist Xubin Zeng describes four steps: measure the event from observed data, check the models against the observations, run the same models on a baseline with no climate change, and compare the results statistically. Because the methods are already peer reviewed, studies of new events can be published within a news-cycle time frame.

The results are mostly one-directional but not always. A November 2024 review of more than 600 studies found that climate change made 74% of events more likely or severe, and that about 9% were made less likely or severe. For the 2003 European heatwave, a study found it very likely that human influence had at least doubled the risk of a heatwave exceeding that threshold. Hurricane Harvey in 2017 was found to be three times more likely and 15% more intense.