Last November the spring weather in South America jumped from cold to searing. Usually at that time of year people would have been holding backyard barbecues, or asados, in the lingering evening light. But on December 7 the temperature in northern Argentina, near the borders of Bolivia and Paraguay, hit 115 degrees Fahrenheit, making it one of the hottest places on Earth. The heat exacerbated a three-year drought, baking the soil and shriveling vast wheat crops before harvest.
A decade ago the Intergovernmental Panel on Climate Change said attribution science was not yet “fit for purpose.” In contrast, a 2021 IPCC report called attribution science “robust.” The growing ability to say decisively just how much climate change is to blame could have implications for everything from insurance claims and court cases to international negotiations over which nations should pay for climate adaptation.
As the field grew, two methods developed. One, known as probabilistic event attribution, is used to estimate how much human behavior has contributed to the odds of a certain kind of event occurring, such as a heat wave. Scientists compare models of extreme weather dynamics with simulations of a world in which climate change is not happening, revealing whether factors such as increased emissions made an event more likely.
The field made further headway in 2017 after Hurricane Harvey stalled for days over the greater Houston region, dumping as much as 60 inches of rain in places, far exceeding previous records. Trenberth found that superwarm ocean water in the Gulf of Mexico created greater evaporation than normal, leading directly to excessive precipitation. A separate analysis by Otto and her colleagues indicated that climate change had added 15 percent more rainfall.
The South American heat wave caught the team's attention because it met several of the WWA's criteria for heat waves: record-breaking temperatures occurring in very early summer in a vulnerable area. Although the first video conference to discuss it in late November 2022 offered some windows into the researchers' lives—Madrid's sunshine, London's gloom—the meeting began without small talk.
Otto asked the team whether it could do something fast and meaningful based on just temperature and precipitation. The group bandied the options around: Heat waves are fairly straightforward to analyze because many have been studied and linked to climate change. But some WWA members advocated for a larger, slower analysis of the drought.