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When floods swept through parts of China’s Henan province last month, killing at least 302 people, a group of scientists who specialise in analysing the drivers of extreme weather events found themselves unable to help. Like everyone else, they were horrified by the images of people trapped in water-filled subway stations, as whole blocks of the city of Zhengzhou were flooded by record rainfall. Their work involves two core questions: did climate change make this disaster more likely? And did it make it worse? But by the time the storms hit China, the scientists were already fully engaged trying to untangle why the floods in Germany and Belgium earlier in July had been so devastating. Demand is intensifying for the work of the World Weather Attribution initiative (WWA), a team founded in 2014 which comprises seven core volunteer researchers, all of whom also have day jobs, and which has been unfunded for years.
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Two weeks before the German floods, the same questions were being asked about a freak heatwave in North America. This week, fires sparked by record temperatures are blazing across Turkey, Greece and Italy. “We are already struggling to get the manpower,” says Geert Jan van Oldenborgh, co-lead of WWA. As the world warms up, he expects the workload to increase, because “events will get worse”. Is climate change to blame? To the casual observer, the flurry of extreme weather events over the past two months can feel like the obvious consequence of rising average temperatures — and something that many scientists have been warning about for years. Yet establishing a direct causal link between an individual case of flood, fire or storm and the broader climate is an evolving science — and something that is still desperately hard to do in practice. A litany of factors can influence a natural disaster, including local weather conditions — which may be changing — the shape of the landscape, human choices and natural variability. Even without climate change, extremes such as heatwaves would occur. That climate change is making extreme weather more frequent and intense is a connection that has been “well made”, says Peter Stott, an expert in climate attribution at the UK’s Met Office. But he adds: “The science gets more difficult when we ask versions of the question to do with a specific [event] . . . and say ‘is this due to climate change’, or maybe more meaningfully, ‘how did climate change contribute to this?’”
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If the science of climate attribution becomes more accurate, the implications could be substantial. It could allow more precise predictions about future events and identify areas that are particularly at risk, which would help societies prepare and adapt. Stronger attribution could also fortify the legal cases that are already being brought against companies and governments seen as partly responsible for climate change. But the challenge is considerable. “It’s quite a tricky question to say ‘how much more rain fell in [the European] floods due to climate change’,” says Stott. “That is really pushing at the frontiers of science.” Scientists are making some headway. In July, the WWA made the striking pronouncement that the North American heatwave that sent temperatures in the Canadian village of Lytton soaring to 49.6C would have been “virtually impossible without human-caused climate change”. The group reached a similar verdict about a heatwave last year in Siberia, concluding that the 38C recorded in Verkhoyansk would have been “almost impossible without climate change”. “We know that weather varies a lot from day to day,” says FlavioLehner, a climate scientist at Cornell University who works with the WWA. “The question is when [extreme weather events] occur, are they stronger, longer and more severe than they would have been without climate change?” Yet they are wary about pushing their conclusions too far. “Every time we publish something, I wake up at 4am in the morning to make sure that the numbers are correct,” admits van Oldenborgh. “It’s essential that we worry . . . [to get] as close to the truth as possible.”
