Climate Change Made Summer Hotter and Drier Worldwide, Study Finds

A field destroyed by a brush fire in the drought-ravaged village of Xinyao in Jiangxi Province, China, in August. Credit…Thomas Peter/Reuters

Raymond Zhong

Human-caused global warming has made severe droughts like the ones this summer in Europe, North America and China at least 20 times as likely to occur as they would have been more than a century ago, scientists said Wednesday. It’s the latest evidence of how climate change caused by the burning of fossil fuels is imperiling food, water and electricity supplies around the world.

The main driver of this year’s droughts was searing heat throughout much of the Northern Hemisphere, the researchers reported in a new study. Such high average temperatures, over such a large area, would have been “virtually impossible” without the influence of greenhouse gas emissions, the scientists said.

Across the Northern Hemisphere north of the tropics, soil conditions as parched as they were this summer now have a roughly 1-in-20 chance of occurring each year, the scientists found. Global warming increased this likelihood, they said, but cautioned that because of the challenges involved in estimating soil moisture at a global scale, the exact size of the increase had a wide possible range.

“In many of these countries and regions, we are clearly, according to the science, already seeing the fingerprints of climate change,” said Maarten van Aalst, the director of the Red Cross Red Crescent Climate Center and one of 21 researchers who prepared the new study as part of the World Weather Attribution initiative, a research collaboration that specializes in rapid analysis of extreme weather events.

“The impacts are now very clear to people, and they’re hitting hard,” Dr. van Aalst said.

Extreme summer dryness that ravages cropscripples river commerce and strains hydropower generation across so much of the planet would be hugely problematic on its own. This year, though, global food and energy prices had already been rising for other reasons, including Russia’s war in Ukraine.

The Castaic Lake reservoir in Los Angeles County was at 35 percent capacity in October, well below its average of 43  percent. Credit…Mario Tama/Getty Images

Record heat began smothering Europe in May, and roasting temperatures dried out rivers and fueled wildfires for prolonged stretches over the next few months. The heat might have contributed to 11,000 excess deaths in France and 8,000 in Germany, according to estimates. Across the European Union, summer wildfires burned a total area more than twice as large as the average over the previous 15 years.

China had its most brutal summer since modern records began in 1961, according to the country’s meteorological authority, with hot and dry weather reducing hydropower output in the manufacturing-heavy south. To keep production lines running at car and electronics factories, China dug up and burned more coal, increasing its contribution to global warming.

And in the United States, nearly half of the area of the lower 48 states experienced moderate to extreme drought this summer, according to the National Oceanic and Atmospheric Administration. Parts of the Southwest and California remain stuck in a 20-year-plus megadrought.

To gauge the influence of global warming on droughts and other extreme weather events, scientists use computer simulations to examine both the real-world climate and an alternate climate in which humans hadn’t burned fossil fuels and emitted greenhouse gases for more than a century. They see how often weather events as severe as the one in question occur in both worlds. The differences suggest how much global warming was to blame.

Scientists with World Weather Attribution found last month that climate change had quite likely worsened this summer’s devastating floods in Pakistan, which have killed 1,600 people, damaged two million homes and submerged large stretches of farmland. Earlier, they found that global warming had made Britain’s record-shattering July heat wave both hotter and more likely to occur.

The 14th-century Puente de la Mesta in central-western Spain in August. The bridge has been mostly submerged since the creation of a reservoir in the 1950s.Credit…Thomas Coex/Agence France-Presse — Getty Images

Droughts are harder to study than hot spells. Scorching temperatures and weak rainfall aren’t the only factors that influence them. Local landscape features also play a role. Plus, while sensor technologies are constantly improving, estimating the amount of moisture in the soil across large areas is hard to do reliably compared with measuring temperature or precipitation.

The authors of the new report looked at soil moisture levels from June through August across two geographic areas: the entire Northern Hemisphere north of the tropics, and a swath of continental Europe from France to Ukraine. They also looked at this summer’s temperatures and precipitation in both areas.

For the Northern Hemisphere region, the scientists found that, because the planet has already warmed by 2.2 degrees Fahrenheit (1.2 Celsius) since the late 1800s, this summer’s low moisture levels in the first few feet below the soil’s surface, where many plants’ roots draw water, had been at least 20 times as likely to occur compared with a hypothetical world with no burning of fossil fuels.

This has already made this summer’s drought a “relatively frequent” occurrence in the present climate, said Sonia I. Seneviratne, a scientist at the Swiss university ETH Zurich and another author of the study. But if the globe warms to 3.6 degrees Fahrenheit (2 Celsius) above preindustrial temperatures, as is likely under governments’ current policies, such dryness will become an additional 15 times as likely, she said.

“Basically, it would happen every year, every other year, more or less,” Dr. Seneviratne said.

For Western and Central Europe, global warming increased the chances of this summer’s dryness by a factor of three to four, the researchers found. This doesn’t mean Europe is less affected by climate change than other parts of the Northern Hemisphere, they said. Because it is a smaller area than the Northern Hemisphere above the tropics, natural variations in the weather cancel each other out less than they do for the larger region, said Friederike Otto, a scientist at Imperial College London and another study author.

“There is absolutely no doubt that climate change did play a big role here,” Dr. Otto said. But, she continued, “the exact quantification of that role is more uncertain for soil moisture than, for example, when we look at heavy precipitation.”


Raymond Zhong is a climate reporter. He joined The Times in 2017 and was part of the team that won the 2021 Pulitzer Prize in public service for coverage of the coronavirus pandemic

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