Many regions of China are just emerging from a record-breaking heatwave that lasted for two months, and which was followed by one of the worst droughts in nearly 40 years. Researchers are now trying to understand the conditions that fuelled such extreme events, because many projections came up short.
“We did not predict that the intensity would be so high,” says Sun Shao, a climatologist at the Chinese Academy of Meteorological Sciences in Beijing.
The severity of the heatwave and drought also highlights the importance of improving climate models and studying multiple extreme events together, to better assess their impacts. “Compounding events cause more disaster,” says Wang Aihui, an atmospheric scientist at the Institute of Atmospheric Physics, Chinese Academy of Sciences in Beijing. For instance, the drought in the Yangtze River basin worsened quickly because of the combined effects of high temperature and a lack of rain.
Heat and drought
From mid-June to late August, a heatwave spread across central, eastern and southern China — the longest-running and most intense since records began in 1961. Close to one billion people experienced temperatures exceeding 35 °C, and 360 million felt temperatures of more than 40 °C at some point during the heatwave.
And between July and August, the Yangtze River basin in southern China experienced the worst drought since 1972. The basin, home to nearly one-third of China’s population, received up to 80% less rain than the 30-year average for that period, and temperatures were 2–4 °C higher than the average, says Sun. The drought was so extreme that water levels in China’s largest freshwater lake, Poyang Lake, dropped from 19 metres in June to 9 metres by the end of August, says Wang Aihui.
These extreme trends were interspersed with out-of-season forest fires and regional pockets of heavy rains that caused flash flooding. “It was a very abnormal year,” says Fang Keyan, a climate scientist at Fujian Normal University in Fuzhou, China.
The immediate driver of the cloudless skies and high temperatures was a high-pressure system known as the Western Pacific subtropical high, which was strong and hovered above the Yangtze River basin, instead of sitting further east as a smaller pressure system, as is typical for this region, says Wang Aihui. The system prevented cold air from the north, and moisture from the Indian Ocean, from reaching the basin, she says.
Subtropical highs are typically broken up by tall mountain ranges, such as the Himalayas, but this year, multiple systems connected to encircle the entire globe at a certain latitude, which could explain the heatwaves in Europe and the United States, too, says Wenju Cai, a climate scientist at the Commonwealth Scientific and Industrial Research Organisation in Melbourne, Australia.
The high also coincided with unusual behaviour of the westerly winds that blow around the globe in the mid-latitudes, which brought rain to northern China but mostly bypassed the south, says Wang Huijun, a climatologist at Nanjing University of Information Science and Technology in China.
Ocean surface temperatures also contributed to the extreme events, such as the cooler water in parts of the Pacific Ocean that result in the La Niña climate event. Meteorologists have proposed that a third consecutive year of La Niña is under way, which, together with cooler sea surface temperatures in the Pacific Ocean, typically brings less rain to the Yangtze River, says Wang Huijun.
The convergence of multiple extreme events has been an eye opener for some researchers in China. More focus should be placed on studying compounding climate events, says Wang Aihui, who plans to move away from analysing individual climate extremes. To study complex events such as droughts, he says researchers will increasingly rely on complex models that simulate Earth systems.
Sun also plans to investigate how overlapping events can amplify disasters, from heatwaves and droughts to gales, heavy rainfall, hailstorms and cold snaps. “In the past, we focused on individual hazards, but in the future we should pay more attention to compound hazards,” he says.
This year’s extreme events took place against a backdrop of global warming. As global temperatures rise, the northern, arid regions of China are expected to become wetter and the southern, humid regions will become drier1. Zhang Qiang, a hydrometeorologist at Beijing Normal University, says this year’s events so far agree with those projections.
Since 1951, temperatures in China have risen by 0.26 °C a decade — more than the global average. Global warming’s role in the recent events has yet to be analysed, but generally, warmer temperatures increase evaporation, which can prompt a quicker transition to drought conditions and make them more intense, says Cai. Climate models suggest that droughts will worsen across China, and become more frequent in certain regions2.