Many regions of China are recovering from a record-breaking heatwave that lasted two months and was followed by one of the worst droughts in nearly 40 years. Researchers are now trying to understand the conditions that fueled such extreme events because many projections have fallen short.
“We didn’t predict the intensity would be that high,” says Sun Shao, a climatologist at the Chinese Academy of Meteorological Sciences in Beijing.
The severity of the heatwave and drought also underscores the importance of improving climate models and studying multiple extreme events together to better assess their impact. “Compound events cause even more catastrophes,” says Wang Aihui, an atmospheric scientist at the Institute of Atmospheric Physics of the Chinese Academy of Sciences in Beijing. For example, the drought in the Yangtze River basin quickly worsened due to the combined effects of high temperatures and lack of rain.
heat and drought
From mid-June to late August, a heatwave swept across central, eastern and southern China – the longest-lasting and most intense since records began in 1961. Nearly a billion people experienced temperatures above 35°C, and 360 million felt Temperatures in excess of 40°C at some point during the heatwave.
And between July and August, the Yangtze River basin in southern China experienced its worst drought since 1972. The basin, which is home to nearly a third of China’s population, received up to 80% less rainfall than the 30-year average for that period. and temperatures were 2-4 degrees Celsius above average, Sun says. The drought was so extreme that the water level in China’s largest freshwater lake, Lake Poyang, dropped from 19 meters in June to 9 meters in late August, Wang Aihui said.
These extreme trends were punctuated by off-season wildfires and heavy regional rainfall that caused flash flooding. “It was a very unusual 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 hovering over the Yangtze River basin, rather than sitting farther east as a smaller pressure system typical of that region, says Wang Aihui . The system kept cold air from the north and moisture from the Indian Ocean from reaching the basin, she says.
Subtropical highs are usually punctuated by high mountain ranges like the Himalayas, but this year multiple systems are orbiting the entire globe at one latitude, which could also explain the heatwaves in Europe and the United States, says Wenju Cai, climate scientist at the Commonwealth Scientific and Industrial Research Organization in Melbourne, Australia.
The high also coincided with the unusual behavior of westerly winds blowing in mid-latitudes around the globe, bringing rain to northern China but mostly bypassing the south, says Wang Huijun, a climatologist at Nanjing University of Information Science and Technology in China .
Ocean surface temperatures also contributed to extreme events, such as the cooler waters in parts of the Pacific Ocean that led to the La Niña climate event. Meteorologists have suggested a third straight year of La Niña is underway, which combined with cooler sea surface temperatures in the Pacific Ocean usually brings less rain to the Yangtze River, says Wang Huijun.
The coincidence of several extreme events has opened the eyes of some researchers in China. More focus should be placed on studying intensifying climate events, says Wang Aihui, who plans to move away from analyzing individual climate extremes. To study complex events like droughts, researchers will increasingly rely on complex models that simulate Earth systems.
Sun also plans to study how overlapping events can amplify disasters, from heat waves and droughts to storms, heavy rains, hailstorms and cold snaps. “In the past we have 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 the background of global warming. China’s northern, arid regions are expected to become wetter and southern, wet regions drier as global temperatures rise1. Zhang Qiang, a hydrometeorologist at Beijing Normal University, says this year’s events so far are consistent with those forecasts.
Temperatures in China have risen by 0.26°C per decade since 1951 – more than the world average. Global warming’s role in recent events has yet to be analyzed, but in general, warmer temperatures increase evaporation, which can cause a more rapid transition to drought conditions and make them more intense, Cai says. Climate models suggest that droughts are getting worse across China and will become more frequent in certain regions2.