The weather in some of the world’s most densely populated cities is swinging from droughts to floods and back again as rising temperatures play havoc with the global water cycle, a study commissioned by the charity WaterAid showed on Wednesday.

South and Southeast Asia face the strongest wetting trends, while Europe, the Middle East and North Africa are becoming ever drier, researchers found in a study of 42 years of weather data drawn from more than 100 of the world’s most populous cities.

“There will be winners and losers associated with climate change,” said Michael Singer of the Water Research Institute at Cardiff University, one of the authors of the study. “It’s already happening.”

China’s eastern city of Hangzhou and Indonesia’s capital of Jakarta topped the list of cities suffering from “climate whiplash”, or a rapid succession of prolonged floods and droughts, the study showed.

As much as 15% of the cities surveyed also faced the worst of both worlds, with extreme flood and drought risks rising at the same time, among them the Texan city of Dallas, the Chinese commercial hub of Shanghai and Baghdad, the capital of Iraq.

“You can’t just assume that every place can have a similar response to atmospheric warming,” added Singer. “It doesn’t care who you are, whether you’re wealthy or poor or you have great infrastructure or not.”

China’s coastal city of Hangzhou set a record with more than 60 days of extreme high temperatures last year, and was also hit by severe floods that forced tens of thousands to evacuate.

A fifth of the cities have seen a reversal in climate extremes, with the Sri Lankan capital of Colombo and the Indian financial hub of Mumbai “flipping” to become far wetter, while the Egyptian capital of Cairo and Hong Kong are steadily drier.

Many cities that built infrastructure either to maximise scarce water supplies or mitigate flood damage are now facing entirely different circumstances, and will need to invest to adapt, Singer warned.

The few experiencing favourable changes include the Japanese capital of Tokyo, London and China’s southern Guangzhou, which had significantly fewer wet and dry months over the period from 2002 to 2023 than in the two prior decades.