LONDON – While Pakistanis count the cost of one of the country’s worst recorded floods, heavy rain is hitting south-western China as the Texas city of Dallas recovers from a 10-inch deluge in a single day last month.

Each of these rain-fuelled disasters followed a heatwave, suggesting the regions have been swinging wildly between two contradictory extremes. But extreme heat and extreme rainfall are closely related – and being gassed-up by climate change, scientists say.

Sweltering spring temperatures in South Asia, topping 50 degrees Celsius, are likely to have warmed the Indian Ocean. That warm water would then have fuelled what the UN Secretary-General Antonio Guterres this week called “a monsoon on steroids” over Pakistan – dumping more than three times as much rain as the 30-year average for August and inundating a third of the country.

More than 1,100 people have been killed, crops are ruined, and homes destroyed, prompting urgent pleas for aid.

It will take weeks if not months to determine exactly how much of a role climate change may have played in this year’s floods, but scientists agree it is supercharging extremes.

Heatwaves are already more frequent and intense worldwide, increasing evaporation from both the land and the ocean. Because a warmer atmosphere can also hold more moisture, the water vapour builds until clouds eventually break and send down heavier rain.

“The same places can be expected to experience both flooding and drought in a hotter climate,” said climate scientist Deepti Singh at Washington State University.

Flash floods

The area around Dallas had been bone dry for three months, with more than half of Texas suffering extreme drought. Cotton crops withered in the fields. Ranchers were forced to kill off much of their cattle for lack of feed. Soils hardened and cracked, forming a parched checkerboard across the landscape – the perfect setup for flash flooding.

It eventually rained on Aug 21, dropping nearly 10 inches within 24 hours, but the ground was too hard to absorb the deluge, leaving much of the water to flow through the city.

Interstate traffic came to a halt. Flights were cancelled. And apartments in the historic area of Old East Dallas were swamped.

In a drought-stricken area, “the ground can almost act like concrete in an urban environment”, said climate scientist Liz Stephens at the University of Reading in Britain.

Unlike flooding that comes from rivers gradually overflowing their banks, flash floods are triggered by intense rain in a short period – usually less than six hours – giving little warning before the water swells into a raging torrent. In an urban population centre, they pose the most risk. But flash floods also often rip through desert canyons in Utah and Arizona, threatening hikers.