San Antonio, Texas, which added more to its population than any other US city in the year to July 2021, has already had more than a dozen days over 100F this summer and hit 104F on Tuesday.

Phoenix in Arizona, second on the population growth rankings compiled by the US census, also hit 104F on Tuesday and has suffered a record number of heat-related deaths this year. Meanwhile, Fort Worth, Texas, third on the population growth list, has a “red flag” warning in place amid temperatures that have reached 109F this week.

Cities that stretch across the “sun belt” of the southern and south-western US have in recent years enjoyed population booms, with people lured by the promise of cheap yet expansive properties, fine weather and plentiful jobs, with several large corporations shifting their bases to states with low taxes and cheaper cost of living.

But this growth is now clashing with the reality of the climate emergency, with parts of the sun belt enduring the worst drought in over 1,000 years, record wildfires and punishing heat that is triggering a range of medical conditions, as well as excess deaths.