“There are a number of studies on health impacts that show that nighttime temperatures are particularly important,” said Ben Zaitchik, a professor in the department of Earth and planetary sciences at Johns Hopkins University who studies extreme heat. «Cumulative heat stress in the body can lead to all sorts of complications, and the body’s ability to relax overnight can be critical.»

Texas state climatologist John Nielsen-Gammon said temperatures during this heat wave in the state have threatened records at both ends of the spectrum, including highs and lows, in its urban centers.

“We have had a couple of stations, mostly urban, that have come close to having periods of time with record low temperatures: San Antonio and Houston,” he said.

Nielsen-Gammon said several areas, including Midland, San Angelo and Del Rio, have set records for average weekly temperatures, an indicator of the duration and severity of this heat wave, which was triggered by a prolonged stretch of high pressure.

Kristie Ebi, a professor at the University of Washington Center for Global Health and Environment who specializes in heat and climate change research, said deaths during heat waves typically start after 24 hours, as Stress builds up in the body.

«It takes a while for our heart to warm up before we see something like a heart attack,» he said. “We have behavioral mechanisms: it’s hot, we try to find a place to cool off. We have physiological mechanisms: sweating. There is a real effort to lower that core temperature on the behavioral and physiological side.»

Ebi said the high nighttime temperatures and protracted nature of the Texas heat wave are particularly troubling.

The cost of heat is often underestimated in part because its cumulative stress can exacerbate underlying health conditions. Once a heat wave is complete, researchers will compare death data to previous years, control for other factors, and then count the number of «excess deaths» — people who otherwise wouldn’t have died if it weren’t for the temperatures. extreme.

“A very small percentage of death certificates during a heat wave indicate that ‘heat’ is an underlying cause,” Ebi, who studies heat deaths, adds that about half of excess deaths, on average, are due to cardiovascular disease.

Climate change is causing temperatures to rise in Texas. Average daily minimum temperatures they have increased from 51.9 F in 1970 to 54.2 F in 2020, a change of about 2.3 F, which is roughly in line with the general rate of warming in the state.

“Everything has been going up at about the same rate: daily lows, daily highs, winter versus summer,” Nielsen-Gammon said. “We are now 2 degrees above the 20th century average in all seasons.”