A predicted consequence of anthropogenic climate change is that the air won't cool as quickly overnight, leading to ever-increasing temperatures overall. It's happening, and it's dangerous:
Nationwide, summer nights have warmed at nearly twice the rate of days, with overnight low temperatures increasing 0.8°C per century since 1895, when national temperature records began, compared to a daytime high increase of 0.4°C per century. (Nights have warmed faster than days during other seasons, too.)
While warm summer nights may seem less concerning than scorching afternoons, “the combination of high daytime and high nighttime temperatures can be really lethal because the body doesn’t have a chance to cool down during the nighttime hours,” said Lara Cushing, professor of environmental epidemiology at San Francisco State University.
Those risks are higher in places where temperatures have historically been cooler, like coastal California. There people are less physiologically acclimated (the body can get used to higher temperatures up to a point) and less behaviorally adapted to hot weather.
“A hundred and five degrees in San Francisco is going to have a bigger impact probably than 105 degrees in Houston, Tex., where everybody has air conditioning and people are accustomed to dealing with high temperatures,” Dr. Cushing said.
Older people, the sick, and young children are especially at risk. So are agricultural, construction and other outdoor workers, who can no longer avoid the heat by shifting their hours to work earlier or later in the day. Similarly, homeless people who bear the full brunt of the elements get little relief.
Never mind the rest of the local environment. Slower overnight cooling puts stress on plants as well.