The Atlantic reports:

When the heat index—the temperature multiplied by humidity—reaches 80 degrees, the National Weather Service advises Americans to take caution. When it reaches 90, that advisory gets bumped to possibly dangerous; at 100, it’s likely so. At a heat index of 125 or above, the National Weather Service warns of “extreme danger” and describes its effect on the body concisely: “heat stroke highly likely.”

Until now, that kind of extreme heat has been limited to relatively small parts of the country. But that might not always be the case. According to a new heat model released yesterday by the First Street Foundation, a nonprofit organization that assesses future climate risk, more than 100 million Americans live in counties that are expected to experience at least one day with a heat index of 125 degrees or above in the next 30 years. That’s 13 times more than the 8 million people who are forecasted to experience such world-melting temperatures this year, the group notes. And it helps to illustrate just how much of the country will need to start preparing today for more regular periods of intense heat.

The nonprofit, which has previously modeled wildfire and flood risk, predicts that an “Extreme Heat Belt” will form along the Mississippi River, enveloping most of Louisiana, Mississippi, Arkansas, Missouri, Illinois, Iowa, and Indiana—as well as portions of Oklahoma, Texas, Kentucky, and Tennessee. [Continue reading…]