Dangerous heat predicted to hit 3 times more often in future

What's considered officially “dangerous heat” in coming decades will likely hit much of the world at least three times more often as climate change worsens, according to a new study.

In much of Earth's wealthy mid-latitudes, spiking temperatures and humidity that feel like 103 degrees (39.4 degrees Celsius) or higher -- now an occasional summer shock — statistically should happen 20 to 50 times a year by mid-century, said a study Monday in the journal Nature Climate Change.

By 2100, that brutal heat index may linger for most of the summer for places like the U.S. Southeast, the study's author said.

And it’s far worse for the sticky tropics. The study said a heat index considered “extremely dangerous” where the feels-like heat index exceeds 124 degrees (51 degrees Celsius) — now something that rarely happens — will likely strike a tropical belt that includes India one to four weeks a year by century's end.

“So that’s kind of the scary thing about this,” said study author Lucas Zeppetello, a Harvard climate scientist. “That’s something where potentially billions of people are going to be exposed to extremely dangerous levels of heat very regularly. So something that's gone from virtually never happening before will go to something that is happening every year.”

Zeppetello and colleagues used more than 1,000 computer simulations to look at the probabilities of two different levels of high heat -- heat indexes of 103 degrees (39.4 Celsius) and above 124 degrees (51 Celsius), which are dangerous and extremely dangerous thresholds according to the U.S. National Weather Service. They calculated for the years 2050 and 2100 and compared that to how often that heat happened each year across the world from 1979 to 1998.

The study found a three- to ten-fold increase in...