A new online tool explores the number of days per year in places worldwide when heat is likely to exceed a deadly threshold if nothing is done about climate change.

Deadly heat waves—already a risk for 30 percent of the world’s population—will spread around the globe, posing a danger for 74 percent of people on Earth by the end of this century if nothing is done to address climate change, according to a new study.

Nearly as alarming, the researchers project that even if greenhouse gases are aggressively reduced, at least 48 percent of the population will still face deadly heat waves by 2100 because of the amount of long-lived heat-trapping gases that already have accumulated in the atmosphere.

“We’re running out of good options for the future,” said lead author Camilo Mora, a biologist at the University of Hawaii at Manoa. “For heat waves, our options are now between bad or terrible.”

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