Just when you thought the environmental news couldn’t get any worse, a shocking new report warns Americans that they may be facing temperatures of 127 degrees Fahrenheit. By the year 2100, more than one-third of the U.S. population could be exposed to this kind of heat and its accompanying health risks.

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The county-by-county survey predicts that by mid-century, more than 250 American cities will suffer from at least one month of days where the heat index surpasses 100 degrees. Historically, this number has been 29 cities. The heat-index scale combines temperature with humidity to arrive at what the temperature feels like.

Related: Extreme heat wallops the US

“We were very surprised at how steeply and quickly the number of days of dangerous heat increased in such a short time,” said Kristina Dahl, senior climate scientist at Union of Concerned Scientists (UCS). Dahl co-authored the dire new report, “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days.”

Extreme heat will hit the Southeast and Southern Great Plains the hardest. Texas and southern Arizona will also be sweltering. But even temperate areas, such as Seattle, will notice a significant increase in hot days.

Humans, with a normal internal temperature of 98.6, function poorly in extreme heat. Climate researcher Tom Crowther said the increase in heat events “represents a terrifying prospect” and “a direct and immediate threat to human health and wellbeing.”

Droughts, floods and wildfires will accompany the extreme temperatures. Sea level rise and compromised food production will also make life tougher.

Humankind’s best hope? A fast and drastic reduction in carbon emissions. “Our past climate can no longer guide us,” said meteorologist Richard Rood. “We have to build and adapt to what’s coming.”

+ Union of Concerned Scientists

Via National Geographic

Image via Mark Hemmings