As the climate crisis progresses, the number of extremely hot days around the US could more than double, according to a peer-reviewed study and report from the Union of Concerned Scientists.

By mid-century, an average of 36 days a year could feel like 100F (37.7C) or hotter. Toward the end of the century, 54 days a year could feel that hot, researchers with the science advocacy group found.

The new data comes as an oppressive heatwave spreads across the eastern two-thirds of the United States, with heat warnings and advisories issued in nearly half of all US states. The heat index – or how hot people feel from air temperatures and moisture combined – will be as high as 110F (43C) in some places, according to the National Weather Service.

“We basically are looking at increases in the kinds of conditions we’re seeing this week across the country,” said Erika Spanger-Siegfried, lead climate analyst for the UCS.

The group’s report “shows a hotter future that’s hard to imagine today”, said Kristina Dahl, a co-author. “Nearly everywhere, people will experience more days of dangerous heat even in the next few decades.”

The data shows what would happen under current climate policies. Global reductions in the heat-trapping pollution from power plants, cars and other human activities would keep summers from becoming as hot.

For example, even slow action to cut emissions could help Washington DC avoid 11 days a year that feel hotter than 100F by the middle of the century. It could help the city avoid 32 days a year of heat that high by the end of the century.

Average global temperatures have already risen about 1.8F (1C) since industrialization, and research shows they are on track to rise as much as almost 6F (3.3C) by the end of the century.

Guardian Graphic | Source: Union of Concerned Scientists Guardian Graphic | Source: Union of Concerned Scientists

Without further action to reduce global heat-trapping pollution, parts of Florida and Texas could feel like 100F or hotter for a total of five months out of the year. Most of those days would surpass a heat index of 105F.

Some days could exceed the upper limit of the National Weather Service’s heat-index scale, which tops out at 127F. These “off-the-charts” days in the US now occur only in the Sonoran Desert, on the border of southern California and Arizona. By the end of the century more than one-third of the US population could experience at least a week of those days.

The south-east US and southern Great Plains could bear the brunt of the extreme heat, the study found.

Phil Duffy, a climate scientists and director of the Woods Hole Research Center, called this week’s heatwave “a sweltering demonstration of how growing carbon pollution in our atmosphere is pushing summer heat to dangerous extremes”.

In preparation for rising temperatures, the Red Cross Red Crescent Climate Centre this week launched a heatwave guide for cities.

Julie Arrighi, a lead author, said cities will need to create more green spaces, paint roofs white to reflect heat, and design buildings to stay cooler, as well as providing places for vulnerable people to cool off.

“Heat risk is on the rise,” Arrighi said. “The positive message is that we can prevent the deaths that are associated with [heat waves] with relatively simple, low-cost actions. We just need to take those actions and to be ready for that hotter future.”