Already-hot regions can expect even more heat in the future. The San Antonio area could expect just under four months of at least 90-degree weather on average in 1960. By the end of this century, such heat may last for an extra month in any given year. Likewise, Phoenix could go from five months of 90-degree heat in 1960 to six months by the end of the century.

Large relative increases in heat will be felt in more moderate climates, too. The Baltimore area, which averaged 18 days of at least 90-degree heat in 1960, could see these very hot days nearly triple by the end of the century.

Americans are likely to feel this extra heat even if the world takes action to reduce greenhouse gas emissions, according to the analysis conducted for The New York Times by Climate Impact Lab, a group of climate scientists, economists and data analysts from the Rhodium Group, the University of Chicago, Rutgers University and the University of California, Berkeley. If countries continue emitting at historically high rates, the future could look even hotter.

The future projections assume countries will curb greenhouse gas emissions roughly in line with the world’s original Paris Agreement pledges (although most countries do not appear on track to meet those pledges).

How different cities and regions experience an increase in 90-degree days depends in part on how well adapted to heat they already are, said Kelly McCusker, climate scientist at the Rhodium Group.

More frequent hot days will be less disruptive in Phoenix, where residents are already used to blistering temperatures, than in Seattle, where an estimated 60 percent of households don’t have air conditioning.

But how much hotter it gets matters, too. In Phoenix, more 90-degree days will most likely also mean more days in the high 90s and above. (This summer, temperatures in the city soared to 115 degrees.) In Seattle, such scorching heat will most likely remain rare, even as days in the low 90s become more common.

High temperatures have been found to increase the risk of illness and death, especially among older people, infants and people with chronic medical conditions. Lower-income populations, which more often lack access to air conditioning and other adaptive technologies, are also more likely to suffer the impacts of extreme heat. So are people of color.

Outdoor workers are particularly vulnerable to more frequent hot days, but excessive heat has consequences for indoor factory workers, too, especially in developing countries, where work spaces are less likely to be cooled.

An increase in 90-degree days will also be more painful in humid regions than in dry ones.

“A very important factor for how humans experience heat is how humid it is,” McCusker said. “If it’s also humid, humans can’t physiologically evaporate sweat as easily, and we can’t cool down our bodies effectively.”

Humidity can make even less-extreme temperatures dangerous.

High heat also affects food production (including lowering crop yields and dairy production) and can increase the demand for electricity. More frequent 90-degree days could also exacerbate drought and fire conditions in certain regions.

“More very hot days worldwide bring direct and dangerous impacts on people and the systems on which we depend,” said Cynthia Rosenzweig, head of Climate Impacts Group at the NASA Goddard Institute for Space Studies. “Food, water, energy, transportation and ecosystems will be affected both in cities and the country. High-temperature health effects will strike the most vulnerable.”

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This article originally appeared in The New York Times.

Nadja Popovich and Blacki Migliozzi © 2018 The New York Times