This story originally appeared on New Republic and is part of the Climate Desk collaboration.

A young, fit US soldier is marching in a Middle Eastern desert, under a blazing summer sun. He’s wearing insulated clothing and lugging more than 100 pounds of gear, and thus sweating profusely as his body attempts to regulate the heat. But it’s 108 degrees out and humid, too much for him bear. The brain is one of the first organs affected by heat, so his judgment becomes impaired; he does not recognize the severity of his situation. Just as his organs begin to fail, he passes out. His internal temperature is in excess of 106 degrees when he dies.

An elderly woman with cardiovascular disease is sitting alone in her Chicago apartment on the second day of a massive heatwave. She has an air conditioner, but she’s on a fixed income and can’t afford to turn it on again—or maybe it broke and she can’t afford to fix it. Either way, she attempts to sleep through the heat again, and her core temperature rises. To cool off, her body’s response is to work the heart harder, pumping more blood to her skin. But the strain on her heart is too much; it triggers cardiac arrest, and she dies.

Such scenarios could surely happen today, if they haven’t already. But as the world warms due to climate change, they’ll become all too common in just a few decades—and that’s according to modest projections.

This is not meant to scare you quite like this month’s cover story in New York magazine, “The Uninhabitable Earth.” That story was both a sensation and quite literally sensational, attracting more than two million readers with its depiction of “where the planet is heading absent aggressive action.” In this future world, humans in many places won’t be able to adapt to rising temperatures. “In the jungles of Costa Rica, where humidity routinely tops 90 percent, simply moving around outside when it’s over 105 degrees Fahrenheit would be lethal. And the effect would be fast: Within a few hours, a human body would be cooked to death from both inside and out,” David Wallace-Wells writes. “[H]eat stress in New York City would exceed that of present-day Bahrain, one of the planet’s hottest spots, and the temperature in Bahrain ‘would induce hyperthermia in even sleeping humans.’”

These scenarios are supported by the science. “For heat waves, our options are now between bad or terrible,” Camilo Mora, a geography professor at University of Hawaii at Manoa, told CNN last month. Mora was the lead author of a recent study, published in the journal Nature, showing that deadly heat days are expected to increase across the world. Around 30 percent of the world’s population today is exposed to so-called “lethal heat” conditions for at least 20 days a year. If we don’t reduce fossil-fuel emissions, the percentage will skyrocket to 74 percent by the year 2100. Put another way, by the end of the century nearly three-quarters of the Earth’s population will face a high risk of dying from heat exposure for more than three weeks every year.

Even the best-case scenario shows that nearly half of humanity will be exposed regularly to deadly heat by the year 2100.

This is the worst-case scenario. Even the study’s best-case scenario—a drastic reduction in greenhouse gases across the world—shows that 48 percent of humanity will be exposed regularly to deadly heat by the year 2100. That’s because even small increases in temperature can have a devastating impact. A study published in Science Advances in June, for instance, found that an increase of less than one degree Fahrenheit in India between 1960 and 2009 increased the probability of mass heat-related deaths by nearly 150 percent.

And make no mistake: Temperatures are rising, in multiple ways. “We’ve got a new normal,” said Howard Frumkin, a professor at the School of Public Health at the University of Washington. “I think all of the studies of trends to date show that we’re having more extreme heat, and we’ve having higher average temperatures. Superimposed on that, we’re seeing more short-term periods of extreme heat. Those are two different trends, and they’re both moving in the wrong direction.” Based on those trends, the US Global Change Research Program predicts “an increase of thousands to tens of thousands of premature heat-related deaths in the summer ... each year as a result of climate change by the end of the century.” And that’s along with the deaths we’ve already seen: In 2015, Scientific American noted that nine out of the ten deadliest heat waves ever have occurred since 2000; together, they’ve killed 128,885 people.