Large swaths of Earth may soon be too hot for humans to handle.As early as mid-century, roughly a billion hectares of land about the land area of the United States or more could hit temperatures hazardous for human health, scientists estimate February 4 in Nature Reviews Earth & Environment. Thats triple the land area that currently can experience these health-threatening heat levels, says Radley Horton, a climate scientist at the Columbia Climate School in New York City.When global temperatures average 2 degrees Celsius above preindustrial levels, massive stretches of South Asia and South America, parts of West Africa and spots across the U.S. Southeast, may become so hot and humid that even young, healthy people face extreme danger, Horton says.That 2-degree threshold is not far from current global temps. In 2024, Earths temperature was already 1.6 degrees higher than the preindustrial average of 13.5 C. Its sobering to see how close we already are to the brink, he says.To play out possible future scenarios, Hortons team used global climate models and linked real-world temperature and humidity data with heat mortality risk. That let the researchers estimate how much warming it would take for a place to become life-threateningly dangerous or unsurvivable.In many tropical and subtropical locations around the world, hitting the 2-degree threshold means experiencing an extreme heat event at least once every 30 years. In such an event, most people would not be able to bring down their core body temperature, putting them at risk of organ damage or death.Additional warming would make conditions even more grim. In Earths hottest places, 4 or more degrees over the preindustrial average could prompt extreme heat events that warm people up so fast that their body temperature exceeds 42 C, which is lethal for more or less everybody, Horton says.In both scenarios, people over 65 fare far worse than younger people. Older adults tend to be more vulnerable to heat, so a 2- or 4-degree global temperature rise will make vast regions of the world blisteringly dangerous or deadly. The teams work offers just one look at a possible future, not a certainty. Adaptations like air conditioning can offer protection. Theres also hope for a different path if we dramatically cut the amount of greenhouse gas huffed into the atmosphere each year, Horton says. Its a very compelling argument for the urgency of emissions reduction as if we didnt already have enough arguments.