This summer, as record-breaking heat sweeps across Europe and the United States, millions of people are searching for a way to cool off. They crowd into public pools, and libraries turned into cooling centers. They run their air conditioners around the clock—if they have them. They flee to the mountains or the coast for relief, only to find that many of those traditional escapes are under heat advisories.

For many, the very real dangers of extreme heat (early analyses of Europe’s June heatwave estimate a death toll of at least 20,000) will pass with the season. But for millions of others, heat is not just something to survive for a few weeks. It has become a reason to leave home altogether.

Scientists have identified a “human climate niche”—the range of rainfall, humidity, and temperatures in which human societies have thrived for thousands of years. Climate change is rapidly shrinking that niche. 

By 2070, up to one-third of humanity could be living outside these conditions most suitable for human survival if greenhouse gas emissions continue unabated, according to a study published in Proceedings of the National Academy of Sciences. Extreme heat is central to that shift. Scientists have long warned that sustained wet-bulb temperatures of 35°C (95°F) or more—when heat and humidity combine to prevent the human body from cooling itself—are not survivable. Several cities have already breached this threshold, and emerging evidence suggests the body’s breaking point may be even lower. Long before entire regions become physiologically uninhabitable, however, they become economically, socially, and practically impossible to live in.

People rarely move because of heat alone. They move because heat makes it too costly to stay. Whether someone can remain in place increasingly depends on whether they can afford to adapt. Extreme heat is not just a climate hazard, it is a........

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