Why are cities so susceptible to heat waves? Well, in part because that’s where most people live (obviously). But there’s another factor, too: Cities tend to run much hotter than nearby rural areas. This is due to what’s known as the urban heat island effect. There are fewer trees and plants in the city to enable evaporation. Buildings and pavements absorb more warmth from the sun. And factories and automobiles give off waste heat. That all adds up. On a hot summer afternoon, a large city can easily run 5°F to 18°F hotter than surrounding rural areas, enough to turn an unpleasant heat wave into a deadly calamity.

And as global warming pushes up temperatures around the country, this urban heat island effect is only getting stronger. A new study in the journal Landscape and Research Planning finds that many large U.S. cities are warming twice as fast as the rest of the country. Between 1961 and 2010, rural areas in the United States heated up at a rate of roughly 0.29°F per decade. Yet three-quarters of the biggest U.S. metro areas were heating up at an average rate of 0.56°F per decade, thanks in part to increased sprawl. …

These sweltering cities can pose a problem for public health. Climate scientists say that if the planet keeps warming, we can expect more frequent and severe heat waves around the United States. Heat stress will get especially severe in cities, where the bulk of the population lives. What’s more, as demand for air conditioning in cities surges, that will put increased stress on electric grids, which are already buckling under this summer’s heat.