What's the deadliest natural disaster? In the United States, it's heat. Between 1979 and 2003, heat waves killed at least 8,015 Americans, according to the Centers for Disease Control and Prevention. That's more than hurricanes, lightning, tornadoes, floods and earthquakes combined. And it's largely an urban problem—the bulk of those deaths occur in cities.

I've got to get out of this town. (Linda Davidson -- The Washington Post)

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.

"It's not surprising that cities are heating up more rapidly than surrounding areas," says Brian Stone of Georgia Tech, one of the paper's authors. "But the extent to which they're amplifying warming trends did come as a surprise."

Georgia Tech's Urban Climate Lab has a very useful resource page with data from cities across the country. Here's how the urban heat island effect has been slowly growing in D.C. over time:

Source: Georgia Tech Urban Climate Lab

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.

Yet many localities aren't prepared for this eventuality: Stone and his co-authors examined the climate plans of 50 major cities and found that only one-quarter of them even addressed this growing urban heat island effect.

In theory, there are steps that cities can take. More plants can cool a town down — both New York City and Los Angeles are trying to plant one million trees, for instance. Cities can also try to use more reflective material for their roofs and pavements, in order to reflect more sunlight rather than absorbing it. (Rooftop solar panels could help, as well.) Improved energy efficiency could reduce the amount of waste heat from buildings and factories. On average, Stone says, an aggressive strategy could cut the urban heat island effect in half, shaving 5°F to 7°F off temperatures on a hot summer afternoon for a large city.

So why aren't more cities doing this? Stone says that part of the reason is the single-minded focus on greenhouse gases when policymakers think about climate change. Some 500 U.S. cities now have plans to cut their carbon emissions. Yet even if greenhouse-gas emissions were zeroed out tomorrow, there's already enough extra carbon in the atmosphere that heat waves will extend their reach for years to come.

"If you’re worried about heat waves in the near term," Stone says, "reducing the heat island effect is the most important thing we can do."