Heat waves are America's deadliest weather disaster. They don't have to be.

Heat waves are quickly becoming one of the world's deadliest weather phenomena. In the United States, extreme heat now kills more people each year than tornadoes, hurricanes, or flooding. And a massive heat wave, like the one that hit Europe in 2003, can kill tens of thousands in a blow.

Some studies project that heat wave deaths could rise to 27,000 per year by mid-century

What's more, without big changes, scientists are predicting that heat waves will get far more lethal as global warming unfolds. One study projects that US heat-wave deaths could rise to 3,500 per year or more by mid-century if the planet keeps heating up.

One reason for that? More and more people are moving to cities. And cities are heating up even faster than surrounding rural areas due to the "urban heat island" effect. All that concrete and asphalt tends to absorb additional heat. On top of that, as cities get hotter, more people use air conditioning — and the waste heat from that A/C can warm a city up a couple of extra degrees, making things even hotter for people who don't have it.

Heat waves can can kill in a variety of ways. When it's especially hot and humid, people can't shed heat quickly enough through sweating and their core temperature starts rising, leading to heat stroke. (That's why hot nights are a big contributor to heat deaths, because people can't cool down.) But extreme heat can also exacerbate other health conditions — which is why the elderly and sick are most at risk.

Fortunately, we don't have to just sit by idly and die. An interesting recent study in PLOS One by researchers at Georgia Tech found that there were a few relatively straightforward tweaks that cities could make to reduce heat wave deaths by up to 60 percent. Here's a rundown:

How to cool cities down — and prevent heat-wave deaths

Chill out. (Photo by Brett Deering/Getty Images)

Broadly speaking, there are two major things that can help lower the overall temperature of a city:

1) Adding more trees and green space: Planting more vegetation throughout a city can have a couple of major effects. Trees can offer more shade, obviously. But trees, shrubs, and grass all increase the rate of evapotranspiration, which cools the surrounding air.

The big trick is actually finding more space for plants. The PLOS One paper notes a couple of different strategies that cities could employ — planting more grass or vegetation on the roofs of commercial buildings, converting grass on public land to tree cover, or making sure there are more trees surrounding major roads, especially downtown.

2) Increasing the reflectivity of rooftops, roads, and parking lots: The logic here is pretty straightforward. All the dark-colored roads, parking lots, and buildings in a city absorb a lot of heat (in the form of short-wave radiation) during the day.

One way to cool things off would be to make those surfaces more reflective — so that they absorbed less heat. There have been plenty of studies on the effects of "white roofs" and New York City is starting to build a bunch. (Note, though, that these roofs can have some unexpected side effects — such as heating surrounding buildings — so there's still lots of ongoing research here.)

Other researchers have been looking into ideas like covering parking lots with a reflective layer of paint. And so on.

3) Figuring out the right mix of the above strategies: The researchers then used modeling to look at three different cities — Philadelphia, Atlanta, and Phoenix — to see how big an effect each of these strategies would have.

Different cities would need a different mix of cooling strategies

Different cities would need to make different modifications on public and private lands to have the biggest impact on temperature and heat-wave deaths. In Philadelphia, for instance, a combination of increased tree cover over roadways and reflective roofs was most effective at reducing heat-wave deaths. In water-scarce Phoenix, meanwhile, reflective pavement and roofs had the biggest effect.

The Georgia Tech researchers did manage to devise scenarios for each of these cities that, under their modeling, would reduce the number of projected heat-wave deaths compared to what was expected under significant global warming. In Atlanta, they came up with a scenario in which heat-wave deaths didn't increase at all between today and 2050:

(Stone et al 2014)

Now, there's a caveat here: It's not free to plant trees or install white roofs (the material for reflective surfaces can be more expensive — and they can increase heating costs in the winter). In future papers, the researchers will try to calculate which of these strategies are the most cost-effective.

The next step is to figure out which of these strategies are cost-effective

What's more, this paper only looked at three cities in the United States. Other metropolitan areas will undoubtedly need their own specific approaches to cut down on heat-wave deaths. Cities in water-scarce areas, for instance, may not be able to rely as much on increased tree cover.

The paper also doesn't look into other approaches to mitigating heat-wave deaths — such as public cooling centers that offer air conditioned space to those who don't have A/C, or better public-health infrastructure or monitoring to help those most affected by high heat. (For instance, researchers have noted that the 2003 heat wave in Europe, which killed 72,000 people, mostly affected the elderly with weak social networks and poor health care.)

It's also worth noting that slowing down the pace of global warming — say, by reducing greenhouse-gas emissions — would also reduce the number of heat wave deaths. Still, at this point, the world is locked in to some amount of future warming no matter what, and many of these strategies would likely make sense no matter what we do on emissions.

Further reading: Heat waves have replaced drought as the world's deadliest climate disaster.

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