Cities in the United States are getting hotter much faster than the rest of the country. Experts say that's a combination of climate change that's affecting the entire planet and the urban heat island effect that turns pavement and buildings into an urban cauldron. They warn that killer heat waves are likely to strike more in cities unless planners and residents take some simple steps to beat the heat.
"The situation is going to become rather dire in the lifetime of people making decisions today," said Brian Stone, associate professor of geography at Georgia Tech's Urban Climate Lab. "Cities are much less resilient to large scale warming trends. They don't have anywhere to dump the heat."
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Stone says that global climate change is making the heat island effect -- which has been studied for decades -- even worse. Especially during the night. In fact a paper published last month in the journal Natural Climate Change forecast that cities like Phoenix will heat up by an average of 7 degrees Fahrenheit by 2050.
Stone and others are pushing city leaders to plant more trees, use new highly-reflective building materials, and figure out ways to reduce the vast amount of waste heat that comes from building's air conditioning units, for example.
The fastest-baking cities in the United States are Louisville, Phoenix, Atlanta, Greensboro, N.C.; Detroit, Indianapolis, Las Vegas, Syracuse, N.Y.; Oklahoma City, Toledo, Ohio, and Portland, Ore., according to figures from Stone's lab. Louisville is warming so fast that "it's off the charts," he said, mainly because the city has no tree ordinance and hence almost no trees.
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Tree-planting programs are underway in the New York, Los Angeles and Denver, with the stated goal of getting more than 1 million trees in the ground. There are also projects to boost the acreage of urban gardens, wall gardens and grassy areas. But Stone says a more cost-effective method is to use reflective paint on building roofs. The paint can either be white, or a slate-colored material that has high reflectivity and high emissivity, or is able to emit heat that it absorbs back to the air.
"Painting a roof white is cheap, putting on simulated slates costs a lot more," Stone said. "But the materials will pay for themselves in a short amount of time."
Seattle and Austin are factoring in albedo, or reflectivity of surfaces, before granting building permits for homes and businesses.
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Conserving water also makes a difference. That's because water acts as "heat sink" and cools the city. Chicago has started a "Green Alleys" program that uses porous cement and other permeable surfaces for alley pavements. Rainwater soaks right into the ground and recharges the local water table, rather than running off into storm water or sewer systems.
J. Marshall Shepherd, director of the atmospheric sciences program at the University of Georgia modeled the heat island effect by either greening Atlanta with more trees or making it more reflective.
"The most significant reduction in the heat island effect came from a tripling of the reflectivity of the Atlanta area basically by painting the city white, and having white asphalt," Shepherd said. "Of course you just can't do that, you need a combination of strategies."
The glare of white road surfaces would be a hazard to drivers, he said.
Both Stone and Shepherd say more energy efficient air conditioning units in both homes and commercial buildings serve a two-fold purpose: they cut the amount of waste heat that contributes to the heat island effect, and they reduce the overall load of greenhouse gas emissions.