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More than 20,000 high-temperature records have been broken so far this year, across the United States. And the heat has been especially bad in cities. A recent study shows that urban areas are heating up about twice as fast as the rest of the country. Higher temperatures increase health risks - everything from asthma to allergies. But a researcher in Atlanta also sees this urban heat wave as an opportunity to do something about our warming planet. NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: Ebenezer Baptist Church is arguably the most famous place in Atlanta - Martin Luther King's church, and the heart of the civil rights movement. And now, it's playing an unexpected role in a new movement: the struggle against rapidly rising urban temperatures. Cities are, literally, global hot spots.

BRIAN STONE: We'll go ahead and cut through here.

HARRIS: To tell this story, Brian Stone from Georgia Tech leads us into a huge green space - two full, urban blocks - which Ebenezer Baptist leases out as a community garden. [POST-BROADCAST CORRECTION: The land is adjacent to Ebenezer Baptist Church, but it is leased out by Wheat Street Baptist Church.]

STONE: The impetus for this was to have local food production, as we see in a lot of communities. But the centralization of this - literally, less than a mile from the center of downtown Atlanta - is unique.

HARRIS: And it's not simply a garden. It's a summer camp, at the moment - and really, an urban farm. Amakiasu Ford-Howze greets us as we invite ourselves in.

AMAKIASU FORD-HOWZE: Right now, we have okra; we have onions; we have garlic; we have tomatoes; we have some arugula; we have peppers.

HARRIS: And it's an island of calm that actually helps offset some of the heat that builds up so dramatically, in the concrete all around us - not to say it's actually cool on a hot summer day.

FORD-HOWZE: You know, it feels good here. People come here and they go, ah - you know. It's just - because you do see green, and you see the flowers. We have a lot of flowers. It just feels good. So it helps.

(LAUGHTER)

HARRIS: Brian Stone says it actually accomplishes much more than meets the eye. Open, vegetated space - like this - helps water evaporate throughout the day. And evaporating water carries away heat - like sweat. It's nature's air-conditioning, but we've managed to interrupt that process in cities. It's called the urban heat island effect, and it's adding to our warming woes.

STONE: In addition to having increasing concentrations of greenhouse gases globally - that are driving warming at the global scale - at the scale of cities, we have lots of changes in land use that are also contributing to rising temperatures.

HARRIS: Cutting down trees is a big one. Pavement also stores heat during the day, and makes cities hotter at night. And as cities heat up, air-conditioners run harder, and their exhaust heat also pushes up the temperature. It all adds up - a lot, according to Stone's research on American cities.

STONE: Not only are most cities heating up more rapidly than the planet, they tend to be heating up at double the rate.

HARRIS: And if this trend continues - if the planet heats up four degrees, in the coming decades - cities will heat up a blistering eight degrees.

STONE: That raises some significant public health issues; significant infrastructure issues within cities. This isn't just a problem for a hundred years from now. This is a problem today.

HARRIS: But Stone isn't all gloom and doom because there are ways to ease this. His own lifestyle is at the extreme end of what you can do: solar panels on his roof, an electric scooter for commuting. His car is powered by biodiesel. And he drives with the windows open - no air-conditioning - which is especially noticeable when we pull up next to an accelerating bus.

(SOUNDBITE OF BUS ENGINE)

HARRIS: It's like sitting next to a space heater here, driving down the middle of the road in the summertime.

Stone says yep - something like 80 percent of the fuel that vehicles burn, simply turns to heat; another important element of the urban heat island effect.

STONE: In some cities, it can be 20, 25 percent of the total heat load is from engines.

HARRIS: But Atlantans don't have to be as dedicated as Brian Stone is, to make a difference. We pull up to a beautiful new park behind one of the city's defining avenues.

STONE: You might say Ponce de Leon in another part of the country, but we call it Ponce de Lee-on.

HARRIS: Stone walks past grassy areas, toward a large pond with a fountain in the middle - a far cry from Ponce de Leon's fabled fountain of youth, but still beneficial.

STONE: This is a good example of what cities should be doing, to deal with climate change at the urban scale - is not just think about energy efficiency, which we need to think about; but we also need to be bringing in this green space and even more surface amenities, like this pond here, within the city itself - which will directly contribute to cooling benefits for the neighborhood.

HARRIS: Water evaporates from the pond, and cools the area. That's not why the city built it. It's part flood control, part neighborhood beautification, part recreation site. But Stone says if you plan with just a little care, these sorts of amenities can also counteract urban warming. Trees can help more than anything else; both by providing shade, and by evaporating water through their leaves. But Stone has no illusions that this will be a simple task.

STONE: We will need to plant millions of trees around Atlanta to really - measurably - reduce temperatures around the city. And that's a tremendous challenge. But it's - what's advantageous about that is, that's fully within the control of the city itself.

HARRIS: You don't need an international agreement on climate change, or even consensus that humans are causing climate change. And if you can reduce urban heat, you can reduce demand for air-conditioning - which, in turn, lowers energy demand and greenhouse gas emissions. So cities can help themselves and the globe, at the same time. Tackling this problem requires some creativity.

(SOUNDBITE OF SHEEP)

HARRIS: Across town in a 58-acre swatch of urban forest, the challenge isn't simply to plant trees, but to preserve the ones that are already here. This is where a flock of sheep and goats comes in.

GREG LEVINE: The sheep out here are eating kudzu right now. And of course, kudzu is the most invasive plant in the Southeast.

HARRIS: Greg Levine, from a nonprofit called Trees Atlanta, explains that the kudzu vines can climb trees and kill them. So his organization has hired an urban shepherd to fight back. It's an uphill battle not only against kudzu but with developers who, in boom times, chop down 50 acres of trees a day around Atlanta, according to the U.S. Forest Service.

LEVINE: We're one of the most forested states in the country. We have beautiful forests. But we also are known for having the most tree removal in the country as well.

(SOUNDBITE OF SHEEP)

HARRIS: A thick canopy of trees can easily drop air temperature by 20 or 30 degrees, compared with a paved parking lot, Levine says. His small organization can manage to plant 3,000 trees, and 3,000 saplings, a year. That's far short of the millions that urban planner Brian Stone says the city could ultimately use. But it's a start.

(SOUNDBITE OF PEOPLE CHATTING)

UNIDENTIFIED MAN #1: They were follow - they were actually running after us, as we drove over here.

UNIDENTIFIED MAN #2: Like - like the ice cream truck.

HARRIS: Neighbors wandered down to see the sheeps and goats, which are guarded by dogs and corralled by a portable electric fence - turned off, at the moment. Levine grabs a doughnut that they've laid out for the gathering, and tries a gentle fundraising pitch.

LEVINE: All the sheep are adoptable, everybody. (Clapping hands) Pick out a sheep - donation for Trees Atlanta.

HARRIS: Here, in the humid grips of a midsummer Atlanta day, the expression "think global, act local" doesn't seem quite so worn.

Richard Harris, NPR News.

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