“It’s a win-win,” says Koetter, the sustainability director. “Doing a lot of green roofs would be the fastest way to impact our urban heat island.”

Louisville is already making some headway. Seven years ago, there were no green roofs in Louisville; today, there are 18, covering more than 3.3 acres. This is thanks, in part, to subsidies provided by Louisville’s Metropolitan Sewer District, which prizes the roofs because of their ability to reduce stormwater run off. The Louisville city government will soon begin to provide its own incentives, chipping in up to $10,000 to match the MSD’s grant.

But green roofs aren’t feasible on every type of building and they’re expensive. They’re heavier than a traditional roof, so the structure has to be able to support the additional weight and water. A 2008 study from the University of Michigan estimated a 21,000-square-foot green roof would cost $464,000 to install, versus $335,000 for a conventional one. But over its lifetime, the study’s authors calculated the green roof would ultimately save the building’s owner $200,000 through reduced energy needs.

“The people that do green roofs are often doing it maybe with less care for the cost,” Koetter says. “They accept and embrace that it’s going to cost a little bit more. But it’s the right thing to do.”

***

In every big city, there are “hot spots” beyond the central downtown district that contribute more than their fair share to the urban heat island effect. In Louisville, these include the city’s two airports and several giant manufacturing facilities.

Skip Miller says he can see and feel the effects of one of the city’s hot spots everyday on his way to work.

“I can tell you, it is almost predictable, about any time of year, that when you get near the Watterson Expressway, you can watch the thermometer in my truck go up,” he says. The expressway circles the older part of Louisville, which includes the urban core. Miller says he usually can see an immediate temperature difference of anywhere from one to four degrees.

Miller is director of the Louisville Regional Airport Authority and oversees operations at Louisville’s two airports: the larger international airport—Standiford Field—and Bowman Field, which is mainly used by private planes. Outside his office window at Standiford is the airfield—a vast expanse of concrete. The entire property covers 1,500 acres. Directly adjacent, is the UPS Worldport Hub, with a facility covering the equivalent of 90 football fields.

Miller says the airport already includes some features that reduce the heat island effect: The runways are lighter concrete—not black asphalt—and part of the terminal was re-roofed with lighter material for energy-saving reasons. But there are limits when working at an airport.

“By definition, airports are large, flat open spaces,” he says. “And I think by definition, any large flat open space is more of an attractant for heat than spaces that have topography associated with it or that have a lot of tree growth associated with it.”

Downtown Louisville, Kentucky. | Mark Peterson/Redux

But trees are problematic at airports. There’s a worry, for example, that too many trees will attract birds, which create a safety risk for airplanes. Any trees planted too close to the UPS cargo areas also run the possibility of harboring pests that could contaminate cargo.

And at the nearby Bowman Field, the airport authority actually needs to remove mature canopy trees. Miller says some have gotten too big, and will have to be trimmed or removed to comply with new, stringent federal safety standards. He’s waiting for an environmental assessment to determine how many trees will be affected, but says for each one removed, two smaller species will be planted in its place.

Two of the city’s largest employers—Ford and General Electric—have also taken steps to cool their campuses. At Ford’s Kentucky Truck Plant, the company is covering an expansion project with one million square feet of white roofing. A spokesman says this roofing material costs about 30 cents more per square foot than traditional black roofing. At General Electric’s Appliance Park, the company has turned 20 acres of parking lots into a park-like walking trail.

Much of Louisville's tree loss can be attributed to the city's extreme weather and ice storms. | Mark Peterson/Redux

Koetter says these efforts are important, since the heat emanating from these facilities can affect temperatures throughout the surrounding area. But ultimately, she says the greatest mitigation efforts will come elsewhere in the county.

“What we want to do is look at it holistically and come up with that big picture strategy to reduce the total contribution,” Koetter says. “The airport—we’re never going to [mitigate the urban heat island effect] on site, but we want to figure out how we can do it around the county to impact that contribution. It’s not necessarily about ‘This is a hotspot. We have to fix that hotspot.’ It’s the whole county’s issue.” And beyond.

That’s a concept not lost on Sarah Fosnight, the bartender-cum-citizen forester. Fosnight understands that planting trees in one part of the city can have positive aesthetic, environmental and health effects throughout the community. And long into the future.

“Ten years down the road, you can be like, ‘I planted that tree. And it looks awesome,’” she says. “It’s one of those things. You can volunteer three times a year and have a real impact on your city. A lasting impact.”

Correction: An earlier version of this article incorrectly reported the rate at which the disparity in temperature between Louisville’s city center and outer areas is growing. It is widening by 1.67 degrees each decade—not each year. The text has been corrected.