Four years ago, Louisville, Kentucky, awoke to an alarming urban trend. This forward-thinking southern city—recently ranked as America’s most livable by the U.S. Conference of Mayors—had landed at the top of a decidedly less welcome list: researchers at Georgia Tech’s Urban Climate Lab reported that Louisville had the most rapidly growing urban heat island in the United States.
Simply put, compared to its outlying rural areas, Louisville’s temperature was soaring. Driven by a variety of factors—few street trees, stagnant air masses, and miles of hot, impervious asphalt and rooftops—Louisville’s heat island was increasing by an average of 1.67°F per decade. That’s almost twice the rate of Phoenix, the next fastest growing urban heat island, and nearly seven times that of New York City.
Then there are trees: nature’s cooling machines. Meter for meter, trees are the most effective urban cooling tactic. Street trees shade buildings and roads—not to mention pedestrians trudging through the heat—and cool the air through evapotranspiration, generating significant energy savings. Research suggests that a mature tree provides about 3 tons of free cooling as well as a range of ecological benefits, from stormwater absorption to biodiversity.
“The most radical thing we could do is implement a landscape ordinance,” said Laurie Kerr, now the director of policy for the Urban Green Council in New York City and formerly the lead researcher for DDC’s work on cool city policies. Kerr pointed out in an interview that New York lacks comprehensive regulations for landscape design that could require a minimum amount of vegetative cover for a particular site.
Other cities, such as Washington, D.C., have implemented a “green area ratio” that sets standards for sustainable landscape design. In Seattle, meanwhile, a points system allows property owners to select from a menu of strategies including green roofs, tree planting, or permeable paving to satisfy the requirement.
Such broad-based strategies, however, haven’t been widely implemented due to a lack of basic research about their impacts and benefits. “In order to really move forward,” Kerr said, “we need to model a range of possible policies and look at the cost-benefit not only in terms of energy but in terms of health impacts.”
That approach is now playing out in Louisville, where Brian Stone and the Urban Climate Lab are working on an urban heat mitigation plan that will make neighborhood-level recommendations to reduce heat mortality through tree planting, white and green roofs, cool paving materials, and other strategies, targeting each one to make the most impact in a particular location. The effort is focused on identifying factors that Louisville agencies can control. “The city wanted us to actually model feasible, land-cover changes that are tied to policy,” Stone told me.–by Jeff Biles on Broken Sidewalk.
NewTerrain February 15, 2016.