Cities Are Trapping More Heat, and Even Planting More Trees Won't Cool Them Down

They're planting trees in Louisville, Kentucky, and they're not for making baseball bats. They're for fighting back against the summer in one of the fastest growing "urban heat islands"  in the United States. But according to the researchers behind a study just published in Nature, if Louisville wants the plan to work, it's pretty much going to need to plant a downtown forest.

As the US Environmental Protection Agency explains “the term 'heat island' describes built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with one million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.”

Researchers from Yale University's School of Forestry and Environmental Studies found that the urban heat island effect was most pronounced in the warm and wet parts of the country. According to the study, it's because the smooth surfaces of the city don't release heat into the atmosphere, through a process known as convection, as efficiently as the rough surfaces of the forests and trees.

“In wet climates (such as warm damp southeastern US), we have lush forests,” Lei Zhao, a doctoral student and lead author of the study, told me. “Forests are efficient 'heat convertors' that dissipate heat away from the ground to the lower atmosphere. In comparison, cities in these climates are not as efficient in convecting heat—our estimate is that they are 58 percent less efficient than the natural landscape—and hence we have a strong urban heat island.”

In addition to the convection issue, the buildings and pavement in cities store more heat than trees and dirt, which especially contributes to the heat island effect at night. Other human activities like industry and air conditioning also don't help, but the big one is that the aerodynamic smoothness of urban areas doesn't create the turbulence that leads to convection like the vegetation-heavy natural areas of wet climates do.

Maria Koetter, Louisville’s director of sustainability, told WFPL that since the mayor took office, 10,000 trees have been planted in Louisville. In that same article though, she admits that no one seems to know how many trees are supposed to planted, or how many need to be planted to make a difference.

As Louisville is in the southeast, Zhao allowed that “planting trees could of course result in localized cooling spots due to shading effect and evaporative cooling,” but the plan to plant a lot of trees probably can't completely cool the city's heat island.

“According to our results, since the daytime urban heat island intensity is largely determined by the city background climate, therefore small-scale planting trees in the city, if not effectively altering the city’s average roughness, would not have a strong effect in mitigating the heat island,” he said. “Louisville might need to plant a large number of trees to mitigate the average heat island effect on the whole city scale.”

Perhaps the best way to cool a city, then, is to make the city shinier. “By using reflective construction materials for roofs, pavements, parking lots, you can increase the albedo (the proportion of sunlight or radiation reflected by the surface) of a city,” Zhao said. “A good case in point is Chicago. In response to the notorious 1995 heat wave (which killed hundreds of people), the City of Chicago has been promoting use of reflective roofs through its building code, resulting in a detectable city-wide increase in albedo (by ~0.02) over the period 1995-2009.”

A shadier and also shinier Louisville, then, would be the coolest.