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Louisville Needs More Trees to Curb Fast-Growing Urban Heat Island Effect, Researcher Says

Downtown Louisville residents live in one of the fastest growing urban heat islands in the U.S., according to preliminary results of an ongoing study.

A city’s urban heat island effect is defined as the temperature difference of rural areas compared to downtown areas.  That temperature difference in Louisville ranked highest among 50 of the country’s largest metropolitan areas, according to the study by Brian Stone, a professor of City and Regional Planning at Georgia Tech.

In a public presentation on Tuesday, Stone said the root cause of Louisville's excessive urban heat island is the lack of tree cover downtown.

The study found that Louisville has room to add about 1.2 million trees, though he added it isn’t feasible to plant that many trees, he said.

Louisville is working on growing the tree canopy in Louisville, but the process will take time, said Maria Koetter, Louisville’s director of sustainability.

“Since the mayor took office, in his term, we have planted about 10,000 trees in the community,” she said.

Despite that effort, the downtown area has just about an 8 percent tree canopy cover, she said.

Neither Stone or Koetter could say what the recommended number of trees would most effectively decrease the urban heat island effect while maintaining a high quality of life, but there is a consensus that there needs to be more.

“A big part of the issue is that Louisville has a very sparse tree canopy downtown and is one of the only large cities that doesn’t have a tree ordinance,” Stone said.

As WFPL previously reported, an expertbelieves the city could benefit from such an ordinance.

In addition to the need to plant more trees, Stone said his study found that different approaches to residential and commercial construction and upkeep could also lower the surrounding air and surface temperatures.

He said incorporating more reflective material and living material, such as garden rooftops, are good ways to combat excessive heat in downtown areas, which can often be 5-20 degrees warmer than surrounding rural areas.

Excessive heat can lead to a number of issues for urban dwellers— increased mortality rates, heightened utility costs, poorer air and water quality, and added stress to existing infrastructure, Stone said.

From 1979–2003, excessive heat exposure contributed to more than 8,000 premature deaths in the U.S., according to the Centers for Disease Control.

“Heat kills many more people than any other form of extreme weather,” he said.  “This is obviously a problem that is significant.”

He said for his study to have any impact, the entire community will have to take responsibility.

“The city cannot do this alone,” he said.  “It is a relatively slow moving problem, but it is something that needs to be worked on year after year from this point on.”

Stone said he is about five months into the 12-18 month study of Louisville’s urban heat island.  The $135,000 study is the first of its kind in the country, and Stone said it may be the first in the world.

The study was completely funded by private grants, Koetter said.

Here are slides from Stone's presentation:

Jacob Ryan is the managing editor of the Kentucky Center for Investigative reporting. He's an award-winning investigative reporter who joined LPM in 2014. Email Jacob at jryan@lpm.org.

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