Louisville, fastest-warming city in U.S., reaches for the brakes

Two years ago, the home of the Kentucky Derby, Kentucky Fried Chicken and the Louisville Slugger received an unwelcome distinction: fastest-warming heat island in the United States.

Researchers at the Georgia Institute of Technology found that since the 1960s, urban Louisville, Ky., saw its temperature rise above that of its surroundings at a rate greater than any other city in the country and more than double the warming rate of the planet as a whole.

This trend puts Louisville on track to receive another unwanted crown: most heat-related excess deaths in the United States. A 2012 report from the Natural Resources Defense Council found that years of rising temperatures due to climate change will lead to an additional 18,988 deaths by the end of the century (ClimateWire, July 9, 2012).

The reports made national news at the time, but the headlines were unwanted attention and a serious headache for Maria Koetter, the city's newly minted sustainability director.

"If we can't get it under control, we're going to be jeopardizing the quality of life for the people in our city," she said.

Hired in 2012 as a fulfillment of Mayor Greg Fischer's campaign promise, Koetter was tasked with implementing the city's sustainability plan. The heat island findings soon dominated her radar.

The concept of a heat island is neither new nor unique to Louisville. Urban areas around the world accumulate more heat than their rural surroundings as vegetation yields to sunlight-absorbing pavement and heat-trapping pollution.

However, Louisville faces some unique challenges. Located in the Ohio River Valley, the geography acts as a basin for urban and industrial pollution, trapping it over the city. The region is notorious for air quality problems, and urban emissions can drive up local temperatures.

Heat's up, trees are down


Another issue is the city's tree canopy. Advocates have long lamented the lack of shade in the metropolitan region, a problem that goes beyond aesthetics; moisture transportation from trees and other vegetation plays an important role in regulating air temperatures (ClimateWire, July 11, 2012).

Over time, foliage diminished as concrete and asphalt crept over the land. "We have a lot of impervious surface downtown," Koetter said. "We've got these five- and six-lane streets that hold a lot of heat."

As the climate warms, these problems are poised to get worse. Though the global average temperature may only rise by 2 to 3 degrees Celsius, urban temperatures could rise by 10 C or more.

As people migrate from rural areas to urban centers around the world, a larger share of the planet's population will be exposed to this accelerated warming, which can have devastating health effects, ranging from increased cardiovascular distress to worsening neurodegenerative diseases to kidney stones (ClimateWire, July 21).

Without any intervention, Louisville may suffer the worst of these problems. But with grant support and some private financing, this city is gearing up to fight back.

The city of Louisville last year launched comprehensive assessments of its tree canopy and its heat island.

Erin Thompson, urban forester for the city of Louisville, was hired in 2013 to help tackle the tree problem. Finding shade turned out to be a more complicated process than initially expected.

Aerial photographs don't show the age of trees and leave out details about siting, air quality and actual temperatures in the vicinity. Researchers are now probing property records and conducting on-site measurements to verify just how much tree cover the city has.

"We hope to have the results of that urban tree canopy assessment done by the end of the year," Thompson said.

The preliminary numbers showed that the Louisville downtown area has a tree cover of 8 percent, just over half of what foresters recommend for such regions.

Where to plant new ones?

Results like these can then help city officials craft informed policy around trees. The current dearth of tree cover is in part a consequence of poor regulations.

In particular, Louisville lacks a comprehensive tree ordinance to ensure there is no net loss of canopy, guidelines that most other major cities have in place. There are provisions in Louisville for protecting trees in new developments, but these rules overlook tree attrition from old age, damage and construction on existing properties.

Though another preliminary estimate found there are 1.2 million potential plantable sites in Louisville, the solution isn't simply to plant more oaks, maples, pines and cottonwoods all over the place.

Thompson said new plantings need to be staggered by age and species so no part of the city gets overwhelmed by just one type of tree. Arborists need to ensure there is adequate space for the tree to grow. The trees then need sufficient water and sunlight. With a limited budget, tree plantings also have to be ranked in order of where they will have the greatest impact.

Even as the tree canopy assessment proceeds, city officials and activists have rolled forward with tree plantings. "The recent statistics I've heard, since Mayor Fischer has been in office, 10,000 trees have been planted," said Chris Chandler, a restoration ecologist and arborist at Eco-Tech Consultants, a firm helping Louisville assess its tree canopy.

Still, challenges remain and some new ones have emerged, in part due to climate change.

The emerald ash borer is a major emerging threat to Louisville's 2.3 million ash trees. Warmer winters have allowed this insect to survive and thrive in greater numbers.

"It's wreaking havoc on our ash trees," Chandler said. "This is the first year you can really see landscape-level canopy declines. Now we're seeing huge mortality events."

The city is also limited in how aggressively it can respond. "Most of the places [to plant trees] are not on city-owned property. The city is not going to mitigate the urban heat island [only] on their property," Chandler said. "That's going to take everybody."

"The whole community is going to have to get engaged to fix our problem," echoed Henry Heuser, a local businessman who serves on the city's tree commission.

"We could use a lot more money from the city budget, but everybody wants more," he added. "I think when we get all of our ducks in a row, then it can be a funding priority."

Preparing a model plan for other cities

Getting a handle on Louisville's urban heat island is another important component of fighting warming and works in tandem with the tree canopy assessment, helping Louisville denizens more effectively deploy cooling strategies like tree planting along with cool roofs and reflective pavement (ClimateWire, July 10).

Brian Stone Jr., an associate professor of city and regional planning at Georgia Tech, is working to establish a base-line heat model for the city to track hot spots and how they will change over time.

An assessment like this requires measuring not just surface temperatures but air temperatures, which are more difficult to measure throughout the city. "We're doing that, but to do that you need to build a data set that captures all of the land cover throughout the city," Stone said. "Then you can identify the neighborhoods that have the greatest vulnerability."

Stone said the next step is to merge the heat model with data on health impacts to tease out specific medical vulnerabilities in different parts of the city. "This is the first study looking at specific health impacts and stand-alone heat management plans," he said, adding that he plans to deliver his results next spring.

The process is tedious, but the end result could make Louisville a climate change adaptation pioneer. "We're hoping this could be a model for other cities," Stone said.

While metropolises like New York, New Orleans and Los Angeles focus on rising sea levels and storm surges from a warming world, Louisville may rise as the first city in the Untied States with a comprehensive, lifesaving plan to mitigate heat and its health effects.

"The changes that we're seeing with climate change -- the weather patterns are going to be more erratic and unpredictable," Koetter said. "Taking action right now is within our realm of capability."

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