With fewer trees and more concrete, Nashville is heating up. What’s the plan?

Humans seem to instinctively understand the problem of too much concrete and not enough trees. Typical science fiction portrays dystopias as grey scapes of massive buildings, while magical fairylands are almost always green with towering trees.

In a nutshell, this grey-green divide reflects the temperature, and often race and wealth, of communities.

Nashville contains both images. The built environment, however, has been rapidly expanding, smothering soils with impervious surfaces, removing trees and raising local temperatures in the process.

This type of heating has been uneven, but residents will soon know the location of the hottest spots in town, thanks to a new study that will get its data from citizens.

In August, ideally a hundred-plus volunteers will drive or ride bikes around the city with sensors to identify heat pockets, according to the National Oceanic and Atmospheric Administration, which is organizing the effort with local support.

“It’s this really wonderful opportunity to democratize science,” said Adelle Monteblanco, a heat researcher at Middle Tennessee State University who will help analyze the data.

Courtesy NOAA

The National Oceanic and Atmospheric Administration is organizing heat studies in more than a dozen cities across the U.S. this year.

The sensors read temperature, humidity, time and the volunteers’ locations every second. Afterwards, the data will be transformed into a heat map.

Nashville leaders plan to use the info to help prioritize efforts for future heat mitigation and adaptation, said Kendra Abkowitz, the city’s sustainability manager.

“We want to get as many people involved as possible because just participating in the event is a great educational opportunity, but then we also genuinely need them to collect the information that we need to come up with the heat maps,” said Abkowitz, and added that the city is still accepting volunteers.

Heat waves are the deadliest weather event 

Urban landscapes can be dangerous and even deadly during heat waves. The biggest threat is a blackout, which occurs when the electric grid becomes overstressed. But, some vulnerable groups, like construction workers and homeless persons, are more routinely exposed.

Risks are neighborhood dependent, and most people can feel the difference between boroughs or even blocks. Between bordering dense and rural areas, the temperature split can be upwards of 20 degrees.

This phenomenon is called the “urban heat island effect.”

There are four main causes of a heat island. Loss of natural land cover reduces evapotranspiration. Paving, streets, parking lots and buildings absorb solar heat and release it back into the air. Electricity, air conditioning and industries compound the problem by releasing waste heat. Then, there is the morphology aspect: tall buildings trap heat.

“These are all drivers of climate change, and this is what climate change looks like in cities,” said Brian Stone, a heat researcher and director of the Urban Climate Lab at Georgia Tech.

The heat threat is worsening because of Nashville’s growth and warming climate. The mean temperature of the city increased in every month of the year in NOAA’s latest 30-year climate normals.

Cities are unprepared

Most cities, including Nashville, do not have comprehensive heat management plans, Stone said.

At this time, Nashville’s mitigation approach has been more about education and encouragement rather than requiring developers to take specific action to reduce urban heat. For example, Nashville encourages the use of green infrastructure, like green roofs or permeable pavement, to meet stormwater regulations that require developers to infiltrate, evapotranspire or use the first inch of rainfall.

Experts say solutions need to be mandated, instead.

“Given the real estate demand in Nashville and the strong local economy, we think that you have a very strong hand and urgent need to ask more from developers,” Adam Freed, of Bloomberg Associates, told Nashville leaders during a panel on heat mitigation last year.

Green roofs and reflective paving lessen heat

Fortunately, there are very feasible solutions available now to lower temps, according to Stone.

“Cities have full control over it. They don’t need national coordination or international coordination. Cities can cool themselves down, Nashville can cool itself down, by quite a bit, if it wants to invest,” he said.

Courtesy CHUTTERSNAP/Unsplash

Green roofs can lower ambient temperatures and reduce building energy demand.

The key solution is planting trees to shade streets, sidewalks and parking lots, focusing on where people actually live.

Nashville is planting 500,000 trees by 2050. But, to reduce heat with those trees, there must be special engineering to pick the placement and species of trees, as well as maintenance.

“You have to know exactly the problem you’re trying to solve,” Stone said.

This effort should be focused on preventing people from getting sick, Stone added, instead of how to make our landscape prettier — though that is a side effect.

‘A generation of canopy loss’

Tree ordinances can help slow further warming, as developers routinely clear trees for construction. Nashville lost at least 13% of its tree canopy between 2008 and 2016. Metro Water Services is working on a new tree canopy study, currently estimated to be out next summer, but the rate of tree loss has likely increased during an unprecedented building boom in the last five years.

“We have experienced a generation of canopy loss,” said Jim Gregory, chair of the Nashville Tree Conservation Corps.

Tree loss is happening across the city, with exclusions for some wealthier neighborhoods able to fight against development. This can be seen on the Tree Equity Score, a map that compares canopy cover with factors like income, employment, race and temperature, measured as a neighborhood aggregation of surface reflectance with a Landsat satellite.

A census block in Belle Meade, for example, has 58% canopy, with 1% people of color, 4% people in poverty, and a temperature of 83 degrees. An East Nashville census block that contains Nissan Stadium has 8% canopy, with 65% people of color, 72% people in poverty, and a temperature of 91 degrees.

“The tree canopy does absolutely reflect equity and financial resources,” Monteblanco, the MTSU researcher, said. “There are these larger historical policies that have shaped which neighborhoods are cooler and which neighborhoods are hotter.”

Reducing urban heat also requires that cities change roofs and pavements to be reflective, permeable or green. Chicago requires green roofs on most major developments and incentivizes them for other projects, for example, and Los Angeles is using cool pavement.

The new study could help shape this work in Nashville.

“Nashville and cities across the globe have to prepare for hotter climates,” Monteblanco said. “There’s no way around it. If we want to safely live here, we have to prepare, and we have to have the data to understand which neighborhoods are facing the highest heat exposure.”