How Trees Act As NYC’s “Natural Air Conditioning Units”

You may have noticed that it’s been a tad bit zesty outside this week—New York City has been under a heat advisory since Monday as record-breaking temperatures have made the city feel like the gooey, trash-filled center of a hot pocket (and it’s even worse in other parts of the country). If you have access to air conditioning or one of the city’s cooling centers, then you’ve likely planted your sweat-stained butt there. But the city has its own “natural air conditioning units” as well—and all you have to do is spend some time in nature to access them.

Throughout the city, tree canopy helps to cool down areas during heat waves such as this. As Curbed pointed out, the four coolest parts of the city in recent summers have all been areas neighboring major parks, including Brooklyn Heights (Brooklyn Bridge Park), Prospect Heights (Prospect Park), Concourse in the South Bronx (Joyce Kilmer, Macombs Dam, Franz Siegel, and Mullally parks), and the Upper West Side (Central Park and Riverside Park).

This phenomenon is perhaps most acute in Central Park, which is home to over 18,000 trees that help cool the surrounding areas by absorbing extreme heat and providing shade.

Peter Haupt, who has been a Tree Care Manager with the Central Park Conservancy for over a decade, explained to Gothamist that the cooling happens due to transpiration, a process in which trees absorb rainwater through their roots, and then release it into the air via their leaf surfaces. The release of this vapor lowers the temps around it.

“In conjunction with the shade that trees provide, in a densely populated urban area like NYC and Central Park, the temperatures inside the park can be as much as 10 degrees cooler on hot days than anywhere outside of the park,” he said.

Read More: The City Is ‘Absorbing The Heat And Making It Warmer’—Explaining The Urban Heat Island Effect

Haupt also referenced the Urban Heat Island Effect. “Outside of the park, you have a lot more paved surfaces and buildings that absorb sunlight through the day and slowly release it overnight and the next day, and that’s what essentially keeps densely populated urban areas warmer than surrounding areas—so inside the park the tree canopies reduce that affect quite significantly,” he added.

Haupt works with a team of six arborists who inspect, monitor, and maintain the trees and other plant life in the park.

“Depending on need, some days our arborists will literally scale trees to perform up-close, minimally invasive examinations and prunings,” said Arica VanBoxtel, a spokesperson for the Central Park Conservancy. “This requires extensive safety precautions, deliberate rope systems, and, of course, a tolerance for heights. Other days, team members can spend more time on the ground—diagnosing diseases and removing invasive plants, among other tasks.”

Of course, not all parts of the city are created equally. The 843 acres of green that is Central Park is one of the five largest parks in the city, just behind Pelham Bay Park in the Bronx (2,765 acres), Greenbelt in Staten Island (1,778 acres), Van Cortlandt Park in the Bronx (1,146 acres) and Flushing Meadows/Corona Park in Queens (898 acres). There are plenty of areas in the city that don’t have the proximity to parks and trees needed to produce this cooling effect.

“What we find is that different parts of the city heat up at different rates,” Dr. Lewis Ziska, a professor of Environmental Health Sciences at the Columbia University Irving Medical Center, told Gothamist. “The degree of heating, which can be as high as 7-10 degrees, really depends on the vegetation in the area. If you have an area that’s well-vegetated, if you’re living adjacent to Central Park for example, the chances are you’re not going to experience the kind of extreme heat that someone in the South Bronx is going to experience.”

This, he says, is a reflection of historical redlining segregation in NYC from the 1930s and 1940s. Redlined neighborhoods typically received less investment in greenery than wealthier, whiter neighborhoods.

“Those were the areas that became the most built-over, they have the most asphalt and buildings, the least amount of green space,” Ziska said. “What we find is that because of that, they retain heat to a greater extent. Because they retain heat to a great extent, from a climate change point of view, they are the ones most likely to be hit by extreme summer temperatures going forward.”

Access to clean air and outdoor activities is limited across the country for lower-income communities, as an incisive Times op-ed explained this week.

Neighborhoods with a majority of people in poverty have 25 percent less tree canopy on average than those with a minority of people in poverty, according to American Forests’ Tree Equity Score,” they write. “In the most extreme cases, wealthy areas have 65 percent more tree canopy than communities where nine out of 10 people live below the poverty line.”

As for what can be changed in the future, developers can look to invest more in greenery in diverse neighborhoods, though space may limit what can be changed; Ziska cited Detroit as an example of a city where, because of the economic downturn of the last decade and the increase in vacancies, there were opportunities to take different kinds of land and turn it back into forest, gardens, and greenery.

However, adding new parks is only one option for the city. There are other opportunities for increasing greenery by installing green roofs (starting a garden on a rooftop), adding reflective tape to roofs (a material that can reflect heat back), or changing blacktop to something less absorbent.

Most of all, Ziska advocates for communities investing in green spaces of any variety, because climate change will be an unrelenting disruptor of life moving forward.

“I can’t emphasize this enough: we all know there’s a heatwave on right now and most of us feel it, but for people who live in those areas without green space, they feel it several degrees warmer than you or I do,” he said. “I think this is a really important part of dealing with climate change that doesn’t get enough attention. It is really fundamental in terms of improving our abilities to cope with heat stress, especially for urban dwellers.”

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