NYC’s Trees: A Natural Defense Against Heat, But Not Equally Shared

By John Upton, Climate Central, and Clarisa Diaz, for Science Friday

Prospect Park goers find shade under trees. Credit: Clarisa Diaz

Taina was sitting with a friend in the shade cast by a large tree in Prospect Park in Brooklyn, NY on a Saturday afternoon in late July. Throughout the city, it was another scorching day for a population that had endured one of the world’s deadliest COVID outbreaks just months earlier.

“The heat is way too unbearable to be doing anything else but sitting in the park in the shade and kind of just enjoying that fresh breeze and that shadow,” said Taina, who asked Science Friday to use only her first name. “I live in Bushwick, and it takes me a while to get all the way across Brooklyn.”

Brooklyn residents Taina and Cherise in Prospect Park. Credit: Clarisa Diaz

As pollution traps heat and pushes temperatures upward around the world, residents of its cities are experiencing the sharpest changes, with forests and other cooling landscapes replaced by pavement and other hard surfaces that absorb heat. But the heat is being felt unevenly, with the legacy of redlining and urban development keeping wealthier and whiter neighborhoods cooler than poorer and browner ones.

Luis Ortiz, a researcher at the New School’s Urban Systems Lab, used data obtained by a satellite passing over New York City several days before Science Friday spoke with Taina. It showed Bushwick, which is a working class industrial area on Brooklyn’s outskirts, in some places was more than 20°F warmer than Prospect Park and the tree-lined neighborhoods that surround it. The park is a nearly 600-acre behemoth of green space surrounded by a mixture of neighborhoods including the heavily gentrified Park Slope, where incomes and property prices are high. 

Plantings on the perimeter of Prospect Park. Credit: Clarisa Diaz

Preliminary findings from survey-based research involving some of Ortiz’s Urban Systems Lab colleagues indicate that park visitorship jumped this summer in New York City compared with before COVID triggered shutdowns. But not everybody could find luxuriant respite. The researchers also found that many New Yorkers didn’t feel they could travel safely to a park during a pandemic, perhaps because parks near their homes are run-down or nonexistent and reaching larger ones require multiple bus trips.

The U.S. Forest Service estimates that New York City is home to 7 million trees overall. Such trees help counter the urban heat island effect, in which cities are warming more rapidly than surrounding countryside.

“We live in a pre-war building and it kind of traps the heat in during the summer months,” Will, a seven-year resident of Brooklyn, told Science Friday as he worked on his laptop beneath a tree in Prospect Park. “If we can get out here, even when it’s hot, it feels nice to be out in the breeze and near the trees and in the shade and everything.”

Air temperature modeling showing Prospect Park is significantly cooler than surrounding areas on an average summer day. Credit: U.S. Forest Service

Average nighttime temperatures are rising more quickly than those during the day, making it harder for people to cool off at night and recover. And a slew of scientific papers published this year have been confirming that low-income neighborhoods throughout the U.S. lack tree canopy cover and are consistently hotter than those that house wealthier neighbors nearby.

“As the day heats up, the buildings and the sidewalks absorb heat and store it,” said David Nowak, a senior scientist at the U.S. Forest Service. In vegetated areas, by contrast, “trees are evaporating water and cooling the environment.”

Heat is America’s deadliest weather hazard, killing an estimated 12,000 every year. Seniors and those who can’t afford air conditioning are among those most at risk. Hot nights can be particularly insidious, because they rob people of sleep and deprive their bodies the opportunity to recover from heat experienced the previous day.

New York City heat trend. Credit: Climate Central

This year, large parks have provided neighbors with an opportunity to escape the heat while observing social distancing guidelines, while smaller parks can hold fewer visitors and are more likely to cram those who do visit together.

“The heat island effect tends to be greatest just after sunset when the sun goes down,” Nowak said. “The rural surface and natural surfaces tend to cool out faster than the urban surfaces. So you get the rural area cool off quicker than the urban area.”

Entering the core woodlands of Prospect Park. Credit: Clarisa Diaz

After Nowak led efforts to model summertime air temperatures across the city more than a decade ago, clear differences could be seen following the contours of its parks, water bodies and other green spaces.

You can see these pockets of cooler surface air temperatures across New York City,” Nowak said. “You get a huge cool island effect in the center of Staten Island. But also you can see pockets of cool around some of the bay areas. Also in Prospect Park, some other park areas such as Central Park, the surface temperatures tend to be cooler.”

A typical plant palette for native woodland restoration in Prospect Park. Illustrations by Clarisa Diaz with information provided by Prospect Park Alliance

It’s common for neighborhoods to be sharply divided in terms of the tree cover they contain, with those home to those with the lowest incomes faring the worst. These neighborhoods contain fewer trees and tend to be denser, allowing less air circulation. The problem is not unique to the U.S.—”This is a global phenomenon,” said Chandana Mitra, an associate professor at Auburn University in Alabama who has studied such patterns around the world.

U.S. Forest Service research forester Morgan Grove helped pioneer research into the distribution of canopy cover and heat in Baltimore. He worked on a study published this year comparing tree cover in neighborhoods that were “redlined” a century ago by federal policies, depriving mortgages and curtailing investments from areas home to Blacks and immigrant communities.

The study examined 37 cities and Grove said wealthier neighborhoods had more than twice the amount of canopy cover than did the formerly redlined areas. “That was true regardless of the age of the cities, or in terms of the climate where the cities were throughout the United States,” he said.

Archival map of Prospect Park in 1865. Credit: Brooklyn Public Library/Prospect Park Alliance

Reshaping an urban environment to usher in more greenery can be a challenging endeavor. With New York City built out and up over hundred of years into a remarkable metropolis, there aren’t many places left here where big new parks could be plonked down. The good news? Grove and other experts said small clusters of trees spread out through a city offer advantages over large parks when it comes to cooling as many residents as possible.

In ongoing research that has been presentedpublicly by Grove’s research partners but not peer reviewed or published, cooling benefits for surrounding neighborhoods were found to increase with the size of a park until it reached roughly the size of a football field, then the increase in benefits tapered off.

To translate it, it’s better to have lots of small air conditioners throughout the city than big air conditioners,” Grove said.

The finding didn’t surprise other experts interviewed for this story. It suggests abandoned or redeveloped land wield vast potential to help New Yorkers adapt to climate change by helping cool down their neighborhoods. Retrofitting cities with more pocket parks and trees comes with substantial added maintenance costs in addition to up-front capital costs. 

We have not come to terms with the real cost of what solutions for building resilience to climate change look like,” said Timon McPhearson, director of the Urban Systems Lab at the New School in New York City. “The fact that nature is going to cost money to invest in should not be surprising to us.”

John Jordan of Prospect Park Alliance inspects a native sugar maple. Credit: Clarisa Diaz

Individual trees in cities can cost thousands of dollars to purchase, plant and establish. A tree growing in a dense part of a city is harder and more expensive to keep alive than one growing in a forest within a sprawling park. Street trees can be particularly difficult to keep alive.

One of the most important things we can do is to plant trees to transform vacant lots into vegetated and small pocket parks, to take basically the availability of any space that’s either being redesigned or reimagined or is still available, and make those spaces green,” McPhearson said. “This is critical for climate resilience in low-income and minority neighborhoods.”

One of the richest cities in the world, New York City is home to initiatives to increase tree cover in neighborhoods where it presently is sparse and expand parkland. Since 2007, its parks department says it has planted more than 500,000 native trees and shrubs where canopies had been lost, damaged or jeopardized by age. New trees are being planted where they can shade seating and walking areas.

Still, the inequity in tree coverage is a stubborn one, locked in place by urban planners of the past. And amid the pandemic, trees and parks are falling victim to municipal cost-cutting, even as fiercening summers and social distancing rules highlight their importance. New York City’s parks department’s budget this fiscal year was set at $503 million—$84 million down from last year, preventing it from hiring seasonal maintenance and operations staff.

A lack of tree coverage in a neighborhood jeopardizes more than physical wellbeing. It also contributes to stress and anxiety. “There is a booming body of literature on the benefits of green space exposure for both physical and mental health,” said Alessandro Rigolon, an assistant professor in the University of Utah’s city planning department. 

Passing time under trees in Prospect Park. Credit: Clarisa Diaz

Yet the intended beneficiaries of new and improved parks can end up becoming their victims, leading to gentrification that displaces longtime residents from an area as it improves.

Rigolon said larger parks and open space projects, such as the revitalization of the Los Angeles River, tend to have particularly strong effects on gentrification. So, too, do parks built close to downtowns and linear parks like the Highline that can connect cyclists and pedestrians with different parts of a city. “We don’t know about the gentrifying effects of pocket parks yet,” he said.

Because of these gentrifying impacts from parks, Rigolon said that “when we do invest in those amenities,” it’s important to be “upfront in our efforts to do something about affordable housing, both in terms of maintaining existing affordable housing and building new.”

Rigolon said the disparity between rich and poor areas can be even greater when talking about the number of trees than the number of acres of parkland, partly a legacy of nonprofit and volunteer groups prioritizing street tree plantings and maintenance in wealthier and whiter neighborhoods.

Environmental nonprofits were mostly helping those more affluent communities, because that’s where the early environmentalists started from,” Rigolon said. “The momentum movement is fortunately evolving, and it’s evolved probably in the last 20 years to recognize issues of justice and equity.”

To maximize the likelihood that trees planted in neighborhoods can survive and provide cooling and other benefits for years and decades ahead, Rigolon said neighbors should be involved in decision-making alongside environmental nonprofits that are thoughtful about issues of equity.

If it’s top down from the city, it’s harder to get buy-in from those communities,” Rigolon said. “There’s a distrust for public institutions among communities that have been the object of systematic racism for generations.”

The Prospect Park Alliance, a nonprofit group that normally operates with an annual budget of $11 million (its estimated shortfall this year amid the pandemic is $3 million) has been improving the nearly 600-acre urban treasure since 1980s, following a decline in public spending that began during an economic crisis in the 1970s.

Core woodlands in Prospect Park. Credit: Clarisa Diaz

There was a lot of decline and neglect that happened in the city parks,” said alliance spokeswoman Deborah Kirschner. “The budget levels never went back to whatever they had been previously, and a lot of the big parks in the city do have private nonprofits that help raise funds and employ staff and provide resources.”

The alliance employs park workers and landscape architects, helps maintain the park’s natural areas, works on forest restoration projects and runs volunteer and education programs, enriching the neighborhood with a sweeping escape from its hot and congested buildings and streets.

I think everyone deserves access to a park and access to have that moment of peace and tranquility away from the city, the noise, the hustle and bustle, and just being around green,” said Liz, who was enjoying a picnic under a tree in Prospect Park on the hot summer day. “It’s like a natural antidepressant.”

SciFri · An Urban Forest In The Concrete Jungle

This story and radio segment were produced through a partnership between Science Friday and Climate Central, a nonadvocacy science and news group, with support from the Pulitzer Center on Crisis Reporting.