Magazine
A Need for Trees: How Central Park Cools the (Urban Heat) Island of Manhattan
Manhattan is defined by its matrix of busy streets, towering buildings, bustling crowds, and (of course!) Central Park. In the summer, however, it’s also defined by its staggering heat. When the island of Manhattan faces an extreme “urban heat island” effect, Central Park’s 18,000 trees and 843 acres of paths, lawns, and woodlands offer a much-needed reprieve.
The urban heat island refers to an area of a city that faces much higher temperatures than its surroundings—creating an “island” of extreme heat. More concrete and less tree cover exacerbate this difference in temperature, which can vary by up to 16 degrees Fahrenheit at the same time of day depending on location. This is because hard, impervious surfaces (like paved roadways and empty rooftops) absorb, magnify, and slowly release heat throughout the course of a day, creating a warming cycle on city streets.
The lush, cool, shaded landscapes underneath the trees at Sheep Meadow stand in stark contrast to the adjacent buildings and streets outside the Park.
Parks Have Their Perks
That’s where Central Park comes in. The Park’s trees, acting like large, leafy umbrellas, have a profound cooling effect on its visitors. As the summer sun shines down and heats up New York City, these trees create a literal buffer, called a tree canopy, that cools and shades our Park and the surrounding blocks, absorbing extreme heat before it can do significant damage.
How exactly does this work? Through a process called transpiration, trees absorb rainwater through their roots, which is transported to their leaves and released as water vapor. The release of this vapor, as well as the shade that a tree’s leafy branches create, lowers the temperature of the surrounding air.
Not surprisingly, the City’s four “coolest” areas in recent summers were near large parks, including Brooklyn Bridge Park, Prospect Park, and Central Park. These greenspaces are always an oasis for New Yorkers but especially this past year, which tied with 2016 as the hottest year on record. As the City endures another record-setting summer of high heat, Central Park’s tree canopy—and the Central Park Conservancy team that cares for it—remains integral to the health of New York City residents and wildlife.
This thermal photo, provided by Dr. Jeremy Hoffman of the Science Museum of Virginia, shows the impact that even one tree makes in cooling its surroundings. Brighter areas indicate hotter spots like sidewalks and roadways, and darker areas indicate cooler spots like tree cover and shrubbery.
A Changing Landscape
Dr. Jeremy Hoffman, Chief Scientist at the Science Museum of Virginia and a leading expert on climate change and the urban heat island effect, has led multiple nationwide studies tracking heat across neighborhood lines that show just how essential greenspace is to the health of a city.
According to Dr. Hoffman, Central Park’s trees act as the City’s “natural air conditioning units” and have a lasting effect on the temperature and air quality within and surrounding the Park. This cooling effect that parks provide to cities is especially important as the planet experiences record heat. “The urban heat island effect is separate from, but intimately linked to, climate change,” Dr. Hoffman explains.
Even outside of human-caused climate change, urban areas are still warmer than rural areas, and certain parts of cities remain warmer than other areas of the same city. “The added factor of global warming,” Dr. Hoffman adds, “basically amplifies the urban heat island effect, including the duration of these heat extremes, how intense they are, and who they most effect.”
As the temperatures rise in New York City, visitors find reprieve—and literally cooler temperatures—on the Park’s grounds. Surfaces like grass, shrubbery, and tree cover have a much more pronounced cooling effect than their concrete counterparts.
Redlines & Greenspaces
In fact, due to discriminatory, race-based policies like redlining—a historic practice in American cities that excluded many communities of color from receiving bank loans—summertime heat is experienced in vastly different ways today.
Redlined neighborhoods received less investment in green infrastructure than their white counterparts, and that disparity is now reflected in the density of the modern urban tree canopy and in park proximity. With this greenspace inequity comes greater exposure to air pollution, higher frequency of heat-related illnesses, and less preparation for the climate crisis.
These compounding factors reiterate the importance of a well-maintained tree canopy locally in the Park and more broadly in the City. “Maintaining and protecting the existing greenspaces that we have is one of the most important steps in mitigating the urban heat island effect,” says Dr. Hoffman.
Trees—like this hybrid elm in the Park—absorb rainwater through their roots, which is transported to their leaves and released as water vapor. The release of this vapor, as well as the shade that a tree’s leafy branches create, lowers the temperature of the surrounding air.
There's a Tree for That
As Central Park’s tree canopy works hard to keep us cool this summer, the Conservancy’s team of arborists works equally as hard to care for these trees. These professionals inspect, monitor, and maintain Central Park’s 18,000 trees to set them up for success, while also planting new trees to keep the Park's canopy growing.
Tree Care Manager Peter Haupt has been working with the Park’s trees for 10 years, building upon his breadth of knowledge about specific tree species and what factors most affect them. “The vast majority of the trees here in the Park are well-suited for urban conditions,” Haupt explains. “They are less sensitive to fluctuations in temperature and in weather, and many have evolved to tolerate extended periods of drought, as well as extensive wet periods.”
Conservancy arborists Peter Haupt and Will Vitagliano in their outdoor office, preparing to climb a tree for inspection.
This resilience bodes well not only for Central Park, but for all who enjoy—and depend upon—its environmental benefits. And as cities around the country face a shrinking tree canopy and an increasingly intense urban heat island effect, the public health implications of greenspaces like Central Park will only grow.
Luckily for New Yorkers, the Park’s 18,000 trees offer clean air, cooling shade, and a needed dose of calm to those who visit. Haupt and his team are happy to provide that space. “Taking care of the Park’s trees is very important to me, now more than ever,” he says. “I want to do my job successfully so that we can provide a respite and relief for everybody that wants to come and enjoy it.”
In the hot summer months, Park visitors naturally flock to areas underneath trees. These shaded spots are significantly cooler than other parts of the Park, showing just how huge an impact the tree canopy has.
A Need for Trees
As much as we need the Park’s trees to stay cool during our hot summers, the Park needs us to help protect them. While the Conservancy’s arborists handle tree maintenance and care, our Park visitors can lend a hand by treating all trees gently and with respect. This means never carving into their bark, removing their flowers, damaging their roots, or climbing their branches.
If the City is a heat island, the Park is an oasis. Stay safe from the heat this summer by following the guidance of public health professionals, keeping hydrated, and if possible, by enjoying the shade in Central Park. You’ll be sure to notice the difference that the tree canopy makes.
