Made in the Shade

Urban greenery as a counterbalance to emissions in growing cities

In the middle of a rapidly growing city, the largest in a rapidly growing country, the powers that be in New York set aside 843 acres of land for the first landscaped public park in the U.S. An aerial view of today’s Manhattan shows a sharp distinction between the greens and blues of Central Park and the urban jungle surrounding it. It’s not the only green space in the country’s most densely populated major city, but it sure stands out. It’s one of the most famous parks in the world, a welcoming place to relax in the midst of a bustling city, and a testament to how urban greenery can improve the quality of life. While the original intent was to have a nice place to retreat in the heart of the city, as things continued to grow, that urban greenery took on an important role in New York’s climate future.

Surprisingly Efficient

New York City emits the most greenhouse gases of any city in the U.S. and third most in the world. But there are many summer days when its urban greenery absorbs more carbon dioxide than the traffic in the city produces, a study published in Environmental Research Letters found. It revealed that tree canopy covers 22% of the city’s area, and grasses cover another 12%.

“Most people have assumed that New York City is just a grey box, that it’s biogenically dead,” Roísín Commane, a Columbia Climate School Lamont-Doherty Earth Observatory atmospheric chemist who coauthored the paper, said in a statement. “But just because there’s a concrete sidewalk somewhere doesn’t mean there’s not also a tree that’s shading it.”

The researchers surveyed emissions from the summer of 2018 and found that oftentimes, the urban greenery would absorb as much as 40% of the city’s total emissions from all sources for the day.

While New York and other cities remain hot in the summertime, the shade and evapotranspiration that trees provide can keep temperatures between 2 and 9 degrees Fahrenheit lower than they would be without them, the EPA says.

A five-city EPA study of heat islands and urban greenery (which did not include New York) found that planting and maintaining trees is relatively cheap, costing between $15 and $65 per tree per year, and provides a return on investment of 1.5-3 times.

Urban Greenery Benefits

“This tells us that the ecosystem matters in New York City, and if it matters here, it probably matters everywhere else,” said Dandan Wei, lead author of the Lamont-Doherty paper.

With more than half of the world’s population living in urban areas, and that figure steadily rising, using urban greenery to curb emissions is hugely important. While cities are major sources of emissions — responsible for at least 40% and as much as 70% of the world’s total – their per capita emissions are much lower than the surrounding countryside.

Wei said there was more urban greenery in New York than the researchers realized, an important revelation because of all the benefits it provides. Trees and vegetation can reduce energy demand by cooling buildings naturally, especially when planted on the west side and cover windows and roofs. The decrease in emissions from this lower energy usage improves air quality. It also improves water quality by filtering rainwater, and reduces runoff. Shade from trees can also make street pavements last longer, again reducing emissions.

The USDA reports that urban trees in the contiguous U.S. sequester more than 700 million tons of carbon, 12.6% of the country’s emissions. Planting 100 million mature trees near residences would save $2 billion annually in energy costs, the agency reports.

All these benefits from urban greenery are a big reason why trees and vegetation should be part of community action plans for cities in the climate fight.

Equitable Distribution of Shade

A Nature Conservancy study found there’s capacity to increase New York City’s tree canopy coverage to 42% without major changes to the city’s infrastructure and land use. Initiatives such as Million Trees NYC and Forest For All NYC strive to increase that coverage.

Planting more trees and vegetation might not be the most important step for cities to take to reduce emissions, but it’s relatively easy and it does make a difference in the quality of life for residents. In addition to the climate benefits, urban greenery can accomplish one of the major goals of Central Park’s establishment: give poorer people more access to green spaces.

The benefits of urban greenery “do not reach everyone equitably,” Forest For All NYC says on its website. “The most heat vulnerable neighborhoods tend to be low-income communities or communities of color with fewer trees and shade.”

This phenomenon is not unique to New York, with major cities all over the world exhibiting a tendency to have more urban forestry in the wealthier parts of town. Given the positive effects urban greenery has on people’s mental and physical health by reducing stress and fostering childhood development, it’s wouldn’t be all that crazy to think of Central Park as the most productive space in the world’s wealthiest city.