New York City's greenery absorbs staggering amounts of carbon emissions

New York City’s greenery absorbs staggering amounts of carbon emissions

Underrated vegetation in backyards and roadsides does a lot of work

A study of the vegetation in New York City and some of its densely populated neighborhoods found that photosynthesis in trees and grasses absorbed all the carbon emissions produced by cars, trucks and buses during many summer months, and then some. The surprising results, based on new hyperlocal vegetation maps, point to an underestimated importance of urban greenery in the carbon cycle.this study just published in the magazine Environmental Research Letters.

Using fine-grained vegetation maps, the researchers documented a large number of previously unrecognized green plants scattered in small dots even in highly developed regions, and found that they play an outsized role in atmospheric gas exchange. They reached their conclusions by modeling the carbon uptake of each patch of lawn and tree canopy, and studying data from instrument towers that continuously measure the amount of carbon dioxide in the air.

The finding is significant because urban areas account for more than 70 percent of human carbon dioxide emissions; New York City is the number one emitter in the United States and the third largest in the world.

The researchers studied carbon uptake by vegetation in and around parts of New York City. Green shows contiguous areas of forest, swamp, or grassland. The remainder was developed, with the highest intensity in purple areas, but a surprising amount of vegetation was also found along sidewalks, backyards and other small places. (Wei et al., Environmental Research Letters 2022)

Zooming in on several fully built blocks in the Prospect Heights neighborhood of Brooklyn, pink areas are buildings; purple are paved surfaces, including sidewalks and parking lots. In backyards and streets, darker greens indicate tree canopies; lighter greens for plants, shrubs, or grasses. (Wei et al., Environmental Research Letters 2022)

“There’s a lot more greenery here than we thought, which is why we came to the conclusion,” said the lead author Dandan WeiPostdoctoral Fellow, Columbia Climate Institute Lamont-Doherty Earth Observatory“What this tells us is that the ecosystem matters in New York City, and if it matters here, it probably matters elsewhere.”

Most previous studies have calculated carbon uptake by vegetation primarily by looking at contiguous expanses of forest and grassland, but these account for only about 10 percent of urban areas. Wei and her colleagues used the latest aerial radar images of New York City to map vegetation on an unprecedented 6-inch grid to include developed areas—the other 90 percent of the region that is missed in most models . Here, they were able to pick out individual street trees, small backyard gardens, overgrown clearings, and other small features. Outlying areas beyond the five administrative districts (approximately one-third of the 2,170 km2 study area) were subdivided into a 30-meter grid, which is still a relatively fine resolution.

“Most people think of New York City as just a gray box that’s biologically dead,” said Lamont-Doherty atmospheric chemist Roísín Commane, a co-author of the paper. “But just because there’s a concrete sidewalk somewhere doesn’t mean there isn’t a tree shading it.”

The researchers determined that tree canopies covered about 170 square kilometers of New York City, or about 22 percent of its area; grasslands covered another 94 square kilometers, or 12 percent. To see how greenery interacts with carbon emissions, they looked at the period from June to August 2018, when the metro area emitted about 14.7 million metric tons of carbon dioxide. The largest sources are the electric power industry and building energy; road transport accounts for about 1.2 million tonnes. Global average carbon dioxide concentrations are currently about 417 parts per million, but around New York they often reach 460 or higher, Commane said.

Levels are higher, if not all, in vegetation, especially in newly mapped developments; they account for nearly 85 percent of daily carbon uptake, according to the study. On many summer days, the total uptake is equivalent to 40% of total emissions from all sources in summer afternoons. The scientists found that carbon dioxide levels rose in the morning as traffic and other activity increased, and fell in the afternoon as grass and trees came to work.

Warning: Carbon uptake of course only happens during the local growing season, which in relatively cold New York is mid-April to mid-October. Vegetation in cities located in warmer climates may play a larger role in carbon sequestration, Wei said.

New York City is actively pushing to increase tree cover. One of the team’s next projects: describing coverage by species and helping to calculate the relative dominance of different species. Hardy, fast-growing oak trees are a common choice in the area, but studies have found that they also emit considerable amounts of isoprene, a volatile compound that reacts with vehicle exhaust to create polluting ozone. Another common gum that produces a similar amount of isoprene but has different growth characteristics. “No matter what kind of tree it is, the more the better,” Wei said. “But we can assess which ones are best.”

The study was co-authored by Andrew Reinmann of the City University of New York and Luke Schiferl of Lamont-Doherty.