The study from Columbia University used aerial radar imagery to map the city and locate small patches of green space in densely urban neighbourhoods—including backyards, individual street trees, and even the shrubbery growing in vacant lots. By doing so down to six-inch spaces, the researchers were able to account for greenery beyond traditional green spaces such as parks.

“Most people have assumed that New York City is just a grey box,” Roísín Commane, a Columbia University researcher and coauthor of 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 modelled the carbon uptake of every bit of this green space and integrating data from instrument towers throughout the city that measure the air’s carbon dioxide content on a continuous basis. Their findings suggest those previously unrecognised areas of green space, in combination with other contiguous tracts of forest and grassland such as the famous Central Park, photosynthesise sufficient carbon emissions to compensate those produced by cars, trucks and buses on summer days (when the process is easier and more efficient for vegetation).

Unfortunately, this level of carbon uptake can only occur during the summer months from mid-April to mid-October. This makes increasing overall vegetation cover (including trees, shrubs and grasses) an even greater priority to compensate for the lack of sequestration that occurs over winter.

The researchers determined that tree canopies cover some 170 square kilometres of New York’s metropolitan area, or about 22 percent of its area; grasses account for another 94 square kilometres, or 12 percent. The city is currently actively seeking to increase its vegetation cover and the team’s next project will be helping to figure out what tree species will provide greatest benefit in terms of sequestering carbon.

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