A new study of vegetation across New York City and densely populated surrounding areas has found that, on summer days, photosynthesis by trees and grasses absorbs a massive amount of the carbon dioxide emissions produced by vehicles and other greenhouse-emitting sources. This surprising discovery highlights the underappreciated importance of urban green spaces in the carbon cycle.
By using fine-grained vegetation maps, the scientists documented massive amounts of previously unrecognized greenery scattered in small spots even in highly developed areas, and found that they are performing a substantial role in the exchange of hazardous atmospheric gases. Since urban areas account for over 70 percent of human CO2 emissions – with New York City as the U.S.’s number one emitter, and third largest in the world – these findings are highly significant, showing that even individual street trees, little backyard gardens, overgrown vacant lots, and other small greenery can play an outsize role in carbon absorption.
“There is a lot more greenery than we thought, and that’s what drives our conclusion,” said study lead author Dandan Wei, a climate expert at Columbia University. “This tells us that the ecosystem matters in New York City, and if it matters here, it probably matters everywhere else.”
“Most people have assumed that New York City is just a grey box, that it’s biogenically dead. But just because there’s a concrete sidewalk somewhere doesn’t mean there’s not also a tree that’s shading it,” added co-author Roísín Commane, an atmospheric chemist at the same university.
The analysis revealed that tree canopies cover about 170 square kilometers of New York City (about 22 percent of its area), while grasses account for another 94 square kilometers (12 percent of the city’s area). By examining carbon emissions from June to August 2018, the researchers found that the total uptake from the city’s trees and grasses equaled up to 40 percent of a summer afternoon’s total emissions from all sources.
Unfortunately, in relatively chilly New York, carbon uptake occurs only during the local growing season, roughly from mid-April to mid-October. However, vegetation in cities with warmer climates most likely plays an even larger role in carbon uptake.
In future research, the scientists aim to characterize coverage by species, and thus figure out the relative benefits of different types of trees and grasses. “More trees are always going to be better, no matter what they are. But we could use an assessment of which ones are the best,” Wei concluded.
The study is published in the journal Environmental Research Letters.
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