Street trees, leafy backyards, and other urban greenery can absorb more carbon dioxide than is commonly thought. Even in a densely built-up city like New York City, urban vegetation swallows up to 40 percent of all CO2 emissions on summer days, as a study shows. The buffering effect of the photosynthesis of city plants is even sufficient to compensate for the entire CO2 emissions from city traffic on such days.
How much green is there in New York City? Most people probably think of Central Park with its forest and grass areas first. The park, which is 460 football pitches in size, is not a bad tip. Not only is it a recreational area for New Yorkers, it also purifies their air at the same time. Because in the course of photosynthesis, the vegetation in Central Park draws climate-damaging CO2 from the air and binds it in trunks and stalks. Despite its enormous size, Central Park is by no means all of the vegetation that New York has to offer.
A green card for New York
Researchers led by Dandan Wei from Columbia University in New York have now determined for the first time how much greenery is growing in the big city and how much CO2 it binds over the course of the day. For the inventory of New York’s vegetation, the team evaluated high-resolution aerial photographs of the city. It was even able to identify individual street trees, small backyard gardens and overgrown wasteland, which are otherwise not taken into account in most models. The researchers entered everything green that they identified into a vegetation map of the city districts and their surrounding areas.
To find out how greening is related to carbon dioxide emissions, Wei and her colleagues also analyzed how much CO2 was in the city air from June to August 2018. To do this, they used instrument towers that continuously measure the CO2 content of the air and thus also reveal fluctuations over the course of the day. Rising values are mainly due to human influences such as road traffic and falling values to the photosynthesis performance of the plants. With the help of a model, the scientists also determined how much CO2 the urban greenery in New York can bind.
Surprisingly many and “hardworking” plants
The result: There is much more greenery in New York than previously thought. Wei and her colleagues found that the treetops alone cover about 22 percent of the city’s area. Another twelve percent are grasses. Most previous studies assumed a total of just ten percent vegetation because they only included contiguous forest and grass areas such as Central Park. “Most people have assumed that New York City is just a gray box, that it is biogenetically dead,” says Wei’s colleague Roísín Commane. “But just because there’s a concrete sidewalk somewhere doesn’t mean there isn’t also a tree shading it.”
The urban vegetation apparently has a significant buffer effect. The scientists’ analyzes showed that trees, grasses and the like in New York absorb up to 40 percent of the CO2 emissions in the metropolis on some summer days. That’s enough to compensate for all of the city’s traffic, among other things. In the months of the study, this was responsible for 1.2 of a total of 14.7 million tons of carbon dioxide. The scientists also observed that the CO2 concentration increased in the morning with traffic and other activities and then decreased slightly in the afternoon. This is the time when vegetation photosynthesis is at its highest.
“This shows us that the ecosystem matters in New York City, and if it matters here, then probably everywhere else,” Wei said. She assumes that cities in warmer climate zones benefit even more from urban vegetation. Because unlike in the relatively cool New York, it is green there all year round and not just from mid-April to mid-October. In the next step, the research team wants to find out whether there are plant species that are particularly suitable as CO2 scavengers when greening cities.
Source: Columbia Climate School; Specialist article: Environmental Research Letters, doi: 10.1088/1748-9326/aca68f