Trees in Central Los Angeles Absorb More Carbon Than Previously Thought: Study

Aerial view of Echo Park Lake in Los Angeles, California. simonkr / E+ / Getty Images

Trees in central Los Angeles absorb much more carbon dioxide than scientists thought. This means they are able to offset a surprising amount of the city’s fossil fuel emissions when the weather is warm and trees are most active.

In a recent study, researchers used densely spaced air-quality sensors to find that vegetation in parts of central LA offset 60 percent of the city’s carbon emissions, especially during the growing season, a press release from University of Southern California (USC) said.

The discovery that urban greenery plays a more substantial role in offsetting LA’s carbon footprint could offer insights to help other cities combat climate change.

“Urban areas are major contributors to greenhouse gas emissions, necessitating effective monitoring systems to evaluate mitigation strategies,” the authors of the study wrote. “A dense sensor network, such as the Berkeley Environmental Air-quality & CO2 Observation Network (BEACO2N), offers a unique opportunity to monitor urban emissions at high spatial resolution.”

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The first-of-its-kind study by Public Exchange and USC Dornsife College of Letters, Arts and Sciences was able to provide more detail than had been previously available by tracking emissions absorption in real time.

The measurements are some of the most detailed on how air quality is impacted by urban trees. Vegetation in the area absorbed as much as 60 percent of daytime carbon dioxide from fossil fuels during the spring and summer months and roughly 30 percent annually. This ranks LA as a city with one of the highest documented uptake rates of carbon dioxide.

In order to track LA’s carbon in real time, the team launched the Carbon Census array, which involved deployment of a dozen high-resolution BEACO₂N sensors over a 15-by-six-mile area of Mid-City.

The sensors mapped changing carbon concentrations in the air as it moved across the urban landscape. This enabled the researchers to take wind direction and speed, as well as urban density, into account to determine the extent to which local vegetation was offsetting emissions.

“You can think of emissions like passengers on a train,” said lead researcher Will Berelson, a USC Dornsife professor of Earth sciences, environmental studies and spatial sciences, in the press release. “As the wind moves pollution through the city, some gets picked up and some gets dropped off. These sensors let us see that process in real time.”

The study, conducted from July of 2021 to December of the following year, measured carbon dioxide directly, unlike other models that estimate carbon levels based on traffic data, fuel sales and other models that depend on carbon landing on individual sensors.

“One of the study’s biggest surprises was that trees absorb the most CO₂ during summer, despite it being L.A.’s driest season. Satellite imagery shows L.A.’s urban greenery is remarkably verdant in summer, likely due to irrigation, groundwater access from leaky pipes and resilient tree species,” the press release said. “Still, trees can’t keep pace with emissions. As expected, CO₂ levels spiked during rush hour, reinforcing the fact that, while greenery helps, it can’t offset pollution from cars, buildings and industry on its own.”

The findings of the study help inform USC’s Urban Trees Initiative, which is a partnership between the City of Los Angeles, USC and community organizations with a focus on expanding urban greenery where it’s needed most. Identifying areas where trees are absorbing the most carbon could be helpful in guiding future planting efforts.

Building on the success of the study, the USC team added eight more sensors to its network outside the original study area.

“Our goal is to monitor more areas of L.A. to define baseline values of CO₂ emission and identify where vegetation is making the biggest impact and where more greenery is needed,” Berelson said.

The fact that urban vegetation only absorbs about one-third of the area’s fossil fuel emissions each year emphasizes the urgent need for improved public transportation, clean energy and broader emissions reductions.

LA has a target of becoming carbon-neutral by 2050. Berelson said that, while the city’s urban greenery gives it a natural boost, reducing the use of fossil fuels is still the most important step in combating climate change.

“Nature is helping us, but we can’t rely on it to do all the work,” Berelson said.

The findings, “Observing Anthropogenic and Biogenic CO2 Emissions in Los Angeles Using a Dense Sensor Network,” were published in the journal Environmental Science & Technology.

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