L.A.’s urban trees are absorbing more fossil fuel emissions than expected
Original story by Ileana Wachtel
In some good news for our climate, a new USC study has found that the urban trees of Los Angeles are absorbing more air pollution than expected. Led by Will Berelson, who holds the Wrigley Institute-endowed Paxson H. Offield Professorship in Coastal and Marine Systems, the study was conducted in collaboration with Public Exchange and USC Dornsife College of Letters, Arts & Sciences.
Researchers found that, over a period of almost two years, vegetation in the study area absorbed up to 60% of daytime carbon dioxide (CO2) emissions from fossil fuels in spring and summer, and about 30% annually.
To track CO₂ in real time, the research team launched what they call the Carbon Census array, deploying 12 high-resolution sensors across a 90-square-mile section of L.A.’s Mid-City neighborhood. The sensors mapped how CO2 concentrations changed as air moved through the urban landscape, enabling researchers to factor in wind speed, wind direction, and urban density to determine how local greenery was offsetting emissions.
“You can think of emissions like passengers on a train,” says Will Berelson. “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.”
Unlike some research that estimates CO₂ levels based on fuel sales and traffic data, or that measures the CO2 landing on a single sensor, this study measured CO2 directly across an array of sensors, yielding a more precise and localized estimation of emissions.
To distinguish CO₂ emitted from fossil fuels versus natural sources, researchers also tracked carbon monoxide, which behaves similarly in the atmosphere and is released when fossil fuels burn, but not by plants or animals.
Although the study focused on a section of L.A., the findings provide valuable insights that could apply to other urban areas.
L.A. trees are helping — but they’re not enough
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.
Still, trees can’t keep pace with emissions. As expected, CO₂ levels spiked during rush hour, meaning that trees were reaching their capacity to absorb emissions. In other words, while greenery helps, it can’t offset pollution from cars, buildings, and industry on its own.
The study’s findings help inform the USC Urban Trees Initiative, a partnership between USC, the City of Los Angeles, and community-based organizations to expand urban greenery in communities that need it most. By identifying where trees absorb the most carbon, the research provides data-driven insights that could help guide future planting efforts.
“Nature is helping us,” Berelson said, “but we can’t rely on it to do all the work.” Even though vegetation in the study area absorbed more emissions than expected, the fact that it absorbed only 30% of annual emissions underscores the urgent need for clean energy, improved public transit, and broader emissions reductions.
Read the full story in USC Dornsife News >>
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