City trees absorb much more carbon than expected

In the heart of Los Angeles, trees are absorbing considerably more carbon dioxide than we ever imagined, and effectively balancing a large chunk of fossil fuel emissions during the warmer seasons, when the trees are at their most productive.

This unexpected carbon uptake highlights the crucial role that urban greenery plays in mitigating the effects of pollution. This finding challenges previous assumptions about the impact of vegetation in cities.

Beyond offering shade and aesthetic value, these trees act as silent workhorses in the city’s climate resilience strategy, capturing emissions at a scale that warrants closer examination.

The silent heroes of Los Angeles

This revelation comes courtesy of a new study, a collaboration between researchers from Public Exchange and the USC Dornsife College of Letters, Arts and Sciences.

The research indicates that LA’s trees generate impressive carbon uptake rates, absorbing up to 60% of daytime CO₂ emissions from fossil fuel combustion in spring and summer, and about 30% annually.

To monitor the CO₂ levels, the researchers launched the Carbon Census array, employing 12 high-resolution sensors across a 15×6-mile section of Los Angeles.

“You can think of emissions like passengers on a train. 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,” explained study co-author Will Berelson.

A new approach to measuring CO₂

Traditional models estimate CO₂ levels based on fuel sales, traffic data, or the amount of CO₂ detected at a particular sensor. For this new study, the experts took a direct measurement approach.

Running from July 2021 to December 2022, the study provided a more precise and localized estimation of emissions, and offered a clearer picture of urban carbon flux.

To distinguish CO₂ generated by fossil fuels from CO₂ emitted by living organisms, the researchers used carbon monoxide (CO) as a tracer. Since CO is co-emitted with CO₂ during fossil fuel combustion and behaves similarly in the atmosphere, it allowed scientists to differentiate human-driven emissions from those generated by natural sources.

While the study focused on a section of Los Angeles, its findings have broader implications. Similar methods could be applied to cities worldwide, and could refine our understanding of urban carbon footprints and inform more effective climate policies.

A global lesson from LA’s trees

One startling revelation from the study was that trees absorb most CO₂ during the summer, despite the fact that this is the driest season in Los Angeles.

This is likely due to irrigation, groundwater access from leaky pipes, and resilient tree species that stay green and active in dry conditions.

Nevertheless, city trees cannot solve the emissions problem alone. Carbon dioxide levels still peak during rush hour, emphasizing that while greenery helps, it can’t offset all pollution from cars, buildings, and industry on its own.

The Urban Trees Initiative

This study is a key driver of the USC Urban Trees Initiative – a collaboration between USC, the city of Los Angeles, and community organizations focused on expanding urban greenery in underserved neighborhoods.

The research is critical for our fight against climate change. It provides a blueprint for how we can effectively track and reduce emissions more effectively. “Nature is helping us, but we can’t rely on it to do all the work,” said Berelson.

Ultimately, the findings will help Los Angeles and other cities worldwide to meet their ambitious targets of becoming carbon-neutral.

The next steps for urban greenery

USC has expanded its sensor network in the quest to monitor CO₂ levels in more areas of Los Angeles. This will help define baseline values of emissions and identify where vegetation is making the most impact and where more greenery is needed.

Ultimately, reducing fossil fuel consumption is the most crucial step in combating climate change. However, as we work toward that goal, let’s take a moment to appreciate city trees – they are working much harder than you think.

The full study was published in the journal Environmental Sciencce & Technology.

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