Published: Oct. 12, 2020

Adapted from a NOAA news story

The Los Angeles Basin is often thought of as a dry, heavily developed landscape. But a new study led by NOAA and the University of Colorado Boulder shows that the manicured lawns, emerald golf courses, and trees of America’s second-largest city play a surprisingly large role in its carbon emissions.

Megacities like Los Angeles contribute significantly to national and global carbon dioxide emissions and are an increasingly important priority for mitigation efforts, the scientists said. What this study showed, however, was that measuring emissions accurately isn’t as simple as, for example, taking a snapshot of carbon dioxide levels from remote sensing data. Plants in urban environments can complicate the picture.

Working as part of the Megacities Carbon Project, the scientists analyzed the carbon dioxide, or CO2, in around 500 air samples collected during 2015 from four sites around the basin for the presence of a rare radioactive isotope known as carbon-14. Because it is radioactive, carbon-14 slowly decays away over time. Carbon-14 is found in living organisms, including vegetation. But fossil fuels, which are millions of years old, are totally devoid of carbon-14. Thus, the isotope can be used to differentiate carbon dioxide emitted during combustion of fossil fuels from that released by plants.

Lead author John Miller, a carbon cycle scientist with NOAA’s Global Monitoring Laboratory, said that when researchers disentangled the carbon dioxide generated by burning fossil fuels from that generated by vegetation, they found that L.A.’s green landscape contributed substantially to the changes in carbon dioxide levels around the city.

“L.A. is a very dry place,” Miller said. “You think of L.A., you think of freeways and sprawl. The natural environment outside the city is not naturally lush. In addition, 2015 was a big drought year, so it was all the more surprising that so much of the CO2 in our measurements came from living plants.”

The timing of the increased carbon dioxide drawdown from the growth of plants was another surprise. In a normal Mediterranean climate, winter rains are followed by a dry season. Plants respond by drawing in carbon dioxide in early spring when rain is available, and emitting it in late summer and fall as they go dormant during the dry season.

“L.A. has a Mediterranean climate but we saw CO2 levels drawn down in the middle of summer, in response to the watering of lawns, golf courses, trees—even though 2015 was a drought year with water restrictions,” he said. “Irrigation was compensating for the lack of rain, and keeping the urban ecosystem active.”

This seasonal fluctuation in ecosystem carbon dioxide amounted to one third of the CO2 level resulting from combustion of fossil fuels.

Riley Duren, a research scientist at the University of Arizona who began this research while at NASA’s Jet Propulsion Laboratory, said that this study is part of a broader program to support science-based decision making at local scales and to develop actionable carbon measurement methods that can be extended to cities globally. “It also helps lay the foundation for using carbon-14 measurements as a reference point to improve and correct other tracers of fossil-fuel carbon dioxide emissions.”

The team is working with colleagues from other pilot projects around the world with a goal of ultimately establishing a sustained, global carbon monitoring system for cities.

The carbon-14 approach, which Miller and INSTAAR scientist Scott Lehman have been developing since 2003, allows them to separately track carbon dioxide from ecosystems and from fossil fuel use. A recent study applied this method to determine US emissions at the national scale, and here it was used to better understand the different sources contributing to the overall urban carbon emissions.

The takeaway, Miller and Lehman said, is that understanding the urban carbon footprint—how much carbon is produced in a city—is a lot more complicated than simply cataloging fossil fuel use and emissions. It highlights the need to more accurately measure and track fossil-fuel emissions, as well as the impact of urban vegetation and greening campaigns to develop and evaluate emissions mitigation strategies.

“If you are the mayor of a major city and you’re interested in your city’s total carbon balance, you should be interested in how active your biosphere is as well,” Miller said. “You can’t just look at CO2 concentrations alone. You can imagine that if the biospheric signal is as large as it is for a dry city like L.A., in wetter places like Mumbai and S̴ão Paulo, ecosystem-produced CO2 could be an even larger part of the carbon budget.”

Lehman concludes, “It’s important to note that the net CO2 signal from the urban biosphere may change from year to year and from place to place, depending on factors such as sunlight, temperature and rainfall and—as our results from LA underscore—urban irrigation. On the other hand, CO2 emissions from fossil fuel use are always net positive and must be reduced drastically if we hope to avoid consequences of manmade warming beyond those that are already baked in. Our results demonstrate the need to track CO2 variations from both the urban biosphere and fossil fuel use—and how one might actually do so.”

Funding for this research was provided by NOAA and NASA’s Jet Propulsion Laboratory. The L.A. Megacity Carbon Project is supported by NASA, the National Institute of Standards and the California Air Resources Board.