When talking about anthropogenic climate change, carbon dioxide usually grabs the attention; however, it is not the only greenhouse gas emitted into the atmosphere by human activity. Neither is it the most powerful.
Another greenhouse gas that is increasing in Earth’s atmosphere due to humans is methane. Although methane is generated by human activity in much smaller amounts when compared to carbon dioxide, it is about 25 times more potent as a greenhouse gas. It is usually associated with cow flatulence, but bovines are not the only human-induced source of methane.
A new study by Caltech researchers indicates that, at least in the Los Angeles Basin, leakage of natural gas used for heating businesses and homes is the main contributor to atmospheric methane.
The study was carried out by Liyin He (MS ’18), a graduate student in environmental science and engineering, while working in the lab of Yuk L. Yung, Caltech professor of planetary science and research scientist at the Jet Propulsion Laboratory, which Caltech supervises for NASA.
She discovered that methane concentrations in the air above Los Angeles change according to the seasons. During winter, when natural gas use is at its peak, methane concentrations are also elevated. During summer, when there is a fall in the use of natural gas, the amount of methane in the air also decreases.
Naturally, methane emissions should be pretty flat across the seasons, but maybe a little higher in the summer period because of a lot of things decompose from higher temperatures. But it seems that in the city, natural gas consumption is so high in the winter that a lot of it leaks into the atmosphere.
Yuk L. Yung, Professor of Planetary Science, Caltech
The study was carried out with the help of a device known as a remote-sensing spectrometer located on top of Mt. Wilson, a mountain whose peak soars a mile above Los Angeles. From its lofty perch, the spectrometer had a view of a broad swath of the urban area beneath.
Although Methane cannot be seen by naked eyes, it can be easily observed by using the spectrometer as it strongly absorbs infrared light, the wavelength of light to which the spectrometer is sensitive.
With the help of this arrangement, the spectrometer was pointed at 33 surface locations around the area and gathered methane measurements six to eight times a day for six years. When those measurements were combined, a clear pattern materialized: every December and January, the methane levels in the atmosphere increased, and every June and July, it dropped to a low level.
Although the work does not find particular sources of methane, the researcher proposes that the whole natural gas distribution system, from storage fields to pipelines to stoves and furnaces, causes the leaks.
As methane is such a powerful greenhouse gas, and as it is comparatively short-lived in the atmosphere, identifying and mitigating those natural gas leaks is one means through which humans might help minimize the impacts of climate change.
“Agriculture and wetlands are still the most important sources of methane when we consider the global scale,” Yung says. “But I think pipeline leakage is the most important one when it comes to cities.”
The paper explaining the study, titled “Atmospheric methane emissions correlate with natural gas consumption from residential and commercial sectors in Los Angeles,” has been published in the July 15 issue of Geophysical Research Letters.
Other co-authors at Caltech are Zhao‐Cheng Zeng, visitor in planetary science; Run‐Lie Shia (PhD ’86), associate research scientist; Paul O. Wennberg, R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering and director of the Ronald and Maxine Linde Center for Global Environmental Science; and Stanley P. Sander (MS ’75, PhD ’80), visiting associate in planetary science.
Other co-authors include Thomas J. Pongetti, Vineet Yadav, Kristal Verhulst, Charles E. Miller, and Riley Duren of JPL; Clare Wong of Cal State Northridge; Sally Newman, former Caltech senior research scientist now at the Bay Area Air Quality Management District; and Jianming Liang and Kevin R. Gurney of Northern Arizona University, Flagstaff.
Funding for the study was offered by the National Institute of Standards and Technology, the California Air Resources Board, the National Aeronautics and Space Administration, and the Resnick Sustainability Institute fellowship program.