by Emil Morhardt
On the occasional foggy day in Claremont, one can be nearly bowled over by the smell of dairy farms—not a smell that mixes well with the usual orange blossom/eucalyptus fragrance permeating the campus of the Claremont Colleges. The smell is wafting over from the eastern South Coast Air Basin (SoCAB), which, along with California’s Central Valley is the focus of a new top-down estimate of methane emissions by Yuyan Cui and colleagues at NOAA and the University of Colorado in Boulder, Harvard University, and the University of Michigan. Top-down estimates are based on measurements of methane made from above—in this case by aircraft—rather than based on ground-based considerations, such as counting the number of dairy cows and multiplying by how much methane each is thought to produce. One object of the study was to provide data for use by the State of California in attempting to assure that statewide greenhouse gas emissions not exceed 1990 levels by 2020. The study used some fancy inverse modeling to trace the concentrations measured aloft to their sources, and to calculate that total emission levels. The results corroborated those of several other recent studies, showing that twice as much methane is being emitted than was estimated by the USEPA in 2005, something on the order of 426 Gg (Gigagrams, millions of kilograms) per year. In the eastern SoCAB it, sure enough, is coming from the dairy cattle. In the western SoCAB, where there aren’t any, it comes from landfills, wells, and other un-described point sources.
Bolstered by this result, the researchers then applied the technique to the Central Valley, where they found emissions three times the 2005 EPA estimates, attributable to the same types of sources, as well as from rice paddies.
The paper doesn’t go into the significance of these results with respect to California meeting its greenhouse gas emission standards, but it adds to the general literature we have been reviewing here that suggest top-down greenhouse gas measurements are likely to be more accurate than bottom-up ones. Perhaps the next question is: What were the 1990 levels? Were they substantially underestimated by bottom-up measurements?
It will be interesting to see results from NASA’s Orbiting Carbon Observatory-2 (OCO2), launched on July 2, which should be available soon. [Note that the tragic loss of the original OCO satellite shortly after launch gave NASA the opportunity for a much better acronym.]
Cui, Y.Y., Brioude, J., Frost, G.J., Peischl, J., Ryerson, T., Trainer, M., Wofsy, S.C., Santoni, G.W., Kort, E., 2014. TOP-DOWN ESTIMATE OF METHANE EMISSIONS IN CALIFORNIA USING A MESOSCALE INVERSE MODELING TECHNIQUE. Presented at the 13th Annual CMAS Conference, Chapel Hill, NC, October 27-29, 2014 https://www.cmascenter.org/conference/2014/abstracts/yuyan_cui_top-down_estimate_2014.pdf