Atmospheric methane levels “shot up” in 2007, according to a paper in this week’s Geophysical Research Letters (MIT news release — Rigby, M., R. Prinn, et al (2008), Renewed growth of atmospheric methane, Geophys. Res. Lett., doi:10.1029/2008GL036037, in press. PDF here for AGU members.). This confirms NOAA’s announcement in April that methane levels were on the increase after a decade of stability, but adds a new twist to the data.
One surprising feature of this recent growth is that it occurred almost simultaneously at all measurement locations across the globe. However, the majority of methane emissions are in the Northern Hemisphere, and it takes more than one year for gases to be mixed from the Northern Hemisphere to the Southern Hemisphere. Hence, theoretical analysis of the measurements shows that if an increase in emissions is solely responsible, these emissions must have risen by a similar amount in both hemispheres at the same time.
A rise in Northern Hemispheric emissions may be due to the very warm conditions that were observed over Siberia throughout 2007, potentially leading to increased bacterial emissions from wetland areas. However, a potential cause for an increase in Southern Hemispheric emissions is less clear.
A possible explanation might be a global reduction in the amount of hydroxyl free radical (OH) in the atmosphere. OH “mops up” methane (and other stuff), and is sometimes called the atmosphere’s cleaner or “detergent”. Unfortunately, measuring atmospheric OH is difficult.
“The key thing is to better determine the relative roles of increased methane emission versus any decrease in the rate of removal,” Prinn said. “Apparently we have a mix of the two, but we want to know how much of each” is responsible for the overall increase.It is too early to tell whether this increase represents a return to sustained methane growth, or the beginning of a relatively short-lived anomaly, according to Rigby and Prinn.
Any increase in atmospheric methane is not good news. A possible reduction in OH is also disturbing, because it might indicate that pollution (from all sources, methane, industry, coal burning etc) is beginning to overwhelm the atmosphere’s ability to cleanse itself. Both together would be very bad news indeed.
[Update: Comments from a CSIRO scientist and co-author here.]