However, the balance has been upset since early 2007, according to research published this week in the American Geophysical Union's "Geophysical Review Letters." The paper's lead authors, Matthew Rigby and Ronald Prinn of the Massachusetts Institute of Technology, say this imbalance has resulted in several million metric tons of additional methane in the atmosphere.
Methane is produced by wetlands, rice paddies, cattle, and the gas and coal industries. It is destroyed in the atmosphere by reaction with the hydroxyl free radical, often referred to as the atmosphere's "cleanser."
"This increase in methane is worrisome because the recent stability of methane levels was helping to compensate for the unexpectedly fast growth of carbon dioxide emissions," said climate modeler Drew Shindell at NASA's Goddard Institute for Space Studies in New York.
"If methane continues to increase rapidly, we'll lose that offsetting effect. We will use NASA's climate modeling capability to improve our understanding of what is causing the increase and project future methane levels."
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 between the hemispheres. 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.
The scientists analyzed air samples collected by the NASA-funded Advanced Global Atmospheric Gases Experiment ground network from 1997 through April 2008. The network was created in the 1970s in response to international concerns about chemicals depleting the ozone layer. It is supported by NASA as part of its congressional mandate to monitor ozone-depleting trace gases, many of which also are greenhouse gases. Air samples are collected and analyzed at several stations around the world.
According to the researchers, a rise in Northern Hemispheric emissions may be a result of very warm conditions 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.
An alternative explanation for the rise may lie, at least in part, with a drop in the concentrations of the methane-destroying hydroxyl free radical. Theoretical studies show that if this has happened, the required global methane emissions rise would have been smaller and more strongly biased to the Northern Hemisphere. At present, however, it is uncertain whether such a drop in hydroxyl free radical concentrations did occur.
"The next step to pin down the cause of the methane increase will be to study this using a very high-resolution atmospheric circulation model and additional measurements from other networks," Prinn said. "The key is to determine more precisely the relative roles of increased methane emission versus a decrease in the rate of removal. 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. Given that methane is about 25 times stronger as a greenhouse gas per metric ton of emissions than carbon dioxide, the situation will require careful monitoring in the near future to better understand methane's impact on future climate change.
