Methane levels in the atmosphere have more than tripled since pre-industrial times, accounting for around one-fifth of the human contribution to greenhouse gas-driven global warming. Until recently, the leveling off of methane levels had suggested that the rate of its emission from Earth’s surface was being approximately balanced by the rate of its destruction in the atmosphere.

However, the balance has been upset since early 2007, the study found. The paper’s lead authors, Matthew Rigby and Ronald Prinn, say this imbalance has resulted in several million metric tons of additional methane in the atmosphere.

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. 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.

—Ronald Prinn

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.

International Siberian Shelf Study. Separately, researchers in the International Siberian Shelf Study (ISSS-08)—the largest field program on the Siberian-Arctic shelves during the International Polar Year (IPY)—returned to shore recently with evidence of large high methane concentrations in both the seawater and overlying air, obviously released from thought-to-be frozen seabed stores off the north coast of Siberia.

Because of the media interest occasioned by a brief report of the findings earlier in the year, Stockholm University published a short set of video interviews with Örjan Gustafsson, Associate Professor at the Department of Applied Environmental Science, who led the Swedish contingent.

Gustafsson cautioned that it is premature to draw any conclusions as to the cause or impact.

We cannot [yet] draw any conclusion whether the methane release is caused by the increase in warming. There are natural mechanisms that can release this methane. What needs to happen is that the [subsea] permafrost lid needs to thaw. It could be geothermal heat, there are several sources in region. The warm energy coming through the river water can also cause methane release—we see that in the extension of the river flow. The third potential cause is the overlying rise in water temperature. We would like to sort out the relative importance of the three.

...one might ask the question why the permafrost on land is not being thawed. The temperature is -18° C over land, the subsea permafrost is covered by sea water. While cold, it is still 0° or plus degrees. There is a much larger warming effect by sea water than by air. That might be one of the reasons.

...we should also keep in mind that there are many other methane sources. I think there is no reason to be an alarmist here, we need to make the studies carefully and be aware of them. It behooves us to focus more on the Siberian sea bottom, let’s focus on resources to do that so we can help IPCC to better assess that particular process.

—Örjan Gustafsson

The preliminary findings of the International Siberian Shelf Study 2008 will also be published by the American Geophysical Union.

Resources

• Rigby, M., R. Prinn, P. Fraser, P. Simmonds, R. Langenfelds, J. Huang, D. Cunnold, P. Steele, P. Krummel, R. Weiss, S. O’Doherty, P. Salameh, H. Wang, C. Harth, J. Mühle, and L. Porter (2008) Renewed growth of atmospheric methane, Geophys. Res. Lett., doi: 10.1029/2008GL036037, in press.