|Methane concentrations at the Mauna Loa observatory. The grey data points are preliminary. Graphic: NOAA.|
We seem to know less about methane emissions than we thought. After a decade of stability, methane concentrations in the atmosphere have been rising strongly in the last 18 months. Early research work suggested that this rise was concentrated in the northern latitudes of the northern hemisphere and was consistent with greater emissions from decaying organic matter in melting permafrost or from the melting of Arctic sea ice. Now this result has been called into question by the publication of a new study showing the concentrations of methane are rising almost everywhere. Since methane takes some time to diffuse around the globe, the later work suggests that the rise in methane may not be directly due to enhanced emissions from biological sources.
Methane is the second most important greenhouse gas, producing about 20% of the radiative forcing of all the main gases. Its concentration in the atmosphere has risen about two and half times since the industrial revolution to about 1750 parts per billion. Although it is present in very much smaller concentrations than CO2, each molecule has a more powerful global warming effect. It also lasts much less long in the atmosphere, typically reacting with the hydroxyl radical (*OH) to form carbon dioxide and water. The average life of a molecule of methane in the atmosphere is about 8 years compared to about a century for carbon dioxide.
The growth rate of methane concentrations in the atmosphere slowed in the second half of the last century. The period between 1999 and early 2007 showed virtually no increase, leading to optimism that methane emissions were under control. Deep coal mines were an important source and much of the industry was closed down in the northern hemisphere in the 1980s and 1990s. Rice farming practices, which floods vegetated areas, allowing plant matter to rot anaerobically and produce methane, were changed in some parts of Asia to reduce emissions. Natural gas is largely methane and pipeline leaks were also reduced.
The recent rise surprised many researchers. In the past, methane concentrations have tended to increase in periods of marked El Niño (high sea temperatures in the eastern Pacific), resulting in greater dieback of vegetation and more methane production from the rotting plant matter. But the last year or so has been a period of lower than average temperatures in the Pacific (La Niña rather than El Niño). So this isn’t a good explanation for the sudden jump of about 10 parts per billion, or about 0.5% increase in the average concentrations.
Dr Rebecca Fisher of Royal Holloway College, University of London, published work with colleagues early this year showing that the rise in methane was particularly great in the Arctic. Some measuring stations saw increases of twice the average global rise. Since methane takes time to diffuse around the world, this suggested a regionally-specific source. It could be from the sea or from rotting vegetation exposed by melting. Dr Fisher’s work was, in a sense, comforting. It suggested we might have an explanation for why methane concentrations were rising. In particular, she showed that increased methane concentrations were associated with rises in the percentage of gas containing the lighter carbon isotope, C12, which is associated with emissions from methane-producing bacteria. It looked as though we could be reasonably confident that at least part of the source of increased emissions was rotting plant matter.
More recent work has dented this belief. MIT scientists have just published work with scientists from Australia and elsewhere that shows that the rise in methane levels has been quite uniform across the globe. This shouldn’t happen if methane was produced by plant sources, since there is far more organic matter in the northern hemisphere. Concentrations should be temporarily higher in the north in the months and years it takes methane to spread uniformly across the globe.
The MIT team speculate that the rise in methane may be a function of decreasing concentrations of hydroxyl, the scavenger radical that mops up methane, perhaps as well as increasing emissions. But we don’t yet have good monitoring of *OH concentrations and it will be some time before we are able to tell if this hypothesis is correct or, indeed, what is causing this change.
The scientific debate about the cause of increased methane is important because it suggests that we do not yet have a good model for what determines changes in concentrations. One of the primary worries about global warming is that it will eventually trigger the eruption of untold millions of tonnes of methane from deep sea water. (This is usually known as the ‘clathrate gun’ hypothesis.) The gas is currently locked into a stable bond with the extremely cold waters in the deep oceans. Continued world temperature increases will eventually cause the methane to burst from its chemical locks within the cold liquid and rise to the surface. This probably happened at times of rapid warming in the far-distant past.
The fact that we cannot immediately know today why the methane rise is occurring, and whether it is likely to continue, raises worries that our understanding of methane’s role in the global carbon cycle is simply not very good.