Rasslin’ swamp gas

It is impossible to measure the global inventory of OH directly because its concentration is so low and so variable. The tracer for the global OH inventory with the longest pedigree is methyl chloroform, CH3 CCl3. Prinn et al [2005] diagnosed 10% changes of OH concentration on decadal timescale, but Krol and Lelieveld [2003] claim that this result is sensitive to small changes in the assumed source function of methyl chloroform. If the stuff were hoarded a bit before the onset of the Montreal Protocol banning its use, the OH change diagnosed by Prinn could go away. Emission of methyl chloroform stopped in 1992, so the signal now comes down to the decay time constant of the atmospheric concentration. This is complicated by other uptake fluxes such as invasion into the ocean [Wennberg et al., 2004].

The concentration of 14C in CO serves as another tracer for the OH inventory [Manning et al., 2005]. 14C is produced naturally, by cosmic ray neutrons impacting nitrogen gas, and it quickly oxidizes to 14CO, then after a few months to 14CO2. Changes in the solar magnetic shielding of the Earth can affect the production rate of 14CO, requiring a correction, and exchange with the stratosphere is important, but the competing source / sink problems do not appear to be as severe as they are for methyl chloroform. The lifetime of CO in the atmosphere is much shorter than that of methyl chloroform, making the 14CO concentration much more sensitive to, and diagnostic of, month-to-year timescale variability in OH. 14CO varies by factor of two or more over the seasonal cycle, whereas methyl chloroform only varies by a few percent. Manning et al [2005] found no long-term trend, but short-term 10% variations from Pinatubo and Indonesian fires. Pinatubo brought a 12% decrease in solar UV flux [Dlugokencky et al., 1996], decreasing OH, while fires, in particular the Indonesian fire in 1991, bring an increase in CO and CH4 emissions, which can also deplete OH [Butler et al., 2005]. One gets a picture of a volatile but self-stabilizing OH cycle, a flickering flame.

Putting them together

One clue that might help unravel past changes in methane sources is that the rate of atmospheric increase of several gases all correlate. Langenfelds et al [2002] found that CO2, 13C, H2, CH4, and CO growth rates all march in step with the Southern oscillation from 1992-99. Simmonds et al [2005] find similar correlations in data from 1996-2003. Both authors point to fires as a potential common source, as opposed to wetland emissions (which don’t produce H2, for example).

Another seemingly useful clue is that, during a period of methane doldrums (no rise) from 1999-2002, the N/S gradient of methane relaxed a bit [Dlugokencky et al., 2003], suggesting that the doldrum was due to a decline in a methane source in the northern high latitudes. Since most fires burn in the tropics, rather than in the high latitudes, this clue would seem to be pointing us toward wetlands, i.e. in a different direction than clue #1.

A recent paper [Bousquet et al., 2006] attempts to bring all these pieces together into an inverse calculation of the methane sources. This is not a fundamentally new approach, but it does have the advantage of including the most recent several years of data, as methane stubbornly continues to refuse to rise. Changes in OH concentration are diagnosed from methyl chloroform. The spatial pattern of CH4 variations, plus 13CH4 data, provides the basis for partitioning the methane changes among the various sources and sinks.

Their conclusion is that rising human emission since 2000 has been masked by a probably temporary natural decline in wetland emission. Their diagnosed source fluxes are consistent with bottom-up models of wetlands and fires, and independent fossil fuel emissions estimates. But I have to wonder what they’d get if they considered some of the other trace gases mentioned above, such as CO, 14CO, and H2.

Bottom line

What are the implications of all this for our ability to predict the future of the methane cycle? Let’s summarize what you’ve just read. According to one set of papers, atmospheric methane could be suppressed in the future by controlling land fires. Or it could be that methane variations are mostly produced by wetland emission, driven by climate change as well as land use decisions, according to another set of papers. Or methane could resume its rise, toward a new steady state, because it is driven by increasing fluxes from melting permafrost peat and hydrates, according to observations on the ground.

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