Rasslin’ swamp gas

In the early 1990’s, in defiance of IPCC projections, the methane concentration in the atmosphere abruptly stopped rising, and has remained nearly constant since then. Methane is a crouching tiger in the carbon cycle, with potentially enough available as hydrates and from peats to really clobber the Earth’s heat budget. The big question is, will atmospheric methane start rising again?

Climate impact of methane release

The climate impact of methane differs from that of CO2 in that methane is a transient gas, while CO2 accumulates. The climate impact of methane release depends on whether it’s released quickly or slowly, relative to the methane lifetime.

If it’s released quickly, over just a few years or less, there would be a decade-timescale warming spike, followed by a recovery toward the lesser warming from the CO2 that the methane would oxidize into. The amount of available methane is staggering. If just 10% of the ocean hydrate reservoir were to escape to the atmosphere within a few years, it would be the radiative equivalent of a ten times increase in atmospheric CO2, truly catastrophic.

On the long term (longer than a few decades) the transient methane concentration is determined by the chronic rate of methane emission to the atmosphere. A higher concentration in the atmosphere accelerates the overall rate of methane oxidation, to balance the greater input. More methane molecules are standing in line to compete for the limiting supply of the reaction catalyst molecule, OH radical. The oxidation product of the methane, CO2, builds up enough to impact the climate as well. In the long term, the radiative forcing from the accumulating CO2 may exceed that of the transient methane concentration [Archer and Buffett, 2005].

Sources

There have been two methane sources in the newspapers in the past few years. One is plants, measured recently [Keppler et al., 2006] to give off prodigious amounts of methane. If this is true, it might imply a small increase in chronic emissions in the future. The seasonal cycle of atmospheric CO2, the breathing of the biosphere, has been getting deeper over the years. If the biosphere is breathing deeper, maybe it’s also farting more.

The other is peats, on which there is a voluminous literature. Peat methane fluxes are notoriously patchy and difficult to generalize, but there are a few things you can depend on. One is that water makes a huge difference; a wet peat soil will emit methane while the same soil, dried, would actually consume methane. Two, there seems to be a reproducible methanogenesis poisoning effect by sulfuric acid deposition (acid rain), caused by the stimulation of sulfate-reducing bacteria displacing the methanogens. Three, melting starts things cooking. Fourth, peats release much more carbon as CO2 than as methane, and their strongest radiative impact will probably be from the CO2.

Methane hydrates are the giant reservoir on Earth. It takes a long time to warm the deep ocean and the clathrate zone, and no one has proposed a mechanism for getting much methane release from hydrates in the coming century. The only place where melting methane hydrates appear to be releasing methane to the atmosphere is on the Siberian margin, where hydrates associated with the permafrost relict from the last glaciation release methane to the shallow water column of the shelf waters.

Industrial emission of methane declined a bit with the collapse of the USSR in 1989 and thereafter. Industrial methane emission arises from leaks, more difficult to pin down than the amounts of deliberately released gases such as CO2. Agricultural emission, from rice farming and ruminant animals, is not so easy to quantify either, but we’ll leave a description of that to the reader’s imagination.

Sinks

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