Posted on: Thursday, June 14, 2012 12:37 PM
|Walter Anthony et al (2012) have made a major contribution to the picture of methane emissions from thawing Arctic regions. Not a game-changer exactly, but definitely a graphics upgrade, bringing the game to life in stunningly higher resolution (/joke).
Katey Walter Anthony draws upon her previous field findings that methane emissions from the Arctic landscape tend to be focused at the intersection between frozen and thawed, in particular in rings around a peripheries of . She also knew what a methane seep looks like in that landscape, leaving visible bubbles frozen into the ice or maintaining an unfrozen hole in the ice. Now she takes to the skies to produce an aerial survey of the Alaskan landscape, data that is so much more voluminous than before that it becomes different in kind.
The methane emission fluxes are higher than previous estimates, but that’s not really the most important point, because emissions from the Arctic are small relative to low-latitude wetlands, and doubling or even nearly quadrupling the Arctic fluxes (in one of their analyzed regions), they would still be small in terms of global climate forcing. And the lifetime of methane in the atmosphere is short, about 10 years, so methane doesn’t build up like CO2, SF6, and to a lesser extent N2O do.
The really interesting take-away from the new paper is how it shows that the near-surface geology and freezing state conspire to control the venting of accumulated gas dribbling up from below, and the decompostion of frozen soil carbon. They have so many methane seep observations that they are able to correlate them with (1) currently melting permafrost, which allow fossil soil carbon deposits from the last ice age called Yedoma to decompose (Zimov et al 2006) and (2) melting ice sheets and glaciers “un-crunching” the landscape as they fade away, making cracks that vent methane from deep thermal sources. Glaciers that melted long ago no longer vent methane, showing that the methane is transiently venting from built-up pools of gas.
What these results do not do is fundamentally change the game, in my opinion. We can now see more clearly that most of the methane flux from the Arctic today are of types of emission that will respond to climate warming. But the general response time of the system is slow, decades to centuries, rather than potentially poised to release a huge pulse of methane within a few years. Earthquakes and submarine landslides are sudden events, but small individually in terms of potential methane release. The new data do not change that. Walter Anthony et al. compare an estimate the amount of methane in the Arctic, 1200 Gton C, with the 5 Gton C of methane in the atmosphere. That’s the nightmare comparison, but it’s only really relevant if the methane comes out all at once. (The Arctic estimate is for methane itself and is mostly methane hydrate, but keep in mind that there is also a comparable amount of decomposable soil carbon.)
In my opinion, the largest impact of all this methane will probably be to the long-term future evolution of climate. Avoiding a peak warming of 2 degrees C or more requires keeping the total emission of carbon down to less than about 1000 Gton C (Allen et al 2009). We have already burned about 300 Gton C, and cut 200 Gton C although the land surface has taken up enough carbon to achieve net wash. So maybe we’re 1/3 of the way there, say 700 Gton C left to go. The 1200 Gton C of Arctic methane hydrates and the permafrost carbon stack up pretty menacingly against our 700 Gton left to go, and the comparison is relevant even if the carbon is emitted slowly, or as CO2 rather than methane, or even if it is released into the ocean rather than into the air (it will still equilibrate with the atmosphere, after a few centuries, converging to the same “long tail” CO2 trajectory that would have resulted from atmospheric release).
Arctic methane, and all that frozen soil carbon, could easily play a huge role, not so much in the near-term evolution of Earth’s climate, but in the long tail of the global warming climate event.
Allen M.R., D.J. Frame, C. Huntingford, C.D. Jones, J.A. Lowe, M. Meinshausen & N. Meinshausen (2009) Warming caused by cumulative carbon emissions towards the trillionths tonne. Nature 458 doi:10.1038/nature08019
Walter Anthony, K.M., P. Anthony, G. Grosse, & J. Chanton (2012) Geologic methane seeps along boundaries of Arctic permafrost thaw and melting glaciers. Nature Geoscience doi:10.1038/NGEO1480
Zimov, S.A., Schuur, E.A.G, and F. Stuart Chapin III, F. (2006) Permafrost and the Global