Much ado about methane

So far there hasn’t been strong evidence presented for detection enhanced methane fluxes due to anthropogenic warming yet. Yet it is certainly believable for the coming century however, which brings us to the next question:

What effect would a methane release have on climate?

The climate impact of releasing methane depends on whether it is released all at once, faster than its lifetime in the atmosphere (about a decade) or in an ongoing, sustained release that lasts for longer than that.

chronic vs catastrophic release cartoon

When methane is released chronically, over decades, the concentration in the atmosphere will rise to a new equilibrium value. It won’t keep rising indefinitely, like CO2 would, because methane degrades while CO2 essentially just accumulates. Methane degrades into CO2, in fact, so in simulations I did (Archer and Buffett, 2005) the radiative forcing from the elevated methane concentration throughout a long release was about matched by the radiative forcing from the extra CO2 accumulating in the atmosphere from the methane as a carbon source. In the figure below, the dashed lines are from a simulation of a fossil fuel CO2 release, and the solid lines are the same model but with an added methane hydrate feedback. The radiative forcing from the methane combines the CH4 itself which only persists during the time of the methane release, plus the added CO2 in the atmosphere, which persists throughout the simulation of 100,000 years.

response of carbon cycle / hydrate model to fossil fuel CO2 forcing

The possibility of a catastrophic release is of course what gives methane its power over the imagination (of journalists in particular it seems). A submarine landslide might release a Gigaton of carbon as methane (Archer, 2007), but the radiative effect of that would be small, about equal in magnitude (but opposite in sign) to the radiative forcing from a volcanic eruption. Detectable perhaps but probably not the end of humankind as a species.

What could happen to methane in the Arctic?

The methane bubbles coming from the Siberian shelf are part of a system that takes centuries to respond to changes in temperature. The methane from the Arctic lakes is also potentially part of a new, enhanced, chronic methane release to the atmosphere. Neither of them could release a catastrophic amount of methane (hundreds of Gtons) within a short time frame (a few years or less). There isn’t some huge bubble of methane waiting to erupt as soon as its roof melts.

And so far, the sources of methane from high latitudes are small, relative to the big player, which is wetlands in warmer climes. It is very difficult to know whether the bubbles are a brand-new methane source caused by global warming, or a response to warming that has happened over the past 100 years, or whether plumes like this happen all the time. In any event, it doesn’t matter very much unless they get 10 or 100 times larger, because high-latitude sources are small compared to the tropics.

Methane as past killing agent?

Mass extinctions like the end-Permean and the PETM do typically leave tantalizing spikes in the carbon isotopic records preserved in limestones and organic carbon. Methane has an isotopic signature, so any methane hijinks would be recorded in the carbon isotopic record, but so would changes in the size of the living biosphere, soil carbon pools such as peat, and dissolved organic carbon in the ocean. The end-Permean extinction is particularly mysterious, and my impression is that the killing mechanism for that is still up for grabs. Methane is also one of the usual suspects for the PETM, which consisted of about 100,000 years of isotopically light carbon, which is thought to be due to release of some biologically-produced carbon source, similar to the way that fossil fuel CO2 is lightening the carbon isotopes of the atmosphere today, in concert with really warm temperatures. I personally believe that the combination of the carbon isotopes and the paleotemperatures pretty much rules out methane as the original carbon source (Pagani et al., 2006), although Gavin draws an opposite conclusion, which we may hash out in some future post. In any case, the 100,000-year duration of the warming means that the greenhouse agent through most of the event was CO2, not methane.

Could there be a methane runaway feedback?.

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References

  1. D. Archer, and B. Buffett, "Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing", Geochemistry, Geophysics, Geosystems, vol. 6, pp. n/a-n/a, 2005. http://dx.doi.org/10.1029/2004GC000854
  2. M. Pagani, K. Caldeira, D. Archer, and J.C. Zachos, "ATMOSPHERE: An Ancient Carbon Mystery", Science, vol. 314, pp. 1556-1557, 2006. http://dx.doi.org/10.1126/science.1136110