Published on April 22nd, 2010 | by Michael Ricciardi
Extensive Release of Methane Gas from Arctic Shelf Confirmed
A team of scientists confirms that sea-bottom and surface waters of the East Siberian Arctic Shelf are “supersaturated” with Methane (CH4) gas and “out-gassing” this potent GHG to the Atmosphere.
A variable quantity known as the “venting flux” was calculated for the ESAS and found to be nearly equal to that amount from the entire World Ocean. Previous calculations by climate scientists estimated that “remobilization” of only a small fraction of of this trapped methane could potentially trigger “abrupt climate warming”.
A research team confirms “extensive out-gassing of methane to the atmosphere” over the Eastern Siberian Arctic Shelf, and confirm its source to be venting from sea-bed sediments. Though acknowledging their findings do not seriously alter climate change predictions, the team also asserts that the sub-sea permafrost layer is failing and advise more urgent investigation.
Large quantities of carbon (C) and methane (CH4)–typically in the form of methyl hydrate (or methyl clathrate)–are trapped in ocean sediments the world over. The source of this trapped methane is commonly through organic decay or from primary production (food-making by planktonic lifeforms, known as autotrophs). Through natural erosion of sediments, shifting of the ocean floor covering, and/or from pressure cracking, bubbles of the gas often escape and rise up through the water column. In the deeper seas and oceans, this natural gas venting to the atmosphere (“out-gassing”) is largely constrained due to the fact that, as the gas rises through the water column, much of it combines with oxygen and forms other, less potent molecules. Also, in colder seas, sub-sea permafrost acts as a “lid” to contain much of the buried carbon.
However, in shallower, warmer seas like the ESAS, where the average depth is only 45 meters, a far larger quantity of this potent GHG is released into the atmosphere, through the dual processes of diffusion and ebullition (pressure conversion of a liquid to a vapor). In the latter process, this transport is in the form of bubbles of methane gas rising up through the water column, and bursting at the surface.
The ESAS is the largest shallow sea in the world. However, this sea is more turgid that other shallow seas and receives less sunlight penetration, which usually means there is less abundance of planktonic life forms, which directly or indirectly produce much of the methane. Despite this, the ESAS generates a methane flux ten times that of the deeper ocean, according to the research findings. Clearly then, something more is going on in this undersea environment.
Many climatologist are concerned that Arctic warming–which is greater than predicted by several degrees C so far this century–will accelerate the thawing of sub-sea permafrost and thus also the release of CH4. Methane, though shorter-lived in the atmosphere than CO2, is a far more potent GHG than CO2, in terms of its heat-trapping capability. Its build-up will increase atmosphere and surface heating, and some of this heat will be down-transported through the water column, resulting in more thawing of permafrost, more methane release, etc. This is known as a positive feedback cycle. Such self-reinforcing feedback loops, it is asserted, are the major driving force for “abrupt climate change” predictions and “runaway greenhouse effect” scenarios.
The team calculated an annual out-gassing from the ESAS at 7.98 Tg (Tera grams, or trillions of grams). This is an amount roughly equivalent to the CH4 emissions estimate for the entire world ocean. In some “hot spots”, the venting was 8300% greater than the background methane estimates.
While advocating a revision of the current, estimated global CH4 flux, the team also acknowledges in their paper that this current estimate is “not alarmingly altering the contemporary global CH4 budget.” However, the report’s authors go on to state: “These finding do not change our view of the vulnerability of the large, sub-sea permafrost carbon reservoir on the ESAS; the permafrost ‘lid’ is clearly perforated, and sedimentary CH4 is escaping to the atmosphere.”
Features known as polynyas (open areas of ocean surrounded by sea ice, where much ocean-air exchange happens) and sea-ice breaks provide the mechanical means for transporting large quantities of the gas between ocean and atmosphere.
The key questions are: As Arctic sea temperatures warm, how will this impact the permafrost lid in its default function of containing trapped CH4? And, will a tipping point be reach in this failure of the permafrost “lid”?
The annual average temperature of the ESAS sub-sea permafrost, typically falling between -1.8° C and 1° C, is still 12° to 17° C warmer than that over on-land permafrost. A combination of bottom up geothermal and top-down seawater heat fluxes is believed to be permitting partial thawing and failure of the sub-sea permafrost, allowing greater permeability of gases.
The study was the joint project of the International Arctic Research Center, University of Alaska at Fairbanks, Russian Academy of Sciences, The Pacific Oceanological Institute, and Stockholm University. The purpose of the study was to achieve an integrated assessment of the entire ESAS in order to determine the total venting flux of CH4 to the atmosphere.
The team tested their hypothesis during a half dozen field campaigns undertaken between 2003 and 2008, and in addition, one helicopter survey and one over-ice winter expedition. In making their assessment, the research team utilized results from over 5100 at-sea observations.
The published reference for this article: Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf, Shakhova et al, Science, 5 March, 2010