Siberian seabed methane: first numbers

Siberian seabed methane: first numbers 

by Gareth Renowden, Hot Topic, March 5, 2011

The latest estimate of methane release from the shallow seas off the north coast of Russia — the East Siberian Arctic Shelf (ESAS) — suggests that around 8 teragrams per year (1Tg = 1 million tonnes) of the gas are reaching the atmosphere. This is equivalent to previous estimates of total methane release from all oceans. The study, led by Natalia Shakhova and Igor Semiletov and published in this week’s Science ("Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf," Science (5 March 2010), Vol. 327, No. 5970, pp. 1246–1250; DOI: 10.1126/science.1182221), is based on fieldwork over 2003–2008. Over 80% of the bottom water over the ESAS was found to be supersaturated with dissolved methane, and 50% of the surface water. More than 100 “hotspots’ were discovered, where large quantities of methane are escaping from the sea-floor. Here’s Shakhova discussing the paper’s findings in a University of Alaska Fairbanks video (press release):

Map of the study region showing fluxes of methane to the atmosphere.

The CH₄ emitted is about 2% of global annual emissions, so it is certainly significant. Ed Dlugokencky of NOAA, who confirmed a couple of weeks ago that recent increases in atmospheric methane were continuing, tells me that the emissions estimates are reasonable, but that the global data is not yet consistent with a large and growing source of Arctic methane:

We saw an increase in atmospheric CH₄ growth rate in the Arctic in 2007, but not in 2008. There were also large increases in the tropics in 2007 and 2008. I can not determine the relative amounts of the increase from the tropics vs. Arctic without a chemical transport model, but much of the increase was because of increased tropical emissions. The 2007 increase in the Arctic likely resulted from terrestrial wetland sources (i.e., not the processes Shakhova discuss), because of warmer than average conditions affecting microbial CH4 production of CH₄. This is supported by measurements of CH₄ isotopic composition. So the bottom line is that the atmospheric data are not consistent with a long-term increase in Arctic sources.

This week’s Science podcast features Shakhova discussing the team’s findings [transcript]:

…subsea permafrost acts as a lid – the seal to prevent this methane escape. And being prevented for a period of time, being sealed for a period of time, means that this gas accumulates, and it accumulates under higher pressure –- this is what we have to give an example, this is what we have, for example, this bottle of champagne. So, you have a lot of gas inside, but it’s sealed for a period of time, and when you uncork this bottle, what you can see – it’s different from a bottle of mineral water left open for period of time, it’s just little bit of different. And I think that release of methane from this kind of seabed deposits disturbed by destabilization of subsea permafrost, provides a pathway for this methane – ready to go methane – because its release does not depend on production. It’s not time-dependent, it’s not temperature-dependent, it only needs the pathway to be released.

The release pathway could be melting of the permafrost on the sea floor, or through faults, reefs and other sea-floor features. This graphic from UAF gives an idea of what’s going on:

In the podcast, Shakhova emphasises that this study establishes a baseline against which future methane fluxes can be judged. But she is in no doubt about the stakes. As she says in the press release (and video):

“The release to the atmosphere of only 1% of the methane assumed to be stored in shallow hydrate deposits might alter the current atmospheric burden of methane up to 3 to 4 times. The climatic consequences of this are hard to predict.”

Sounds like understatement to me… Shakhova and Semiletov are currently working to produce figures based on the summer 2009 fieldwork. More coverage of the paper at New Scientist, Times Online, and forHot Topic’s coverage of methane news over the last three years follow the methane tag.

[Update: Forgot to add that Shakhova and Semiletov's chapter in last year's WWF Arctic report provides an excellent overview of the ESAS methane issue.]

http://sciblogs.co.nz/hot-topic/tag/shakova/