Wednesday 31 July 2013

Discussing the methane threat

Arctic methane catastrophe scenario is based on new empirical observations
Critics of new Nature paper on costs of Arctic warming ignore latest science on permafrost methane at everyone's peril

Nafeez Ahmed

31 July, 2013

Last week, the journal Nature published a new paper warning of a $60 trillion price tag for a potential 50 Gigatonne methane pulse from the East Siberian Arctic Shelf (ESAS) over 10-50 years this century. The paper, however, prompted many to suggest that its core scenario - as Arctic permafrost thaws it could increasingly unleash dangerous quantities of methane from sub-ice methane hydrates in as quick as a decade - is implausible.

The Washington Post's Jason Samenow argued that "most everything known and published about methane indicates this scenario is very unlikely." Andrew Revkin of the New York Times (NYT) liberally quoted Samenow among others on "the lack of evidence that such an outburst is plausible." Similarly, Carbon Brief concluded: "The scientists we spoke to suggested the authors have chosen a scenario that's either implausible, or very much at the upper limit of what we can reasonably expect."
Both the Post and NYT quoted Prof David Archer, an expert on ocean sediments and methane at the University of Chicago:
"For methane to be a game-changer in the future of Earth's climate, it would have to degas to the atmosphere catastrophically, on a time scale that is faster than the decadal lifetime of methane in the air. So far no one has seen or proposed a mechanism to make that happen."
Dr Vincent Gauci, a methane expert at Open University, similarly argued:
"It's not a given all the methane will end up in the atmosphere. Some could be oxidised [broken down] in the water by bacteria, and some could remain in the sediments on the seafloor."
The problem is that these reservations are based on outdated assumptions that sea floor released methane would not make it into the atmosphere - but all the new fieldwork on the levels of methane being released above the ESAS shows this assumption is just empirically wrong.
Atmospheric methane levels in the Arctic are currently at new record highs, averaging about 1900 parts per billion, 70 parts per billion higher than the global average. NASA researchers have found local methane plumes as large as 150 kilometres across - far higher than previously anticipated.
Dr Gavin Schmidt, climate modeller at NASA, was also cited claiming lack of evidence from ice cores of previous catastrophic methane pulses in the Earth's history in the Early Holocene or Eamian, when Arctic temperatures were warmer than today. But the blanket references to the past may well be irrelevant. In the Early Holocene, the ESAS was not an underwater shelf but a frozen landmass, illustrating the pointlessness of this past analogy with contemporary conditions.
Dr Schmidt also overlooked other issues - such as new research showing that the warm, Eamian interglacial period some 130,000 years ago should not be used as a model for today's climate due to fundamental differences in the development of the Arctic ocean. Ice core methane records are also too short to reach back to the entire Cenozoic - another reason suggesting lack of past evidence is no basis for present complacency; and even Prof Archer himself recognises that ice cores will not necessarily capture a past catastrophic methane release due to fern diffusion.
Finally, the Post and NYT refer to a range of scientific publications - a 2008 report by the US Climate Change Science Programme and a 2011 review of the literature by Carolyn Rupple also in the journal Nature - essentially arguing that a catastrophic methane release would be, for all intents and purposes, impossible within such a short time-frame, with actual methane releases taking place over hundreds if not thousands of years.
Yet in my interview with Prof Peter Wadhams, co-author of the Nature study and head of Polar ocean physics at Cambridge University, he told me that the scientists who rejected his scenario as implausible were simply unacquainted with the unique dynamics of the East Siberian Arctic Shelf, the nature of permafrost melting there, and its relationship to ongoing releases of methane in recent years which have been wholly unexpected within established models based on reconstructions of Earth's historical climate:
"Those who understand Arctic seabed geology and the oceanography of water column warming from ice retreat do not say that this is a low probability event. I think one should trust those who know about a subject rather than those who don't. As far as I'm concerned, the experts in this area are the people who have been actively working on the seabed conditions in the East Siberian Sea in summer during the past few summers where the ice cover has disappeared and the water has warmed. The rapid disappearance of offshore permafrost through water heating is a unique phenomenon, so clearly no 'expert' would have found a mechanism elsewhere to compare with this... I think that most Arctic specialists would agree that this scenario is plausible."
In a rebuttal to the original Post article, Wadhams points out that none of the scientists rejecting his scenario understand the unique mechanism currently at play in the Arctic, and all were citing research preceding the empirical evidence which unearthed this mechanism - which has only become clear in recent years in the context of the rapid loss of summer sea ice.
While Wadhams refers directly to an actual empirical phenomenon unique to the Arctic seabed resulting in unprecedented methane venting - uncovered by Dr Natalia Shakhova and Dr Igor Semiletov of the International Arctic Research Center - the critics refer instead to general theoretical dynamics of methane release but show little awareness of what's actually going on in the north pole:
"The mechanism which is causing the observed mass of rising methane plumes in the East Siberian Sea is itself unprecedented and hence it is not surprising that various climate scientists, none of them Arctic specialists, failed to spot it. What is actually happening is that the summer sea ice now retreats so far, and for so long each summer, that there is a substantial ice-free season over the Siberian shelf, sufficient for solar irradiance to warm the surface water by a significant amount – up to 7C according to satellite data.
That warming extends the 50 m or so to the seabed because we are dealing with only a polar surface water layer here (over the shelves the Arctic Ocean structure is one-layer rather than three layers) and the surface warming is mixed down by wave-induced mixing because the extensive open water permits large fetches.
So long as some ice persisted on the shelf, the water mass was held to about 0C in summer because any further heat content in the water column was used for melting the ice underside. But once the ice disappears, as it has done, the temperature of the water can rise significantly, and the heat content reaching the seabed can melt the frozen sediments at a rate that was never before possible. The authors who so confidently dismiss the idea of extensive methane release are simply not aware of the new mechanism that is causing it."
Wadhams thus describes the previous research dismissing the methane threat by Rupple and others as "rendered obsolete by the Semiletov/Shakhova field experiments - the seeing - and the mechanism described above."
So far, cutting edge peer-reviewed research on the link between Arctic permafrost melt and methane release has received no attention from these critics. Indeed, their offhand dismissals are based on ignoring the potential implications of the specific empirical evidence on the ESAS emerging over the last few years, which challenges the assumptions of conventional modelling.

No comments:

Post a Comment

Note: only a member of this blog may post a comment.