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Why the jury’s still out on the risk of Arctic methane catastrophe

Iceberg in Arctic

Arctic Iceberg. Photograph: Delphine Star/getty images


Powered by article titled “Why the jury’s still out on the risk of Arctic methane catastrophe” was written by Nafeez Ahmed, for on Thursday 5th September 2013 19.26 UTC

About a week ago, climate scientist Michael Tobis wrote a critique of my ‘Seven facts about the Arctic methane time bomb‘ following a twitter exchange with him and Chris Colose, author of an article at Skeptical Science arguing that the core scenario of a new Nature paper by Gail Whiteman et. al on the economic costs of Arctic climate change is extremely unlikely.

Much of this debate kicked off because the said Nature paper advances a hypothetical scenario for an abrupt Arctic methane release over either a decade or several decades of about 50 gigatonnes (Gt), and argues specifically that such a scenario is “likely.” My own attempt to understand the literature convinced me that the scenario should be viewed as a serious possibility.

Tobis on the other hand is the latest amongst several scientists offering scathing criticisms of that scenario, which in his own words is “as close to impossible as anything in earth science; actual geophysics refutes it.”

He begins with my first point, 1. The 50 Gigatonne decadal methane pulse scenario was posited by four Arctic specialists, and is considered plausible by Met Office scientists.

Tobis writes that the Review of Geophysics paper I cite says

“Arctic thawing may release in excess of 50 GT of C [Carbon], a very serious matter… But Ahmed refers to the paper in support of a very different assertion, that 50 GT of methane would be released… But the paper to which he points says nothing of the sort. I conclude that he doesn’t really know what he is talking about. Specifically he has already shown that he is confused about the distinction between methane releases and CO2 releases.”

However, the carbon release scenarios from permafrost explored by the paper include both methane and carbon. Here’s what the paper says:

“The most important determinant of whether release of frozen carbon happens as CO2 or CH4 [methane] is whether decomposition proceeds aerobically or anaerobically… In anaerobic conditions, a greater proportion of soil organic carbon decomposition is released as CH4, although not all of it necessarily reaches the atmosphere.”

Following this paragraph, the paper cites several scenarios for large-scale releases from permafrost carbon, including the 50-100 Gt carbon release I mentioned.

Further down, the paper continues:

“Thawing of the terrestrial permafrost will result in CO2 and CH4 emissions on time scales of a few decades to several centuries.”

So Tobis is wrong in assuming that the carbon release scenarios the paper is discussing are only CO2 – that isn’t specified, so I’d assumed the paper was open on whether the 50-100 Gt emissions were methane or carbon.

This was a mistake, however. The paper makes clear that although the scenarios are not clear on the precise quantification of carbon dioxide compared to methane releases from permafrost thawing, methane releases would be only be a small percentage of the overall carbon release scenarios explored. So Tobis is ultimately correct – the paper does not back up the specific scenario endorsed as likely by the Nature paper. I stand corrected on that.

Therefore, the plausibility of the specific 50 Gt scenario rises and falls on the credibility of the four Arctic specialists, including Dr. Natalie Shakhova, who came up with the scenario in the first place. That leaves point 1 only half intact, so we’re left with:

1. The 50 Gigatonne decadal methane pulse scenario was posited by four Arctic specialists

Tobis unfortunately addresses this with only an ad hominem attack on the expertise of these Arctic specialists:


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