Note: This is a response to my piece published yesterday: Methane mischief: Misleading commentary published in Nature.  The author, Peter Wadhams, is a professor of ocean physics at Cambridge University.

I do not agree that our commentary was misleading. What we did was take one prediction of the magnitude and timing of methane emissions from the thawing of Arctic offshore permafrost (a prediction made by the person who has done more field work on this part of the ocean bed than anyone else) and calculated the financial implications over a century for the world economy. Also we calculated its effect on increasing overall global warming, obtaining a 0.6C figure by 2040. We rightly consider these to be substantial figures which deserve wide circulation among climate scientists, and Nature and its referees agreed with us.

In our analysis we showed (a) that for a given total volume of release, the overall cost is relative insensitive to details of the rate of release or, within limits, its timing, BUT that (b) the overall cost is roughly proportional to the overall volume of release. Thus if you or anyone else think that you have a better knowledge of the East Siberian seabed than Shakhova and can come up with a better figure for the total release to be expected, you can substitute your lower figure and scale the 60 trillion dollars accordingly. I suspect the cost will still be substantial – and that is one clear finding, that the planetary cost of Arctic warming far outstrips the benefits (from shipping and oil exploration) that have been talked about so confidently by some politicians.

In support of your skepticism about methane emissions you quote authors who wrote before the enormous retreat of summer Arctic sea ice and its oceanographic effects became so evident. 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.

We can therefore dismiss the 2008 US Climate Change Science Program report for this reason. Equally, David Archer’s 2010 comment that “so far no one has seen or proposed a mechanism to make that (a catastrophic methane release) happen” is also rendered obsolete by the Semiletov/Shakhova field experiments – the seeing – and the mechanism described above – the proposing. Carolyn Rupple’s review of 2011 equally shows no awareness of this new mechanism.

Therefore I robustly defend what we did in this commentary, and we hope that rather than a destructive chorus of denial, the response of the Arctic science community might be to support more intensive research on this problem. We are doing our bit by feeding some financial support to the Semiletov (University of Alaska/Pacific Ocean Lab Vladivostok) field research group from a new European Union programme in which we three authors are partners.