There's a new paper out:
Permafrost soils contain an estimated 1,700 Pg of carbon, almost twice the present atmospheric carbon pool1. As permafrost soils thaw owing to climate warming, respiration of organic matter within these soils will transfer carbon to the atmosphere, potentially leading to a positive feedback2. Models in which the carbon cycle is uncoupled from the atmosphere, together with one-dimensional models, suggest that permafrost soils could release 7–138 Pg carbon by 2100 (refs 3, 4). Here, we use a coupled global climate model to quantify the magnitude of the warming generated by the feedback between permafrost carbon release and climate. According to our simulations, permafrost soils will release between 68 and 508 Pg carbon by 2100. We show that the additional surface warming generated by the feedback between permafrost carbon and climate is independent of the pathway of anthropogenic emissions followed in the twenty-first century. We estimate that this feedback could result in an additional warming of 0.13–1.69 °C by 2300. We further show that the upper bound for the strength of the feedback is reached under the less intensive emissions pathways. We suggest that permafrost carbon release could lead to significant warming, even under less intensive emissions trajectories.Between 68-508 Pg, or 68-508 billion tons of carbon, or, if it all comes out as CO2 (and you'd better hope it does, more or less), 255 to 1,910 billion tons of CO2. For comparison, human emissions in 2010 amounted to 9.1 billion tons of carbon. This feedback could be thought of like continuing on with our current emissions for between seven to fifty-five years, except we don't get any choice in the matter.
This is higher than most of the previous estimates I've seen. Koven et al (2011), for example, estimated 55-69 Pg C of carbon. There are a number of other estimates from a variety of sources using a number of methods. From "Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle":
Risk assessments, based on expert opinion, estimated that up to 100 Pg C could be released from thawing permafrost by 2100 (Gruber et al. 2004). On the basis of laboratory incubation experiments and estimated C stocks, Dutta and colleagues (2006) calculated a potential release of about 40 Pg C over four decades if 10% of the C stock frozen in deep soils in Siberia thawed to 5°C. Tarnocai (2006) estimated that 48 Pg C could be released from Canadian permafrost over this century if the mean annual air temperature increased by 4°C. Model predictions incorporate changes in vegetation and other disturbances, as well as C release from permafrost, to determine the net effect of climate warming. Results for Alaska and for the circumpolar region predict the addition of up to 50 to 100 Pg C to the atmosphere by the end of the century, depending on the particular model scenario (Stieglitz et al. 2003, Zhuang et al. 2006).In other bad news from the Arctic permafrost, Vonk et al (2012) found breakdown of Siberian "Yedoma" permafrost dumping ten times as much CO2 into the Arctic ocean compared to prior estimates.
All of this has, for me, a distinct hint of the Arctic sea ice narrative, to the tune of "Oh-yes-change-will-come-we-see-it-in-the-record-a-few-thousand-years-maybe-OK-maybe-faster-OK-now-we're-getting-good-direct-measurements-could-be-an-issue-in-a-century-or-two-OK-wait-what-WHATTHEHELLISTHAT?"
The analogy is hardly even an analogy at all. The permafrost most at risk, after all, is just another species of Arctic ice. And it should be as clear as crystal that the Arctic is changing faster than experts thought possible just a few years ago. The bits of it impregnated with thousands of gigatonnes of carbon are no exception. Is it a methane bomb, a carbon bomb? It doesn't need to be. When the house is on fire, everything burns.