A number of times in recent months I have found myself explaining my research to somebody when they suddenly interject with the comment “Oh, so you’re studying the arctic methane time bomb.” The first time this happened I was confused and not quite sure how to respond, because while I do study processes that could result in methane release from thawing arctic permafrost soils, I wouldn’t call it a time bomb so much as a slow feedback. As I talked to more people, I slowly realized that this comment actually stemmed from a rather profound misconception. There are two possible sources of methane (CH4) release from the arctic that could be triggered by climate change: methane hydrate release, and permafrost carbon thaw. These two potential methane sources stem from very different pools of carbon and are controlled by very different environmental processes. As a result, they have very different sets of risks associated with them. I study permafrost thaw, but most people who have heard anything about the arctic and methane release seem to have heard about methane hydrates. Let me do my best to explain the difference:
Methane hydrates are concentrated solid deposits of frozen methane which are formed under high pressures and low temperatures. Most (<98.75%) methane hydrates are found in deep ocean sediments, but 1% are estimated to exist at depths greater than 300m below arctic permafrost soils. An additional very small amount (likely >0.25%) exist on arctic continental shelves. Rough estimates suggest that there may be approximately 1800 Pg C stored in methane hydrates globally (Ruppel, 2011). While this total quantity of methane is very large (representing more than the total carbon stored in permafrost soils), most of it is well insulated from the warming effects of climate change by a combination of layers of ocean sediment and deep water above it. The paleo-record gives no indication that such deep methane hydrates have been catastrophically released by previous extreme arctic warming (Colose, 2013). The slightly more than one percent of methane hydrates which exist on land or on continental shelves may be at risk of release due to the extreme warming (7-8ºC) projected for the Arctic (IPCC 2013). However, the process of methane release due to warming will be slow and unlikely to release the entire store of methane instantaneously (Ruppel, 2011). Because methane in the atmosphere only has a 10 year lifespan before it is oxidized, such a small and slow release of methane is unlikely to dramatically increase atmospheric methane concentrations (Ruppel, 2011). Furthermore, the methane released from such deep stores has the potential to be consumed my microbial processes and may never reach the surface, further decreasing its impact (Ruppel, 2011). Thus, the risk of catastrophic methane release from hydrates is very low.
In contrast, methane from permafrost organic matter is not stored as methane itself but as organic molecules in frozen soils either on ocean shelves or on land in permafrost. It is estimated that permafrost soils globally hold approximately 1700 Pg C which is more than twice the amount of carbon currently in the atmosphere (Schuur et al 2008). When permafrost soils thaw, the carbon stored in them is exposed to conditions which promote much faster microbial decomposition (Schuur et al 2008). However, the ultimate fate of this carbon depends heavily on environmental conditions. In dry soils, aerobic decomposition proceeds relatively quickly and produces carbon dioxide (CO2) gas. However if permafrost thaw results in wetland formation, anaerobic decomposition proceeds more slowly and produces a mixture of CO2 and CH4 gases. Permafrost thaw may also result in nutrient release and environmental changes that drive a shift in dominant plant communities to species which store more carbon (Schuur et al 2008). Therefore, the net carbon balance of thawing permafrost systems will depend on the amount of carbon uptake due to plant growth as well as the degree and rate of carbon release due to decomposition. Likewise, the rate of CH4 release from these ecosystems will depend on the area of land which is converted to wetlands as a result of permafrost thaw and the length of time which they remain wetlands. If we consider that globally, wetlands are the single largest source of atmospheric methane (177-284 Tg CH4/yr), and that it has been established that permafrost thaw will result in the creation wetlands in many arctic areas (Schuur et al 2008), arctic warming is very likely to increase CH4 emissions to the atmosphere. These emissions will not occur as a sudden release but as a steady long-term increase. In those areas which do not become wetlands, there is a high risk of greatly increased CO2 emissions from thawed permafrost soils.
So what is the arctic “time bomb,” and why do people confuse it with the possibility of carbon release from arctic permafrost thaw? Last year Whiteman et al (2013) published an article in Nature Commentary describing a “time bomb” consisting of severe global economic impacts from a potential 50 Pg release of methane hydrates either suddenly or over the course of 50 years. The article created a lot of controversy (see articles by Colose, Samenow, and Ahmed) primarily because the authors gave no indication of the likelihood of such an event. While there is not perfect agreement, most scientists consider the likelihood of a sudden catastrophic release of methane hydrates to be very low (Colose 2013). But the prediction of such an extreme event and the ensuing controversy caught the eye of the media. The result is that most people who have heard anything about CH4 release from the arctic have heard about methane hydrates, not the possibility of CH4 release due to the decomposition of organic material in thawing permafrost.
Overall, Arctic warming does have the potential to be a significant positive feedback to global warming, but it is unlikely to come in the form of a sudden, catastrophic release of methane hydrates. The strength and type of the positive feedback will depend primarily on the precise factors controlling the decomposition of carbon compounds released from permafrost soils as they thaw. Decomposition of this material will likely occur very slowly, be incomplete, and produce a mixture of CO2 and CH4 gases. What the precise impacts will be is still very much unknown. That is why groups such as ours are studying the different factors that control the fate of carbon that is released from permafrost.
Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao and P. Thornton, 2013: Carbon and Other Biogeochemical Cycles. In: Cli-mate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://www.ipcc.ch/report/ar5/wg1/
Colose, Chris (2013): http://www.skepticalscience.com/news.php?p=2&t=66&&n=2130
Ruppel, C. D. “Methane hydrates and contemporary climate change.” Nature Education Knowledge 3.10 (2011): 29. http://pm22100.net/docs/pdf/enercoop/energie/gaz/130316_Methane_Hydrates_and_Contemporary_Climate_Change.pdf
Schuur, Edward AG, et al. “Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle.” BioScience 58.8 (2008): 701-714. http://bioscience.oxfordjournals.org/content/58/8/701.short
Whiteman, Gail, Chris Hope, and Peter Wadhams. “Climate science: Vast costs of Arctic change.” Nature 499.7459 (2013): 401-403. http://www.nature.com/nature/journal/v499/n7459/full/499401a.html