Gas hydrates: Entrance to a methane age or climate threat?
Volker Krey1,*, Josep G Canadell2, Nebojsa Nakicenovic1,3, Yuichi Abe4, Harald Andruleit5, David Archer6, Arnulf Grubler1,7, Neil T M Hamilton8, Arthur Johnson9, Veselin Kostov10, Jean-Francois Lamarque11, Nicholas Langhorne12, Euan G Nisbet13, Brian O'Neill1,14, Keywan Riahi1, Michael Riedel15, Weihua Wang16 and Vladimir Yakushev17
(Received 18 May 2009; accepted 20 August 2009; published 7 September 2009.)
Methane hydrates, ice-like compounds in which methane is held in crystalline cages formed by water molecules, are widespread in areas of permafrost such as the Arctic and in sediments on the continental margins. They are a potentially vast fossil fuel energy source but, at the same time, could be destabilized by changing pressure–temperature conditions due to climate change, potentially leading to strong positive carbon–climate feedbacks. To enhance our understanding of both the vulnerability of and the opportunity provided by methane hydrates, it is necessary (i) to conduct basic research that improves the highly uncertain estimates of hydrate occurrences and their response to changing environmental conditions, and (ii) to integrate the agendas of energy security and climate change which can provide an opportunity for methane hydrates—in particular if combined with carbon capture and storage—to be used as a `bridge fuel' between carbon-intensive fossil energies and zero-emission energies. Taken one step further, exploitation of dissociating methane hydrates could even mitigate against escape of methane to the atmosphere. Despite these opportunities, so far, methane hydrates have been largely absent from energy and climate discussions, including global hydrocarbon assessments and the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.
Methane hydrates (or clathrates) are ice-like compounds in which methane is held in crystalline cages formed by water molecules. They are widespread in areas of permafrost such as the Arctic and in sediments on the continental margins where pressure–temperature conditions are appropriate for their formation. Methane hydrates are an energy source of potentially staggering magnitude compared with other known hydrocarbon deposits. It is thus not surprising that a number of scientific inquiries around the world are evaluating gas hydrates as a potential energy source (Nakicenovic et al 2000). Technologies for recovering these resources are likely to become economically feasible in the not too distant future, in which case gas resource availability would increase enormously (MacDonald 1990).
At the same time, methane hydrates might have played an important role in past climates and could have a significant effect in future human-induced climate change. Methane is a potent greenhouse gas which can be released during extraction and use of methane hydrates as an energy source. In addition, methane can reach the atmosphere through abrupt releases, e.g. due to giant submarine landslides, or chronic releases resulting from warming subsurface sediments. Models of methane dynamics in sediments predict significantly lower methane inventories if the ocean were just a few degrees warmer (Buffett and Archer 2004). Therefore there is an increasing interest in assessing the potential of methane release in a warmer world and its consequences for future climate change (Brook et al 2008, Schiermeier 2008, Westbrook et al 2009).
2. Methane hydrate occurrences
Despite the fact that methane hydrates are potentially an enormous energy source and a factor in global warming, the magnitude of global methane hydrate occurrences and, in particular, their geographical distribution and depth profile are very uncertain (see figure 1 for known hydrate accumulations and global distribution of appropriate conditions for methane hydrate formation). At a recent workshop that brought together experts on both the energy and climate dimensions of methane hydrates (Bohannon 2008), an attempt was made to assign likelihoodsNote19 to occurrence estimates: with high confidence a size exceeding 1000 GtC (1 Gt = 1 billion tons) was considered to be very likely. With medium confidence the global methane hydrate inventory is likely to be in the range of 1000–10 000 GtC (equivalent to ~ 2000–20 000 trillion cubic meters, or ~ 70–700 zettajoules of natural gas). For comparison, the global inventory of fossil fuels including coal is estimated to be around 5000 GtC (Rogner 1997), i.e. in the same order of magnitude as the hydrates inventory alone.
|Figure 1. (a) Distribution of known methane hydrate accumulations (courtesy of Council of Canadian Academies (2008), based on data from Kvenvolden and Rogers (2005)). (b) Estimated global methane hydrate stability zone thickness in seafloor sediments (courtesy of Warren Wood, Naval Research Laboratory). The methane hydrate stability zone indicates where appropriate temperature and pressure conditions for the formation of hydrates can be found (see also inlay of Figure 2).|
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Link to abstract: http://www.iop.org/EJ/article/1748-9326/4/3/034007/erl9_3_034007.html