Book 2.indb - US Climate Change Science Program

Abrupt Climate Change1970 to 2000 (Kvenvolden, 2000), and thefraction of terrestrial methane hydrate that isor will be technically and economically recoverableis not well established. In the UnitedStates, the Methane Hydrate Research andDevelopment Act of 2000 and its subsequent2005 Amendment have fostered the NationalMethane Hydrates R&D Program, supporting awide range of laboratory, engineering, and fieldprojects with one focus being on developing theknowledge and technology base to allow commercialproduction of methane from domestichydrate deposits by the year 2015, beginningwith Alaska’s North Slope. Estimates of technicallyand economically recoverable methane inhydrates are being developed (Boswell et al.,2005; Boswell, 2007).5.2 Mechanisms To DestabilizeTerrestrial Methane HydratesTerrestrial methane hydrates in permafrostare destabilized if the permafrost warms sufficientlyor if the permafrost hydrate is exposedthrough erosion (see Box 5.3). Destabilizationof hydrates in permafrost by global warmingis not expected to be significant over the nextfew centuries (Nisbet, 2002; see Sec. 5.4).Nisbet (2002) notes that although a warmingpulse will take centuries to reach permafrosthydrates at depths of several hundred meters,once a warming pulse enters the soil/sediment,it continues to propagate downward and willeventually destabilize hydrates, even if theclimate has subsequently cooled.Terrestrial methane hydrates under an ice sheetare destabilized if the ice sheet thins or retreats.The only globally significant ice sheets nowexisting are on Greenland and Antarctica; mapsof the global distribution of methane hydratesdo not show any hydrates under either ice sheet(Kvenvolden, 1993). It is likely, however, thathydrates formed under Pleistocene continentalice sheets (e.g., Weitemeyer and Buffett, 2006;see Sec. 5.3.1).Terrestrial methane hydrates can also be destabilizedby thermokarst erosion (a melt-erosionprocess) of coastal-zone permafrost. Ice complexesin the soil melt where they are exposedto the ocean along the coast, the land collapsesinto the sea, and more ice is exposed (Archer,2007). The Siberian coast is experiencing veryhigh rates of coastal erosion (Shakhova et al.,2005). Methane hydrates associated with thispermafrost become destabilized through thisprocess, and methane is released into the coastalwaters (Shakhova et al., 2005). Magnitudes ofthe emissions are discussed below.De Batist et al. (2002) analyzed seismic reflectiondata from Lake Baikal sediments, theonly freshwater nonpermafrost basin knownto contain gas hydrates, and infer that hydratedestabilization is occurring in this tectonicallyactive lacustrine basin via upward flow ofhydrothermal fluids advecting heat to the baseof the hydrate stability zone. If occurring, thismeans of destabilization is very unlikely to beTerrestrialmethane hydratesin permafrost aredestabilized if thepermafrost warmssufficiently or if thepermafrost hydrateis exposed througherosion.191