Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5While most methanehydrates aremarine, the size ofthe contemporaryterrestrial methanehydrate pool,although unknown,may be large.ocean rather than reaching the atmospheredirectly as methane. This reduces the centurytime scale climate impact of melting hydrate,but on time scales of millennia and longer theclimate impact is the same regardless of wherethe methane is oxidized. Methane oxidizedto CO 2 in the ocean will equilibrate withthe atmosphere within a few hundred years,resulting in the same partitioning of the addedCO 2 between the atmosphere and the oceanregardless of its origin. The rate and extent towhich methane carbon can escape the sedimentcolumn in response to warming is verydifficult to constrain at present. It depends onthe stability of the sediment slope to sliding,and on the permeability of the sediment andthe hydrate stability zone’s cold trap to bubblemethane fluxes.4.4 Conclusions About Potential forAbrupt Release of Methane FromMarine HydratesOn the time scale of the coming century, itappears likely that most of the marine hydratereservoir will be insulated from anthropogenicclimate change. The exception is in shallowocean sediments where methane gas is focusedby subsurface migration. The most likelyresponse of these deposits to anthropogenicclimate change is an increased backgroundrate of chronic methane release, rather than anabrupt release. Methane gas in the atmosphereis a transient species, its loss by oxidationcontinually replenished by ongoing release.An increase in the rate of methane emission tothe atmosphere from melting hydrates wouldincrease the steady-state methane concentrationof the atmosphere. The potential rate of methaneemission from hydrates is more speculative thanthe rate from other methane sources, such as thedecomposition of peat in thawing permafrostdeposits, or anthropogenic emission from agricultural,livestock, and fossil fuel industries,but the potential rates appear to be comparableto these sources.5. Terrestrial MethaneHydratesThere are two sources for methane in hydrates,biogenic production by microbes degradingorganic matter in anaerobic environments andthermogenic production at temperatures above110 °C, typically at depths greater than about15 km. Terrestrial methane hydrates are primarilybiogenic (Archer, 2007). They form and arestable under ice sheets (thicker than ~250 m)and within permafrost soils at depths of about150 to 2,000 m below the surface (Kvenvolden,1993; Harvey and Huang, 1995). Their presenceis known or inferred from geophysical evidence(e.g., well logs) on Alaska’s North Slope, theMackenzie River delta (Northwest Territories)and Arctic islands of Canada, the MessoyakhaGas Field and two other regions of westernSiberia, and two regions of northeastern Siberia(Kvenvolden and Lorenson, 2001). Samples ofterrestrial methane hydrates have been recoveredfrom 900 to 1,110 m depth in the Mallikcore in the Mackenzie River delta (Kvenvoldenand Lorenson, 2001; Uchida et al., 2002).5.1 Terrestrial Methane Hydrate PoolSize and DistributionWhile most methane hydrates are marine, thesize of the contemporary terrestrial methanehydrate pool, although unknown, may belarge. Estimates range from less than 10 GtCH 4 (Meyer, 1981) to more than 18,000 Gt CH 4(Dobrynin et al., 1981) (both cited in Harveyand Huang, 1995). More recent estimates are400 Gt CH 4 (MacDonald, 1990), 800 Gt CH 4(Harvey and Huang, 1995), and 4.5–400 GtC;this is a small fraction of the ocean methanehydrate pool size (see Sec. 4).Terrestrial methane hydrates are a potentialfossil energy source. Recovery can come fromdestabilization of the hydrates by warming,reducing the pressure, or injecting a substance(e.g., methanol) that shifts the stability line (seeBox 5.1). The Messoyakha Gas Field in westernSiberia, at least some of which lies in the terrestrialmethane hydrate stability zone, beganproducing gas in 1969, and some production isthought to have come from methane hydrates,though methanol injection made this productionvery expensive (Kvenvolden, 1993; Krason,2000). A more recent review of the geologicalevidence for methane production from hydratesat Messoyakha by Collett and Ginsburg (1998)could not confirm unequivocally that hydratescontributed to the produced gas. Due to lowcosts of other available energy resources, therehad not been significant international industrialinterest in hydrate methane extraction from190