Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5important globally, as the necessary geologicalsetting is rare.Mining terrestrial hydrates for gas productionwill necessarily destabilize them, but presumablymost of this methane will be captured, used, andthe carbon emitted to the atmosphere as CO 2 .5.3 Evidence of Past Release ofTerrestrial Hydrate MethaneNo direct evidence has been identified ofpast release of terrestrial hydrate methane insignificant quantities. Analyses related to thePETM and clathrate gun hypothesis discussedin Section 4 have focused on methane emissionsfrom the larger and more vulnerable marinehydrates. Emissions from terrestrial hydratesmay have contributed to changes in methaneobserved in the ice core record, but there are sofar no distinctive isotopic tracers of terrestrialhydrates, as is the case for marine hydrate (Sowers,2006).5.3.1. Quantity of Methane ReleasedFrom Terrestrial Hydrates in the PastWeitemeyer and Buffett (2006) modeled theaccumulation and release of biogenic methanefrom terrestrial hydrates below the Laurentideand Cordilleran ice sheets of North Americaduring the last glaciation. Methane was generatedunder the ice sheet from anaerobicdecomposition of buried, near-surface soilorganic matter, and hydrates formed if the icesheet was greater than ~250 m thick. Hydratedestabilization arose from pressure decreaseswith ice sheet melting/thinning. They simulatedtotal releases for North America of about40–100 Tg CH 4 , with most of the deglacialemissions occurring during periods of glacialretreat during a 500-year interval around 14 kyrbefore present (BP), and a 2,000-year intervalcentered on about 10 kyr BP. The highestsimulated emission rates (~15–35 Tg CH 4 yr –1 )occurred during the dominant period of icesheet melting around 11–9 kyr BP.Shakova et al. (2005) measured supersaturatedmethane concentrations in northern Siberiancoastal waters. This supersaturation is thoughtto arise from degradation of coastal shelfhydrate, hydrate that had formed in permafrostwhen the shelf was exposed during low sealevel of the last glacial maximum. Methaneconcentrations in the Laptev and East SiberianSeas were supersaturated up to 800% in 2003and 2,500% in 2004. From this and an empiricalmodel of gas flux between the atmosphere andthe ocean, they estimated summertime (i.e.,ice-free) fluxes of up to 0.4 Mg CH 4 km –2 y –1 (or0.4 g CH 4 m –2 y –1 ). They assume that the methaneflux from the sea floor is of the same orderof magnitude and may reach 1–1.5 g CH 4 m –2y –1 . These fluxes are low compared to wetlandfluxes (typically ~1–100 g CH 4 m –2 y –1 ; Bartlettand Harriss, 1993), but applied across the totalarea of shallow Arctic shelf, the total annualflux for this region may be as high as 1–5 TgCH 4 y –1 , depending on degree of oxidation inthe seawater. (See Table 5.1 above for globalmethane emissions by source.)5.3.2 Climate Impact of Past MethaneRelease From Terrestrial HydratesMost studies of climate impacts from possiblepast methane hydrate releases have consideredlarge releases from marine hydrates (see Sec. 4above). It is generally not well known whatfraction of the methane released from hydratedestabilization is either trapped in overlyingsediments or oxidized to carbon dioxide beforereaching the atmosphere (Reeburgh, 2004), andthe same considerations are relevant to releasefrom terrestrial sources.Weitemeyer and Buffett (2006) estimatedintervals of 500–2,000 years when methanehydrate destabilization from retreat of the NorthAmerican ice sheet caused increases of atmosphericmethane of 10–200 ppb, with the largestperturbation at 11–9 kyr before present. Anyeffect of methane oxidation before reaching theatmosphere was ignored; this oxidation wouldhave reduced the impact on the atmosphericmethane burden. This atmospheric perturbationis equivalent to about 2–25% of pre-industrialHolocene atmospheric methane burdens, androughly equivalent to a radiative forcing of0.002–0.1 W m –2 (using contemporary valuesfor methane radiative efficiency and indirecteffects from Ramaswamy et al., 2001).192