Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 5DSC_9347.JPGcould be sufficient to cause emissions variationsinferred from ice core data. MacDonald et al.(2006) presented a compilation of basal peatages for the circumarctic and showed that peataccumulation started early in the deglaciation(at about 16,000 years before present), andtherefore emissions of methane from NorthernHemisphere peat ecosystems very likely playeda role in the methane increase at the end ofthe last ice age. The coincidence of peatlanddevelopment and the higher Northern Hemispheresummer insolation of late glacial andearly Holocene time supports the hypothesisthat such wetlands were methane sources atprevious times of higher Northern Hemispheresummer insolation (MacDonald et al., 2006),for example during insolation and methanepeaks in the last ice age or at previous glacialinterglacialtransitions (Brook et al., 1996,2000). In summary, although the sedimentaryrecord of wetlands and the factors controllingmethane production in wetlands are imperfectlyknown, it appears likely that wetlands wereimportant in the pre-Holocene methane budget.The clathrate gun hypothesis is important forunderstanding the future potential for abruptchanges in methane—concern for the nearfuture is warranted if the clathrate reservoirwas unstable on the time scale of abrupt lateQuaternary climate change. However, as anexplanation for late Quaternary methane cycles,the clathrate gun hypothesis faces several challenges,elaborated upon further in Section 4.First, the radiative forcing of the small variationsin atmospheric methane burden duringthe ice age should have been quite small (Brooket al., 2000), although it has been suggestedthat impacts on stratospheric water vapor mayhave increased the greenhouse power of thesesmall methane variations (Kennett et al., 2003).Second, the ice core record clearly shows thatthe abrupt changes in methane lagged the abrupttemperature changes in the Greenland ice corerecord, albeit by only decades (Severinghauset al., 1998; Severinghaus and Brook, 1999;Huber et al., 2006; Grachev et al., 2007). Theseobservations imply that methane is a feedbackto, rather than a cause of, warming, rulingout one aspect of the clathrate gun hypothesis(hydrates as trigger), but they do not constrainthe cause of the abrupt shifts in methane. Third,isotopic studies of ice core methane do notsupport methane hydrates as a source for abruptchanges in methane (Sowers, 2006; Schaeferet al., 2006). The strongest constraints comefrom hydrogen isotopes (Sowers, 2006) and aredescribed further in Section 4.3. Potential Mechanismsfor Future AbruptChanges in AtmosphericMethaneThree general mechanisms are consideredin this chapter as potential causes of abruptchanges in atmospheric methane in thenear future large enough to cause abruptclimate change. These are outlined brieflyin this section, and discussed in detail inSections 4–6.3.1 Destabilization of MarineMethane HydratesThis issue is probably the most well knowndue to extensive research on the occurrence ofmethane hydrates in marine sediments, and thelarge quantities of methane apparently presentin this solid phase in continental-margin marinesediments. Destabilization of this solid phaserequires mechanisms for warming the depositsand/or reducing pressure on the appropriatetime scale, transport of free methane gas to thesediment-water interface, and transport to theatmosphere (see Box 5.1). There are a numberof physical impediments to abrupt release, inaddition to the fact that bacterial methanotrophyconsumes methane in oxic sediments andthe ocean water column. Warming of bottomwaters, slope failure, and their interaction arethe most commonly discussed mechanisms forabrupt release.176