Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeand for abrupt changes in methane at othertimes (Fig. 5.6C). Kennett et al. (2000) foundepisodic negative δ 13 C excursions in benthicforaminifera in the Santa Barbara basin, whichthey interpret as reflecting release of hydratemethane during warm climate intervals. Biomarkersfor methanotrophy are found in greaterabundance and indicate greater rates of reactionduring warm intervals in the Santa Barbarabasin (Hinrichs et al., 2003) and in the Japanesecoastal margin (Uchida et al., 2004). Cannariatoand Stott (2004), however, argued that theseresults could have arisen from contaminationor subsequent diagenetic overprints. Hill et al.(2006) measured the abundance of tar in SantaBarbara basin sediments, argued that tar abundancewas proportional to methane emissions,and described increases in tar abundance andinferred destabilization of methane hydratesassociated with warming during the last glacialinterglacialtransition.As discussed in Section 1, there are severalarguments against the hypothesis of a clathraterole in controlling atmospheric methane duringthe last glacial period. Perhaps the most powerfulso far is that the isotopic ratio of deuteriumto hydrogen (D/H) in ice core methane forseveral abrupt transitions in methane concentrationindicates a freshwater source, rather thana marine source, apparently ruling out muchof a role for marine hydrate methane release(Sowers, 2006). However, the D/H ratio has notyet been measured for the entire ice core record.The timing of the deglacial methane rise wasalso more easily explained by wetland emissionsthan by catastrophic methane release (Brook etal., 2000). The interhemispheric gradient ofmethane tells us that the deglacial increase inatmospheric methane arose in part from highnorthern latitudes (Dällenbach et al., 2000),although more work is needed to verify thisconclusion because constraining the gradientis analytically difficult. The deglacial methanerise could therefore be attributed at least in partto methanogenesis from decomposition of thawingorganic matter from high-latitude wetlands.Regardless of the source of the methane, theclimate forcing from the observed methanerecord (Fig. 5.6C and D) is too weak to arguefor a dominant role for methane in the glacialcycles (Brook et al., 2000).4.3 Review of Model Results AddressingPast and Future Methane HydrateDestabilization4.3.1 Climate Impact of PotentialReleaseProbably the most detailed analysis to date ofthe potential for methane release from hydrateson a century time scale is the study of Harveyand Huang (1995). Their study calculated theinventory of hydrate and the potential changein that inventory with an ocean warming. Theytreated as a parameter the fraction of methane inbubbles that could escape the sediment columnto reach the ocean, and evaluated the sensitivityof the potential methane release to that escapedfraction. Our picture of methane release mechanismshas been refined since 1995, although itremains difficult to predict the fate of methanefrom melted hydrates. Harvey and Huang (1995)did not treat the invasion of heat into the oceanor into the sediment column. Their conclusionwas that the radiative impact from hydratemethane will be much smaller than that of CO 2 ,or even between different scenarios for CO 2release. The calculation should be redone, butit is unlikely that an updated calculation wouldchange the bottom-line conclusion.Schmidt and Shindell (2003) showed that thechronic release of methane from a large hydratereservoir over thousands of years can havea significant impact on global climate. Theaccumulating CO 2 from the oxidation of themethane also has a significant climate impact.New CO 2 from methane oxidation accumulatesin the atmosphere/ocean/terrestrial biospherecarbon pool and persists to affect climate forhundreds of thousands of years (Archer, 2005).If a pool of methane is released over a timescale of thousands of years, the climate impactfrom the accumulating CO 2 concentration mayexceed that from the steady-state increase inthe methane concentration (Harvey and Huang,1995; Dickens, 2001a; Schmidt and Shindell,2003; Archer and Buffett, 2005). After theemission stops, methane drops quickly to alower steady state, while the CO 2 persists.If hydrates melt in the ocean, much of themethane would probably be oxidized in theOur picture ofmethane releasemechanisms hasbeen refined since1995, although itremains difficultto predict the fateof methane frommelted hydrates.189