Book 2.indb - US Climate Change Science Program

Abrupt Climate Changevariations observed in ice cores. Both Kaplanet al. (2006) and Valdes et al. (2005) proposedthat the glacial-interglacial methane changecannot be explained entirely by changes inemissions from wetlands, because in theirglobal climate-biosphere models the differencebetween Last Glacial Maximum (LGM) andearly Holocene methane emissions is not largeenough to explain the observed changes inthe ice core record. Both studies explain thisapparent paradox by invoking increased productionof volatile organic carbon (VOC) fromthe terrestrial biosphere in warmer climates.VOCs compete with methane for reaction withOH, increasing the methane lifetime and thesteady-state methane concentration that can bemaintained at a given emission rate. Neither ofthese studies is directly relevant to the abruptchanges in the ice core record, and there areconsiderable uncertainties in the modeling.Nonetheless, further work on the role of changesin the methane sink on time scales relevant toabrupt methane changes is warranted.The wetland hypothesis has been challengedby authors calling attention to the large marineand terrestrial hydrate reservoirs. The challengewas most extensively developed by Kennett etal. (2003), who postulated that the abrupt shiftsin methane in the ice core record were causedby abrupt release of methane from methanehydrates in sea-floor sediments on continentalmargins. This hypothesis originated fromobservations of negative carbon isotope excursionsin marine sediment records in the SantaBarbara basin, which apparently coincided withthe onset of abrupt warming in Greenland andincreases in atmospheric methane in the icecore record. The “clathrate gun hypothesis”postulates that millennial-scale abrupt warmingduring the last ice age was actually driven byatmospheric methane from hydrate release, andfurther speculates on a central role for methanein causing late Quaternary climate change(Kennett et al., 2003).Some proponents of the clathrate gun hypothesisfurther maintain that wetlands were notextensive enough during the ice age to be thesource of the abrupt variations in methane inthe ice core record. For example, Kennett etal. (2003) maintain that large accumulations ofcarbon in wetland ecosystems are a prerequisitefor significant methanogenesis and that theseestablished wetlands are exclusively a Holocenephenomenon. Process-based studies of methaneemissions from wetlands, on the other hand,emphasize the relationship between annualproductivity and emissions (e.g., Christensenet al., 1996). In this view, methane productionis closely tied to the production of labile carbon(Schlesinger, 1997) in the annual productivitycycle (Christensen et al., 1996). From thisperspective, it has been postulated that the icecore record reflects changes in rainfall patternsand temperature that could quickly influencethe development of anoxic conditions, plantproductivity, and methane emissions in regionswhere the landscape is appropriate for developmentof water-saturated soil (e.g., Brook et al.,2000; van Huissteten, 2004).The hypothesis that there was very littlemethane emission from wetlands prior to theonset of the Holocene is at odds with modelsof both wetland distribution and emissionsfor pre-Holocene times, the latter indicatingemissions consistent with, or exceeding, thoseinferred from the ice core record (e.g., Chappellazet al., 1993b; Kaplan, 2002; van Huissteten,2004; Valdes et al., 2005; Kaplan et al., 2006).Van Huissteten (2004) specifically consideredmethane emissions during the stadial andinterstadial phases of Marine Isotope Stage 3(~30,000–60,000 years ago), when ice coredata indicate that several rapid changes inatmospheric methane occurred (Fig. 5.6C).Van Huissteten describes wetland sedimentarydeposits in northern Europe dating from thisperiod and used a process-based model toestimate methane emissions for the cold andwarm intervals. The results suggest that emissionsfrom Northern Hemisphere wetlands231290main_Permafrostearth_HI.jpgThe “clathrategun hypothesis”postulates thatmillennial-scaleabrupt warmingduring the lastice age wasactually drivenby atmosphericmethane fromhydrate release, andfurther speculateson a central role formethane in causinglate Quaternaryclimate change.175