Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeFigure 5.14. Transition from tundra (left, 1978) to wetlands (right, 1998) due topermafrost degradation over a period of 20 years (Jorgensen et al., 2001). Photographs,taken from the same location in Tanana Flats in central Alaska, courtesy of NOAA(http://www.arctic.noaa.gov/detect/land-tundra.shtml).models, both global and regional, all withvarying degrees of completeness in terms oftheir representation of permafrost, indicateswidespread large-scale degradation of permafrost(and by extension increased thermokarstdevelopment), sharply increasing ALTs, and acontraction of the area where permafrost can befound near the Earth’s surface during the 21stcentury (Lawrence and Slater, 2005; Euskirchenet al., 2006; Saito et al., 2007; Zhang et al.,2007; Lawrence et al., 2008).6.4 Observed and Modeled Sensitivityof Wetland Methane Emissions toClimate ChangeField studies indicate that methane emissionsdo indeed increase in response to soil warmingand permafrost thaw. Christensen et al. (2003)note that a steady rise in soil temperature willenhance methane production from existingregions of methanogenesis that are characterizedby water tables at or near the surface.While this aspect is important, changes inlandscape-scale hydrology can cause significantchange in methane emissions. For example, ata mire in subarctic Sweden, permafrost thawand associated vegetation changes drove a22–66% increase in CH 4 emissions over theperiod 1970 to 2000 (Christensen et al., 2004).Bubier et al. (2005) estimated that in a Canadianboreal landscape with discontinuous permafrostand ~30% wetland coverage, methane fluxesincreased by ~60% from a dry year to a wetyear, due to changes in wetland water tabledepth, particularly at the beginning and end ofthe summer. Nykänen et al. (2003) also foundhigher methane fluxes during a wetter year ata subarctic mire in northern Finland. Walter etal. (2006) found that thawing permafrost alongthe margins of thaw lakes in eastern Siberiaaccounts for most of the methane released fromthe lakes. This emission, which occurs primarilythrough ebullition, is an order of magnitudelarger where there has been recent permafrostthaw and thermokarst compared to where therehas not. These hotspots have extremely highemission rates but account for only a smallfraction of the total lake area. Methane releasedfrom these hotspots appears to be Pleistoceneage, indicating that climate warming may bereleasing old carbon stocks previously storedin permafrost (Walter et al., 2006). At smallerscales, there is strong evidence that thermokarstdevelopment substantially increases CH 4 emissionsfrom high-latitude ecosystems. Mean CH 4emission rate increases between permafrostpeatlands and collapse wetlands of 13-fold(Wickland et al., 2006), 30-fold (Turetsky etal., 2002), and up to 19-fold (Bubier et al., 1995)have been reported.A number of groups have attempted to predictchanges in natural wetland methane emissionson a global scale. These studies broadly suggestthat natural methane emissions from wetlandswill rise as the world warms. Shindell et al.(2004) incorporate a linear parameterization formethane emissions, based on a detailed processmodel, into a global climate model and find thatoverall wetland methane emissions increased by121 Tg CH 4 y –1 , 78% higher than their baselineestimate. They project a tripling of northernhigh-latitude methane emissions, and a 60%increase in tropical wetland methane emissionsin a doubled CO 2 simulation. The increaseis attributed to a rise in soil temperature in197