Modelling CH4 emissions from arctic wetlands: effects of hydrological parameterisation

Together with water vapour and carbon dioxide (CO2), methane (CH4) is an important contributor to the warming of the atmosphere. The atmospheric mixing ratios of so-called greenhouse gases, CO2, and nitrous oxide (N2O) have increased about 31%, and 17%, respectively, above pre-industrial values, whereas CH4 has increased 151 ± 25%, (Watson et al., 2001). The CH4 concentration in 2005 of about 1774 ppb is more than double its pre-industrial value. Increases in atmospheric CH4 concentrations since pre-industrial times have contributed a radiative forcing of +0.48 ± 0.05 Wm. Current atmospheric CH4 levels are due to continuing anthropogenic emissions of CH4, which are greater than natural emissions. Emissions from individual sources of CH4 are not as well quantified as the total emissions, but are mostly biogenic and include emissions from wetlands, ruminant animals, rice agriculture and biomass burning, with smaller contributions from industrial sources including fossil fuel-related emissions (Solomon et al., 2007). About 60% of global CH4 emissions come from human-influenced sources and the rest are from natural sources (Houghton et al., 2001). Natural sources include wetlands, termites, oceans, and hydrates. Natural sources are dominated by wetlands. Where soils are waterlogged and oxygen is absent, methanogenic micro-organisms produce large amounts of CH4 as they respire organic matter to CO2 to derive energy. Wetland CH4 emissions are thought to comprise around 80 percent of the total natural CH4 source. Total annual CH4 emissions from natural sources are estimated to be around 250 Tg (Reay, 2006). In the past decade the overall annual rate of CH4 growth has decreased and become highly variable (Dlugokencky et al, 2003; Ciais et al., 2005). Ciais et al. (2005) attributes the decrease to a temporary reduction in anthropogenic emissions and the increased variability to wetland emission distribution. The largest CH4 atmospheric mixing ratios are north of 40 N (Steele et al., 1987). This distribution coincides with the concentration of wetlands in the northern hemisphere and suggests that wetlands in this area may make a significant contribution to the global CH4 budget (Moore and Knowles, 1990; Aselmann and Crutzen, 1989; Crill et al., 1988; Matthews and Fung, 1987). The magnitude of the CH4 emissions from wetlands is controlled by the dynamic balance between CH4 production and oxidation rates in the peat profile and by transport mechanisms (Bubier and Moore, 1994). Measured emissions demonstrate high spatial and temporal variation (Moore et al., 1990; Whalen and Reeburg, 1992; Dise, 1993) linked to environmental factors such as variation in temperature and ground water level.