Wetlands

Abstract

Wetlands are most likely the largest natural source of methane to the atmosphere accounting for ~20% of the current global annual emission of ~450–550 Tg (10¹² g) (Khalil and Rasmussen, 1983; Cicerone and Oremland, 1988; Fung et al., 1991; Crutzen, 1991; Houghton et al., 1996). Measurements of methane from Greenland and Antarctic ice cores indicate atmospheric concentrations of ~350 ppbv during the Last Glacial Maximum rising to 650 ppbv during the pre-industrial Holocene (Stauffer et al., 1988; Chappellaz et al., 1990). Preindustrial source strengths of methane, consistent with historical concentrations and estimates based on isotopes, have been estimated at ~180–380 Tg methane annually (Khalil and Rasmussen,1987; Stauffer et al., 1985; Chappellaz et al., 1993). Wetlands were the dominant preindustrial source with smaller contributions from wild fires, animals and oceans. During the last two hundred years, atmospheric methane concentrations have more than doubled to ~1750 ppbv (Etheridge et al., 1992). Annual increases of ~0.6% yr⁻¹ in the 1980s declined to ~0.1% yr⁻¹ in the 1990s (Rasmussen and Khalil, 1981; Craig and Chou, 1982; Khalil and Rasmussen, 1982, 1983, 1985; Ehhalt et al., 1983; Rasmussen and Khalil, 1984; Stauffer et al., 1985; Blake and Rowland, 1986, 1988; Pearman et al., 1986; Steele et al., 1987; Blake et al., 1988; Khalil et al., 1989; Lang et al., 1990a,b; Dlugokencky et al., 1994a,b, 1997). Currently, the total annual emission of methane is about twice that estimated for the preindustrial period, but both the relative and absolute contribution of wetlands is smaller than in the past due to increases in anthropogenic sources and reductions in wetland areas. However, climate and related biological interactions that presently control the distribution of wetlands and their methane emissions are expected to change during the next 50 to 100 years.