Wetland vegetation is adapted to the presence of a perennial near-surface water table, which creates standing bodies of water or saturated (waterlogged) soils. The location and areal coverage of wetlands are uncertain, due to the lack of a universally agreed upon definition in view of the wide diversity of wetland vegetation communities and soil types (Williams, 1990). Estimates of the global extent of wetland areas range from 5.26 million km2 (Matthews and Fung, 1987) to 8.56 million km2 (Williams, 1990).
The accumulation of organic-rich sediments in undisturbed wetlands constitutes a significant carbon sink. On the other hand, draining of swamps, bogs, or marshlands leads to oxidation of soil organic matter. Burning of vegetation, including peat for fuel, as well as agricultural conversion also releases C02 from wetlands.
Prior to large-scale conversion of forest clearance for agriculture (c. 1795), temperate wetlands accumulated carbon at rates of 56.9 to 82.6 million tons C/yr (Armentano and Menges, 1986). By 1980, annual carbon accumulation rates had decreased to —6.5 million tC/yr (i.e., a source of carbon) to 19.1 million tC/yr, representing a net reduction of 63.4 tC/yr in carbon storage from temperate wetlands (Armentano and Menges, 1986). Tropical clearance of wetlands, especially during the last several decades, brings the overall global reduction in annual carbon storage rates, relative to preclearance values, to around 150-184 million tC/yr (Armentano and Menges, 1986). If the ratio of annual carbon emission rate in temperate regions to the global rate (c. 1980) is assumed to be proportional to the temperate-to-global ratio of the difference between present carbon storage rates and preclearance rates, then the annual global emissions of carbon from wetland drainage and conversion could range between 22 and 101 million tC/yr.
Water released by drainage and burning of wetlands (computed like that of deforestation; see above) yields 0.03-0.15 km3 H20 per year, equivalent to 0.0001-0.0004 mm/yr SLR. With a dry-to-wet biomass ratio of 0.25, adding in the dry biomass brings the total SLR due to wetland drainage and oxidation to 0.001-0.002 mm/yr (Table 5.4). Possible changes in runoff resulting from wetland clearance have not been included, because of lack of relevant data.
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