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Pressures on wetlands are likely to be mediated through changes in hydrology, direct and indirect effects of changes in temperature, and land-use change. There would be interactions among these pressures and subsequent impacts on services and good from these ecosystems.
Climate change will affect the hydrology of individual wetland ecosystems mostly through changes in precipitation and temperature regimes. Because the hydrology of the surface layer of bogs is dependent on atmospheric inputs (Ingram, 1983), changes in the ratio of precipitation to evapotranspiration may be expected to be the main factor in ecosystem change. However, work on the large peatland complexes of the former Glacial Lake Aggazzi region indicate that the hydrology of bogs cannot be considered in isolation or independent of local and regional groundwater flow systems (Siegel and Glaser, 1987; Branfireun and Roulet, 1998) and that groundwater flow reversals, even in ombrotrophic peatlands, can have an impact on their water storage and biogeochemistry (Siegel et al., 1995; Devito et al., 1996). From the perspective of assessment of climate variability and change of peatlands, these systems need to be viewed in the broader context of their hydrogeological setting.
Fen, marsh, and floodplain wetlands receive additional water influx from the surrounding basin, including underground sources—sometimes from a considerable distance. This means that climate change impacts are partially mediated through changes in the whole basin area or even further afield where groundwater reserves do not correspond with surface basins. These changes also may affect the geochemistry of wetlands. Recharge of local and regional groundwater systems, the position of the wetland relative to the local topography, and the gradient of larger regional groundwater systems are critical factors in determining the variability and stability of moisture storage in wetlands in climatic zones where precipitation does not greatly exceed evaporation (Winter and Woo, 1990). Changes in recharge external to the wetland may be as important to the fate of the wetland under changing climatic conditions as the change in direct precipitation or evaporation on the wetland itself (Woo et al., 1993).
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