Conclusions

Plausible estimates of sea level changes accompanying large-scale anthropogenic modifications of land hydrologic processes have been presented. Increased runoff from groundwater mining and impermeable urbanized surfaces are potentially important human-induced sources of sea level rise. Runoff from tropical deforestation, wetlands clearance, and water released by oxidation of fossil fuel and biomass, are relatively insignificant. Altogether, these processes could augment sea level by some 0.4 to 0.9 mm/yr (Table 5.4). Conversely, sequestration of water by dams, and further losses of water due to infiltration beneath reservoirs and irrigated fields, along with evaporation from these surfaces could retain the equivalent of 1.3 to 1.8 mm/yr over continents. The net effect of all of these anthropogenic processes is to withhold the equivalent of 0.9 ±0.5 mm/yr from the sea. This rate represents a substantial fraction of the observed recent sea level rise of 1 to 2.5 mm/yr, but opposite in sign.

However, it should be pointed out that these estimated impacts on sea level rise represent upper bounds. The historical data base is incomplete and subject to considerable uncertainties. Furthermore, the increased volume of moisture stored in the atmosphere due to evaporation from reservoirs or irrigated fields provides only an upper bound, inasmuch as the atmospheric effects are probably localized and thus are unlikely to represent large-scale averages. Finally, a number of the processes are interrelated, requiring a unified systems approach, using coupled land-surface hydrology and climate models, to track the path of water through the various subsystems, accounting for possible feedbacks.

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