Discussion

The effects on sea level caused by the anthropogenic modifications of the hydrologic cycle are summarized in Table 5.4. Groundwater mining, increased runoff from land-cover changes, and to a lesser extent, burning of fossil fuel and vegetation could raise SLR by 0.6 ±0.2 mm/yr. Conversely, impoundment of water in reservoirs and losses due to infiltration and evapotranspiration from artificial lakes and irrigation could keep the equivalent of 1.5 ±0.2 mm/ yr from reaching the ocean. The combined effect of all of these processes could withhold around 0.9 ±0.5 mm/yr from reaching the ocean. These results imply a reduction in sea level rise comparable in magnitude to the observed recent sea level rise, but of opposite sign.

Figure 5.5 illustrates the evolution of estimated historical changes in sea level resulting from various anthropogenic activities since 1900, based on the midrange estimates of Table 5.4. The effects on sea level become significant only within the last 50 years. The growth in anthropogenic impacts is closely linked to the overall human transformation of the earth's surface, driven in general by the rapid 20th-century expansion in population and economic development (Turner et al., 1990). When the time inevitably arrives that reservoir construction significantly slows, there is a great likelihood of an important increase in the rate of global sea level rise.

The calculations outlined above contain large uncertainties, because of the fragmentary and incomplete nature of the historical data sets. The full extent of groundwater mining may have been underestimated due to limited country information. On the other hand, overestimates in runoff due to deforestation and to urbanization, which tend to raise sea level, may at least partially offset overestimates in rates of atmospheric moisture buildup and losses to deep percolation, yielding a net result roughly comparable to that suggested here.

Several approaches can be taken to reduce uncertainties in the estimates of human-induced changes in terrestrial water storage. A more through investigative search of historical records could provide additional information on groundwater mining and seepage losses beneath reservoirs and in irrigation. However, a unified systems approach using computer models will be needed to trace the path of the water more accurately through the atmosphere, hydro-logic, and biosphere subsystems and to account for various feedbacks. A start in this direction is provided by several recent general circulation model (GCM) experiments which examine the effects of land cover changes, particularly tropical deforestation, on the hydrologic cycle and on regional to global climate

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1900 1920 1940 1960 1980 2000 Year

1900 1920 1940 1960 1980 2000 Year

Figure 5.5 Evolution of potential contributions to sea level rise from various anthropogenic processes, using midrange estimates of Table 5.4. Historical data on urban runoff and reservoirs, after Shiklomanov, 1997; global groundwater withdrawn, irrigation water consumption, after FAO (1997); deforestation (WRI, 1998). See text for further details.

(Zhang et al., 1996; Lean et al., 1996; Polcher and Laval, 1994; Henderson-Sellers et al., 1993). More advanced surface and groundwater hydrology models embedded within GCMs could begin to analyze the movement of water among the various surface and subsurface reservoirs.

Finally, satellite remote sensing offers a potentially useful technology for monitoring the global hydrologic budget (Koster et al., 1999). Sampling of ter rain by TOPEX/POSEIDON, Landsat, SPOT, and other land-surface imaging satellites could provide better estimates of smaller artificial lakes and impoundments, omitted in the current calculations, and also changes in deforestation and other land-use transformations (Chen et al., 1998; Minster et al., 1999).

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