Postel, 2000; Sampat, 2000). According to these authors, only 3 percent of the earth’s water is fresh, and most of this water is tied up in glaciers and ice sheets. Only about 0.01 percent of the total water is available in lakes, the atmosphere, and soil moisture. Water that is “clean” in terms of its suitability for human use is estimated today to be unavailable for 20 percent of the world’s population. Humans have a long history of arguments and conflict over water rights. Water from rivers is impounded for energy generation, agriculture, dilution of wastes, and transport. The construction of dams affects biotic habitats both directly and indirectly; for example, damming of the Danube River has altered the silica chemistry of the entire Black Sea (Vitousek et al., 1997). In the United States only 2 percent of rivers flow unimpeded, and the flow is regulated in over two-thirds of all rivers (Abramowitz, 1996).

The water cycle has important nonlinearities that are relevant to the impacts of abrupt climate change (Kling, 2001). First, the water balance of lakes is extremely sensitive to climate change, and small shifts in the ratio of precipitation to evaporation can result in large changes in lake levels. Extremely large shifts in paleolake levels have been tied to abrupt climate change that is global in extent (Broecker et al., 1998). Lake levels in Lake Victoria and throughout Africa rose suddenly at the beginning of the Bolling warming about 14,700 years ago, a warming that is evident in Greenland, Europe, and North and South America. During the Holocene, rapid lake level shifts are found in western North America (Laird et al., 1996; Stine, 1994) as well as southern South America and Africa. Changes in the Great Lakes water levels have also been linked to climate change, most recently in the last few decades (Sellinger and Quinn, 1999). The declining extent of the Aral Sea and Lake Chad involve both human impact and climate change (Kling, 2001).

Second, the depletion of groundwater is highly nonlinear in its impact on water use when the aquifers dry up (i.e., going from a situation of plentiful water to one of no water or very salty water over a period of a few months). Groundwater deficits, which represent the amount of water withdrawn compared to the amount of recharge water put back into the aquifer, are now equal to at least 163 cubic kilometers per year throughout the world, which is equivalent to about 10 percent of the world’s grain production (Brown et al., 2000). In many cases groundwater is nonrenewable, or fossil water, that took extremely long periods to accumulate and is now being used more rapidly than it is replenished. For example, three-quarters of the water supply of Saudi Arabia currently comes from fossil water-

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