Issues in Ecology Number 9 Spring 2001 Projected Future Changes in AET Proiected future changes in PRcp ange from change from 1961-1990 baseline to 2061-2090 scenario climate 20020406080166 Figure 5 of future cha transpiration(AET BIOME-RGO precipitation (PRCP)generat by ar system mc this atmos h stem m resp r AET projections 89 we. Re th cted for the Sou VEMAP IL Simula n Group Univ.of Montana) Combining this instream use figu removing a dam outweighed the economic benefits of awals puts the al at 6,780 km mns urrppriauns离 operating it As a result of these and other trends.accessible cent of the accessible freshwater runoff of the planet. runoff is unlikely to increase by more than 5-10 percent Global water demands continue to rise with in- over the next 30 years.During the same period,the creases in human population and consumption.Increases earth's population is projected to grow by approximately in accessible runoff.however.can only be accomplished 35 percent.The demands on freshwater systems wil struction of new dams or desaination of seawater continue to grow throughout the coming century atio globa THE WATER CYCLE AND CLIMATE CHANGE ments.it is likely to remain a minor part of global supply for the foreseeable future.Dams continue to bring more A scientific consensus now exists that the con water under human control,but the pace of construction tinuing buildup of human-generated greenhouse gases in has slowed.In developed countries,many of the best sites the atmosphere is warming the earth.The last decade of have alre ady heen Risin the twe centu as the ecord.an ahit tdestruction. pal warm g of the sity.and displacement of hum past50years had no in the past thousand are making further dam construction increasingly dif years.As the earth continues to warm in the coming ficult.About 260 new large dams now come on line world century.a general intensification of the water cycle is wide each year compared with 1.000 a year between the expected to occur.In a warmer climate,greater volumes 1950s and 1970s.Moreover,at least 180 dams in the of water will evaporate from plants,soils and water bod United States were removed in the past decade based on ies globally.lofting more vapor into the atn f afety. ntal impact,ando rain out and in turr sing ru off and making hydr cence. The destruction of the Edwards Dam on Maine' logic extremes such as floods and droughts more com Kennebec River in 1999 marked the first time that fed- mon and more intense.Some decreases in snow and ice eral regulators ruled that the environmental benefits of cover have already been observed.Changes in the tem- 9 9 Issues in Ecology Number 9 Spring 2001 Figure 5 A projection of future changes in actual evapotranspiration (AET) and precipitation (PRCP) generated by an ecosystem model (BIOME-BGC) using a future climate scenario to the year 2100 derived from a global climate model. In this scenario, atmospheric carbon dioxide (CO2) increased approximately 0.5%/yr, and the ecosystem model responded with changes in leaf area index (a measure of plant productivity) based on changes in CO2 , climate, water, and nitrogen availability. In general, these projections suggest higher rainfall and increased plant growth in the arid West, leading to higher AET. Reduced rainfall and the resulting effects of drought on vegetation are the primary causes of lower evapotrans￾piration projected for the Southeast. For additional information, see Box 2 (Results from VEMAP II, courtesy of P. Thornton, Numerical Terradynamic Simulation Group, Univ. of Montana). Combining this instream use figure with estimated global withdrawals puts the total at 6,780 km3 a year. That means humans currently are appropriating 54 per￾cent of the accessible freshwater runoff of the planet. Global water demands continue to rise with in￾creases in human population and consumption. Increases in accessible runoff, however, can only be accomplished by construction of new dams or desalination of seawater. Today, desalination accounts for less than 0.2 percent of global water use and, because of its high energy require￾ments, it is likely to remain a minor part of global supply for the foreseeable future. Dams continue to bring more water under human control, but the pace of construction has slowed. In developed countries, many of the best sites have already been used. Rising economic, environmen￾tal, and social costs including habitat destruction, loss of biodiversity, and displacement of human communities are making further dam construction increasingly dif￾ficult. About 260 new large dams now come on line world￾wide each year compared with 1,000 a year between the 1950s and 1970s. Moreover, at least 180 dams in the United States were removed in the past decade based on evaluations of safety, environmental impact, and obsoles￾cence. The destruction of the Edwards Dam on Maine’s Kennebec River in 1999 marked the first time that fed￾eral regulators ruled that the environmental benefits of removing a dam outweighed the economic benefits of operating it. As a result of these and other trends, accessible runoff is unlikely to increase by more than 5-10 percent over the next 30 years. During the same period, the earth’s population is projected to grow by approximately 35 percent. The demands on freshwater systems will continue to grow throughout the coming century. THE WATER CYCLE AND CLIMATE CHANGE A scientific consensus now exists that the con￾tinuing buildup of human-generated greenhouse gases in the atmosphere is warming the earth. The last decade of the twentieth century was the warmest on record, and paleoclimate records indicate that the warming of the past 50 years had no counterpart in the past thousand years. As the earth continues to warm in the coming century, a general intensification of the water cycle is expected to occur. In a warmer climate, greater volumes of water will evaporate from plants, soils and water bod￾ies globally, lofting more vapor into the atmosphere to rain out and in turn, increasing runoff and making hydro￾logic extremes such as floods and droughts more com￾mon and more intense. Some decreases in snow and ice cover have already been observed. Changes in the tem-