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Global Warming and the Planetary Water Cycle Ruth Curry Research Specialist, Physical Oceanography Woods Hole Oceanographic Institution Rising greenhouse gas concentrations and global warming are altering the processes that control the cycling of freshwater around the planet’s surface: evaporation, precipitation, freezing and melting. In the 21st century, major issues confronting humankind will almost certainly include sea level rise, freshwater resources, shifting weather patterns and drought ― especially in the western U.S. As our planet’s surface temperatures rose in the last half of the 20th century, it became clear that the oceans were warming at a considerably faster rate than the atmosphere [Levitus et al., 2001]. Climate research increasingly points to anthropogenically elevated greenhouse gas concentrations, principally carbon dioxide, as a major contributor to the observed warming [IPCC, 2001]. From ice core evidence, it is known that carbon dioxide levels have fluctuated naturally over the past 400,000 years ―between 190 and 290 ppm― and global temperatures have warmed and cooled synchronously resulting in glacial / interglacial cycles [Petit, et al, 1999]. Atmospheric carbon dioxide levels are presently at 379 ppm with virtually all of the rise (from 290 ppm circa 1750) having taken place over the past 120 years [Etheridge, et al, 1998]. The planetary climate system is responding to this sharp rise in carbon dioxide in more ways than just its temperature. There is ample evidence that the hydrologic engine that cycles freshwater around the planet is changing, too. Although directly measuring evaporation, precipitation, freezing and melting on global scales has not been practical until very recently, the footprints of changes in these processes can be found in ocean salinity distributions. Evaporation and freezing remove freshwater from the ocean, leave salt behind, and raise salinity concentrations; while precipitation and melting both dilute the salt content of seawater. The measurement record, which extends reliably back to the 1950s, reveals salinity increases at low latitudes in the Atlantic, Indian and South Pacific Oceans that contrast sharply with decreases in the North Pacific Ocean and mid- to highlatitudes of both hemispheres [Boyer et al, 2005;Curry et al., 2003; Dickson et al., 2002; Wong et al., 1999]. These global patterns suggest that an overall shift of Earth’s freshwater balance is underway. Evaporation rates over warmer tropical and subtropical oceans appear to have increased by ~10% in the last 20 years. In the Northern Hemisphere, that extra water vapor is bypassing the mid-latitudes ― which have been drying measurably since 1997 and producing severe drought conditions in the western U.S. [Hoerling & Kumar, 2003]. The precipitation is instead falling in the high latitudes across North America, Europe and Asia, elevating the rates of river runoff into the Arctic Ocean [Serreze et al., 2000; Peterson et al., 2002]. As the global thermometer continues its upward trend, the picture emerging in the last decade is of a climate system in transition, especially near Earth’s poles. Ice is melting just about everywhere on the planet. Satellite pictures of Arctic sea ice provide startling evidence of the thinning and shrinking that is occurring [Comiso et al. 2004]. Land ice and permafrost are in decline all around the Arctic [Dyurgerov and Carter, 2004; Zwally et al., 2002; Schiemeier, Nature,2004]. Greenland ― which stores enough freshwater to raise global sea level by ~7 meters ― is now unquestionably losing mass. Its summer melt area has increased by 15 –25% since the 1970s, its ice shelves are beginning to break up, and melt water is percolating downward and lubricating the base of its glaciers. The prognosis is that Greenland will take several centuries to melt [R.Alley, personal communication]. But its release of freshwater will make sea level rise a very significant issue in this century. At the opposite pole, the largest volume of land ice ―Antarctica ―is still gaining mass. However, its ice shelves are becoming unstable because of warming ocean temperatures. The disintegration of the Larson “A” and “B” Ice Shelves changed notions of how fast and how completely ice shelves can break up. The event also confirmed ideas that ice shelves control the rates of land ice movement such that glacial surge results when an ice shelf is removed. References Comiso, J and C. Parkinson, 2004: Satellite-Observed Changes in the Arctic. Physics Today, 57 (8): 3844. Curry, R., R.R.Dickson and I. Yashayaev, 2003: A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 426: 826-830. Dickson, R.R., I. Yashayaev, J. Meincke, W. Turrell, S. Dye and J. Holfort, 2002. Rapid Freshening of the Deep North Atlantic over the past Four Decades. Nature, 416: 832837. Dyurgerov, M.B. and C.L. Carter, 2004. Observational Evidence of Increases in Freshwater Inflow to the Arctic Ocean. Arctic, Antarctic, and Alpine Research, 36 : 117-122. Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola and V.I. Morgan. 1998. Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. Hoerling, M and A. Kumar, 2003: The Perfect Ocean for Drought. Science, 299: 691-695. IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J. T., Y.Ding, D.J. Griggs, M. Noguer, P. van der Linden, X. Dai and K. Maskell (eds.)] Cambridge Univ. Press, U.K. pp 944. Keeling, D.D. and T.P. Whorf, 2003. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. Levitus, S., J. Antonov, J Wang, T. Delworth, K. Dixon, and A. Broccoli, 2001: Anthropogenic Warming of Earth’s Climate System. Science, 292: 267-270. Peterson, B.J. et al., 2002. Increasing river discharge to the Arctic Ocean. Science 298: 2171-2173. Serreze, M.C., J.E. Walsh, F.S.Chapin III, T. Osterkamp, M.B. Dyurgerov, V. Romanovsky, V W.C. Oechel, J.Morison, T. Zhang, and R.G. Barry, 2000. Observational evidence of recent change in the northern high-latitude environment. Climate Change, 46: 159-207. Wong, A.P.S, N.L. Bindoff and J.C. Church, 1999. Large scale freshening of intermediate waters in the Pacific and Indian Oceans, Nature, 400: 440-443.