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esm223_08_Dettinger

esm223_08_Dettinger - Climate Change and Water Supplies in...

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Climate Change and Water Supplies in the West Michael Dettinger, USGS
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During the past 30 years--and in the near-term future--natural climate variability has been augmented by warming trends associated with increases in the global greenhouse effect. Climate-model simulations of global-average temperature Meehl et al., et al., 2004 Observed Natural forcings suffice Greenhouse emissions required
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…and observed temperatures in the Western States have followed pretty faithfully.
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Observed Warming, 1979-2005 Warming, 1979-2005 IPCC 4th Assessmt, WG1 Ch. 4, 2007 Annual-mean warming, degree C/decade
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Warming already has driven observable hydroclimatic change Observed: Less snow/more rain Mote, 2003 TRENDS (1950-97) in April 1 snow-water content at western snow courses Observed: Less spring snowpack Observed: Observed: Earlier Earlier snowmelt runoff snowmelt runoff Stewart et al., 2005 Observed: Earlier greenup dates Cayan et al., 2001
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As a result of these trends, the warm-season fraction of annual streamflow has declined. Sum of natural flows from 8 major rivers in the Sierra Nevada Roos, 1989, 1991; Dettinger and Cayan, 1995; Stewart et al 2005
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20 of 23 in this range Under projected greenhouse forcings, all climate models yield warmer futures for California and the West. Greenhouse forcings 1900-2100 Dettinger, 2005
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To put these levels of of warming into perspective: What fraction of precipitation historically fell on days with average temperatures just below freezing? “Rain vs Snow” More change More change Computed from UW’s VIC model daily INPUTS (Bates et al, 2006) Less Less change
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How many days/year historically were just below freezing? “Duration of Snowpack” More vulnerable Less change Computed from UW’s VIC model daily INPUTS (Bates et al, 2006) To put these levels of of warming into perspective:
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Stewart et al., 2004 Stewart et al., 2004 Center-of-Timing of Streamflow Under even the coolest of these scenarios, historical streamflow- timing trends in snowfed streams are projected to continue in 21st Century.
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