esm223_08_Dettinger - Climate Change and Water Supplies in...

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Unformatted text preview: Climate Change and Water Supplies in the West Michael Dettinger, USGS 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 Natural forcings suffice Observed Greenhouse emissions required Meehl et al., 2004 ...and observed temperatures in the Western States have followed pretty faithfully. Observed Warming, 1979-2005 Annual-mean warming, degree C/decade IPCC 4th Assessmt, WG1 Ch. 4, 2007 Warming already has driven Observed: Less spring snowpack TRENDS (1950-97) in observable hydroclimatic change April 1 snow-water content at Observed: Less snow/more rain western snow courses Mote, 2003 Observed: Earlier greenup dates Observed: Earlier snowmelt runoff Stewart et al., 2005 Cayan et al., 2001 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 Greenhouse forcings 1900-2100 Under projected greenhouse forcings, all climate models yield warmer futures for California and the West. 20 of 23 in this range Dettinger, 2005 To put these levels of of warming into perspective: What fraction of precipitation historically fell on days with average temperatures just below freezing? Less change More change "Rain vs Snow" Computed from UW's VIC model daily INPUTS (Bates et al, 2006) To put these levels of of warming into perspective: How many days/year historically were just below freezing? Less change More vulnerable "Duration of Snowpack" Computed from UW's VIC model daily INPUTS (Bates et al, 2006) Under even the coolest of these scenarios, historical streamflowtiming trends in snowfed streams are projected to continue in 21st Century. Stewart et al., 2004 Center-of-Timing of Streamflow No snow left Most snow left Knowles & Cayan, 2004 Knowles and Cayan 2002 A AD Y EV LE AN AL RR LV RA SIE NT CE Besides changing flow seasonality, these changes may bring more severe winter floods in at least some settings... Changes in Sierra Nevada Outflow, 2060 2000 Knowles and Cayan, 2004; /meetings/agu/pdf2006/dettinger_etal_poster_AGU2006.pdf If the warming continues, where should we expect the largest changes in warm-storm flooding to be manifested? Linear trend 10-yr avg How many historical storms > 1"/day were just below freezing? Computed from UW's VIC model daily INPUTS (ala Bales et al, 2006) Less change More change "Warm-storm flooding" Changes in Sierra Nevada Outflow, 2060 2000 ...and, with earlier runoff/ET and warmer summer temperatures, much drier springs and summers. Knowles and Cayan, 2004 Dettinger et al., 2004 How much would a +3C warming matter to GROWING SEASONS? How many days historically were just below the onset temperatures for growing seasons ? Less change More change Growing Season Length Computed from UW's VIC model daily INPUTS (ala Bates et al, 2006) Overall, though, these seasonal-scale changes pretty much balance each other, so that runoff+recharge may not change much. SIMULATED CHANGES IN RUNOFF+RECHARGE under a uniform +3C warming Simulated with USGS PRMS model: Dettinger et al. 2004 Simulated with the U of WA VIChydrologic model: Hidalgo et al, in prep Aquifers respond preferentially to lowest frequency climatic variations This means that aquifers may express climate TRENDS disproportionately. Changnon, 1987 In arid-semiarid zones, recharge comes preferentially from snowmelt. An example from the Great Basin Isotopic evidence Winograd et al., 1998 Thus, as snowpacks decline and rainfall takes over, RECHARGE may tend to decline in much of the West. SIMULATED CHANGES IN RECHARGE under a uniform +3C warming Widespread 15 to 25% changes projected, mostly as declines in recharge Simulated with the U of WA VIChydrologic model: Hidalgo et al, in prep; Earman et al, 2005 Under projected greenhouse forcings, most climate models yield a fairly narrow range of precipitation changes in California and most of the West. cm/month Greenhouse forcings 1900-2100 19 of 23 in this range cm/month Dettinger, 2005 Across most of the central/north West, current climate models project small changes in precipitation. SW gets drier. cm/month Dettinger, 2005 Only northernmost states show consistency. Zero-change contour thru middle of country. cm/month This is not an accident, but rather is part of a global pattern of projections of precipitation change. Poles and tropical oceans get wetter. Subtropics get drier. Most of conterminous US (& NV) somewhere in between. However, with high-lats getting wetter and desert belts getting drier, the VARIABILITY of precipitation in middle latitudes is projected to INCREASE Results from the US ParallelClimate Model, 2001, which yields small changes in AVERAGE precipitation Precipitation change at the extremes % Dettinger et al., 2004 TO SUMMARIZE... Warming by +2 to +6C, likely more so at higher altitudes Uncertain precipitation changes, maybe more--maybe less, mostly small Significant changes in rain-vs-snow storms snowpack amounts snowmelt timing flood risk streamflow timing low flows growing seasons recharge? just in response to temperatures "The most appropriate response to uncertainty is not denial; it is insurance." --Prof. Michael Hanneman, UCB REFERENCES Bales, R., Molotch, N., Painter, T., Dettinger, M., Rice, R., and Dozier, J., 2006, Mountain hydrology of the western United States: Water Resources Research, 42, W08432, doi:10.1029/2005WR004387, 13 p. Cayan, D.R., Kammerdiener, S., Dettinger, M.D., Caprio, J.M., and Peterson, D.H., 2001, Changes in the onset of spring in the western United States: Bulletin, American Meteorological Society, 82, 399-415. Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21st Century California: San Francisco Estuary and Watershed Science, 3(1), vol3/iss1/art4. Dettinger, M.D., and Cayan, D.R., 1995, Large-scale atmospheric forcing of recent trends toward early snowmelt in California: Journal of Climate 8(3), 606-623. Dettinger, M.D., Cayan, D.R., Meyer, M.K., and Jeton, A.E., 2004, Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, 1900-2099: Climatic Change, 62, 283-317. Earman, S., A. P. Campbell, F.M. Phillips, and B.D. Newman, 2006, Isotopic exchange between snow and atmospheric water vapor: estimation of the snowmelt component of greenwater recharge in the southwestern United States: Journal of Geopysical Research, 111 (D9), Hidalgo, H.G., Cayan, D.R., and Dettinger, M.D., in preparation, Variability of spring-summer drought in regions of high and low evaporative efficiency: for submission to J. Hydrometeorology Intergovernmental Panel on Climate Change, 2007, Climate change 2007, The physical science basis--Summary for Policymakers: IPCC Secretariat,, 21 p. Knowles, N., and D. Cayan, 2002, Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco estuary: Geophys. Res. Lett., 29: 18-21. Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and Watershed. Climatic Change, 62, 319-336. Knowles, N., Dettinger, M., and Cayan, D., 2006, Trends in snowfall versus rainfall for the Western United States: Journal of Climate, 19(18), 4545-4559. Meehl, G.A., Washington WM, Ammann C, Arblaster JM, Wigley TML, and Tebaldi C., 2004, Combinations of natural and anthropogenic forcings and 20th century climate. J. Climate. Mote, P.W., 2003, Trends in snow water equivalent in the Pacific Northwest and their climatic causes. Geophysical Research Letters, 30, DOI 10.1029/2003GL0172588. Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal of Climate,18, 1136-1155. Winograd, I.J., Riggs, A.C., and Coplen, T.B., 1998, The relative contributions of summer and cool-season precipitation to groundwater recharge, Spring Mountains, Nevada: Hydrogeology Journal, 6, 77-93. ...
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This note was uploaded on 08/06/2008 for the course ESM 235 taught by Professor Dunne during the Winter '08 term at UCSB.

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