Climate Change and Water Supplies in the West. Michael Dettinger, USGS

Transcription

1 Climate Change and Water Supplies in the West Michael Dettinger, USGS

2 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

3 and observed temperatures in the Western States have followed pretty faithfully.

4 Observed Warming, Annual-mean warming, degree C/decade IPCC 4th Assessmt, WG1 Ch. 4, 2007

5 Warming already has driven observable hydroclimatic change Observed: Less snow/more rain Observed: Less spring snowpack TRENDS ( ) in April 1 snow-water content at western snow courses Mote, 2003 Observed: Earlier snowmelt runoff Observed: Earlier greenup dates Stewart et al., 2005 Cayan et al., 2001

6 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

7 Under projected greenhouse forcings, all climate models yield warmer futures for California and the West. Greenhouse forcings of 23 in this range Dettinger, 2005

8 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 Rain vs Snow More change Computed from UW s VIC model daily INPUTS (Bates et al, 2006)

9 To put these levels of of warming into perspective: How many days/year historically were just below freezing? Less change Duration of Snowpack More vulnerable Computed from UW s VIC model daily INPUTS (Bates et al, 2006)

10 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 SIERRA NEVADA CENTRAL VALLEY No snow left Most snow left Knowles & Cayan, 2004 Knowles and Cayan 2002

11 Besides changing flow seasonality, these changes may bring more severe winter floods in at least some settings Changes in Sierra Nevada Outflow, Knowles and Cayan, 2004; /meetings/agu/pdf2006/dettinger_etal_poster_agu2006.pdf

12 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

13 and, with earlier runoff/et and warmer summer temperatures, much drier springs and summers. Changes in Sierra Nevada Outflow, Knowles and Cayan, 2004 Dettinger et al., 2004

14 How much would a +3ºC 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)

15 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 +3ºC warming Simulated with USGS PRMS model: Dettinger et al Simulated with the U of WA VIChydrologic model: Hidalgo et al, in prep

16 Aquifers respond preferentially to lowest frequency climatic variations This means that aquifers may express climate TRENDS disproportionately. Changnon, 1987

17 In arid-semiarid zones, recharge comes preferentially from snowmelt. An example from the Great Basin Isotopic evidence Winograd et al., 1998

18 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 +3ºC 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

19 Under projected greenhouse forcings, most climate models yield a fairly narrow range of precipitation changes in California and most of the West. Greenhouse forcings of 23 in this range cm/month cm/month Dettinger, 2005

20 cm/month cm/month Across most of the central/north West, current climate models project small changes in precipitation. SW gets drier. Dettinger, 2005 Only northernmost states show consistency. Zero-change contour thru middle of country.

21 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.

22 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 Parallel- Climate Model, 2001, which yields small changes in AVERAGE precipitation Precipitation change at the extremes % Dettinger et al., 2004

23 TO SUMMARIZE Warming by +2 to +6ºC, 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

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25 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: /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, Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21 st 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), 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, : Climatic Change, 62, 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: Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and Watershed. Climatic Change, 62, Knowles, N., Dettinger, M., and Cayan, D., 2006, Trends in snowfall versus rainfall for the Western United States: Journal of Climate, 19(18), 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 /2003GL Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal of Climate,18, 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,