1. The West’s declining snowpack: is global warming to blame? Philip Mote Climate Impacts Group University of Washington

3. 20th century decline in NH snow cover R.D. Brown, J. Climate, 2000

4. Trends in snowpack Use VIC and met data to evaluate trends, roles of temperature and precipitation

6. Snow course data Collected primarily ~April 1 beginning in 1915 (most in 1940s), added other months Purpose: streamflow forecasting Administered by USDA Natural Resource Conservation Service Mostly discontinued, some replaced by SNOTEL

10. Snow course ???

14. Courtesy USDA NRCS Data: NRCS CA DWR BC SRM

16. Courtesy USDA NRCS

17. decrease increase Trends in April 1 SWE, 1950-2000 824 snow courses 73% – trends Large – trends PNW Some + trends SW

18. decrease increase Trends in April 1 SWE, 1950-2000 Relative to 1950 value

20. Former snow course location

21. Relating SWE to climate data US Historical Climate Network (USHCN) Use nearest 5 stations to form reference time series T(t) and P(t) Regression: SWE(t)=a T T(t) + a P P(t)+  (t)

22. At almost every USHCN station, winters warmed + signs: warming but not statistically significant

23. Winters wetter in much of the West Drier in some of Northwest (PDO)

24. Correlations between Nov-Mar climate and Apr 1 SWE X-direction: precip Y-direction: temp Coldest locations insensitive to temperature Cascades very sensitive

25. April 1 SWE trends, 1930-2002 a P a T

26. April 1 SWE trends, 1950-2002 a P a T

28. SNOTEL data: cold-season melt events vs Apr 1 SWE

29. Relative trends 1950-1997

30. Declining snowpack in the Cascades From a simulation with a hydrology model

31. 1950-1997 relative trends vs DJF temperature Obs VIC

32. Trends in April 1 SWE in VIC simulation with fixed precipitation

33. As the West warms, winter flows rise and summer flows drop Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

34. Simulation of future snowpack with VIC hydrologic model Trend in total April 1 snowpack, 1950-97: -11% Courtesy Andrew W. Wood, University of Washington

36. SNOTEL data: cold-season melt events vs Apr 1 SWE Figure by Martyn Clark, Univ. of Colorado

37. Regional average April 1 SWE Obs o VIC x

38. Changes in SWE vs changes in precip 1930s to 1990s1945-55 to 1990s Obs SWEVIC SWEPrecipObs SWEVIC SWEPrecip Cascades-14%+1%+4%-29%-16%-5% Rockies+11%+2%+9%-16%-9%+1% California+3%-14%+10%-2%-25%-1% Interior+9%-6%+10%-22%-18%+2%

39. Trends/15yr in snow days in Switzerland From Scherrer et al., GRL 2004

40. Trends in Swiss snow days attributed to temperature and precipitation precip and temp attributions; contour lines indicate the sum. Attribution of trends shown by pseudo vectors; altitude and latitude From Scherrer et al., GRL 2004 days/42 yrs.

41. Conclusions Observations show substantial declines in western snowpack; corroboration with VIC suggests they are real and largely temperature-driven Cascades, N. Calif most temp-sensitive Large increases in precipitation have offset warming in some places Is global warming causing the decline? Too soon to tell.