The West’s declining The West’s declining snowpack: is global snowpack: is global warming to blame?warming to blame?
Philip MoteClimate Impacts Group
University of Washington
20th century decline in NH snow cover
R.D. Brown, J. Climate, 2000
Trends in snowpack
Use VIC and met data to evaluate trends, roles of temperature and precipitation
Snow course data
Collected primarily ~April 1 beginning in 1915 (most in 1940s), added other months
Purpose: streamflow forecastingAdministered by USDA Natural
Resource Conservation ServiceMostly discontinued, some replaced by
SNOTEL
Snow course ???
Courtesy USDA NRCS
Data:NRCSCA DWRBC SRM
Courtesy USDA NRCS
decrease increase
Trends in April 1 SWE, 1950-2000
824 snow courses
73% – trends Large – trends PNWSome + trends SW
decrease increaseTrends in April 1 SWE, 1950-2000Relative to 1950 value
Former snow course location
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)=aTT(t) + aPP(t)+(t)
At almost every USHCN station, winters warmed
+ signs: warming but not statistically significant
Winters wetter in much of the WestDrier in some of Northwest (PDO)
Correlations between Nov-Mar climate and Apr 1 SWE
X-direction: precipY-direction: temp
Coldest locations insensitive to temperatureCascades very sensitive
April 1 SWE trends, 1930-2002aP<P> aT<T>
April 1 SWE trends, 1950-2002aP<P> aT<T>
SNOTEL data: cold-season melt events vs Apr 1 SWE
Relative trends 1950-1997
Declining snowpack in the Cascades
From a simulation with a hydrology model
1950-1997 relative trends vs DJF temperature
ObsVIC
Trends in April 1 SWE in VIC simulation with fixed precipitation
As the West warms,winter flows rise and summer flows drop
Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)
Simulation of future snowpack with VIC hydrologic model
Trend in total April 1 snowpack, 1950-97: -11%
Courtesy Andrew W. Wood, University of Washington
Figure 1
Elevation (m) DJF Temp (C) NDJFM PCP (mm)
PNW
CA CRB
GB
SNOTEL data: cold-season melt events vs Apr 1 SWE
Figure by Martyn Clark, Univ. of Colorado
Regional average April 1 SWE
Obs oVIC x
Changes in SWE vs changes in precip
1930s to 1990s 1945-55 to 1990s
Obs SWE
VIC SWE
Precip Obs SWE
VIC SWE
Precip
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%
Trends/15yr in snow days in Switzerland
From Scherrer et al., GRL 2004
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.
days
/42 y
rs.
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.