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