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.