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Indicator: Annual Spring Snowpack

Data and Data Discussion provided by Sustainable Seattle

Annual spring snowpack trends (1945-2006)

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Upstream Indicators
Carbon Footprint --- Since 2004, the state's and King County's emissions have been on the rise. In particular, transportation related GHG emissions and total energy use in King County continue to rise. (view data graph) Urbanization and Impervious Surface Changes --- The number of acres of urban land, clear-cut, and regenerating forest have increased in King County since 1986. Acres of grass and deciduous, mixed and coniferous forest have decreased. Acres of rural forests and privately owned working forests have declined in recent years (view data graph)
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Downstream Indicators
Freshwater Habitat Index --- Overall freshwater quality in King County is below standards except for lakes. These findings are based on 2007 monitoring results for rivers, streams, groundwater, and lakes. Of the total sampled stream sites in King County in 2006-07, 45 percent were considered moderate to high water quality, and 55 percent were rated to be of high concern. This was a decrease in the number of sites considered moderate to high water quality from previous years. (view data graph) Stream and River Flows --- The median of the "flashiness" index scores across all streams measured in King County has increased between 1945 and 2007. (view data graph) State and Federal Listed Threatened and Endangered Species --- Limited historical data exists. However, the rate of global species extinction has accelerated over the past century. Steps are being taken to curtail the rate of species decline in our region, but populations of species intolerant to human development are expected to decline as our region grows.
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Related Goals
The Puget Sound area is very dependent on the annual snowpack in the Cascades as a source of clean water. Small climate changes can have profound effects on snowpack levels which can greatly influence the availability of water as well as the way natural ecosystems function. Clean and Sufficient Water Climate Protection
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Sustainability Snapshot:

Snowpack levels in the Cascades not only reflect changes in the climate but also affect the climate of the region. As the snowpack recedes, solar radiations previously reflected by the snow are absorbed by the darker land mass, causing a faster rise in temperature. As the climate warms, more precipitation will fall as rain than snow in the years to come. Snowpack is critical to adequate water supply as it acts as a natural reservoir for storing water.

Sustainability Trend:

Snowpack levels are decreasing. In the North Cascades, 53 glaciers have disappeared since the 1950s.

Data Discussion

The Indicator Defined
Spring snowpack is measured as the amount of “snow water equivalent” (SWE) on April 1st of any given year. Water equivalent measurements are taken by extracting core samples, or slices, of the snow/ice on the ground, and then melting the sample to calculate the water equivalent of the snow/ice. Snow water equivalent (SWE) totals are recorded on the first day of each month (January, February, March, April, and May) from the SNOTEL (Snowpack Telemetry) network. This network collects snowpack and related climate data from automated stations throughout the mountain regions in the western U.S. and is maintained by the U.S. Dept. of Agriculture Natural Resources Conservation Service (NRCS). Hydrologic modeling is also used to estimate the annual spring snow pack, often in conjunction with observational data.

Data Interpretation/Evaluation
Year to year snowpack is highly variable in the Pacific Northwest. However, there is no doubt that the April 1st snow water equivalent (SWE) in the Cascade Mountains of Washington State has declined 15–35% from mid-century to 2006. The largest declines are at low elevations and smaller declines or increases at high elevations (where temperatures have remained above freezing year round). The exact amount of the decline is hard to pin down. The data used to generate this range includes estimates from observations and hydrologic modeling, reflects a range of starting points between about 1930 and 1970 and also reflects uncertainties about sampling, especially when looking at earlier dates. Skeptics will often point to natural variability as the explanation of snowpack decline over the past century. It is true that natural variability can obscure long-term trends over shorter periods of time. For example, there appears to have been a small increase in SWE for the Cascades for more recent time periods (e.g., beginning about 1975 or later) though SWE in the last five to seven years has been at least 20% below the long-term mean. However, Mote 2006 found that natural variability explains only about 50% of the trends in Pacific Northwest SWE since mid-century (and less from earlier starting points). The vast majority of scientists agree that the remaining portion of the trend is not an artifact of natural cycles and is new phenomenon attributable to the warming climate.

Data Source and Limitations
Data availability is a limiting factor in long-term SWE trends analysis. Prior to the mid-1940s, there were very few snowpack monitoring sites with continuous data sets and those that had continuous data sets tended to be located at high altitudes known to be less sensitive to warming trends. These factors make it difficult to assess SWE trends before the 1940s with high statistical certainty, and results are not consistent with more complete analyses for later periods because of the high elevation bias in the available data. By mid-century, the availability of data and distribution of snowpack monitoring stations is much improved, allowing for a more robust analysis of SWE trends. In addition, there was unusually high snowpack during the 1950s which may skew some of the trend data starting from that time period. Data interpretation is drawn from the following reports and articles: http://www.hydrol-earth-syst-sci.net/12/193/2008/hess-12-193-2008.html http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FBAMS-86-1-39&ct=1 http://oregonstate.edu/dept/ncs/newsarch/2006/Mar06/snow.htm http://www.climate.washington.edu/snowpackdiscussion.html http://wa.water.usgs.gov/projects/glacier/ http://seattletimes.nwsource.com/html/localnews/2008094636_climate06m.html http://www.ecy.wa.gov/pubs/0711016.pdf http://www.ecy.wa.gov/climatechange/ http://www.ecy.wa.gov/climatechange/reducedsnow.htm http://www.cses.washington.edu/cig/res/rc/snowpacktrends.shtml http://www.cses.washington.edu/db/pdf/moteetalvarandtrends436.pdf http://www.cses.washington.edu/db/pdf/cighb1303interim580.pdf http://www.hydrol-earth-syst-sci.net/12/193/2008/hess-12-193-2008.html http://www.cses.washington.edu/cig/pnwc/cc.shtml

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Additional Resources

Facts about Washington's Retreating Glaciers and Declining Snowpack
Fact sheet from the Washington State Department of Ecology: Washington is especially vulnerable to climate change from global warming as we depend on glaciers and mountain snowpack to fed summer streams.

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