Discussions on Recent Pacific Northwest Snowpack Trends

3/14/2007

On February 24, 2007, The Oregonian reported on a debate between researchers at the University of Washington on recent trends in 20th century snowpack in the Washington Cascades. The issue originated with the publication of an op-ed written by the Mayor of Seattle on February 7 stating that “The average snowpack in the Cascades has declined 50 percent since 1950.”. In question was the 50% statistic for the Cascades and the implication that the reported decline was due entirely to anthropogenic (human-caused) climate change; the 50% figure had appeared, erroneously, in the June 2004 report “Scientific Consensus Statement on the Likely Impacts of Climate Change on the Pacific Northwest” (Oregon State University). Mark Albright, of UW Atmospheric Sciences, noted that at the most complete snow courses (a small subset of the total) for the Cascades, the last 10 years were only a little below the long-term average.

To help resolve questions over the statement, a group of University of Washington climate and weather scientists met to review the different statistical approaches used to examine trends in spring snowpack, also referred to as snow water equivalent or SWE. Professor Dennis Hartmann, the Chairman of the Department of Atmospheric Sciences, was asked to prepare a summary statement on the issue. The statement reiterated many of the Climate Impacts Group’s (CIG) research findings on trends in SWE and added additional important insights into recent trends.

In summary:

• 20th century snowpack trends.
  CIG research shows that Pacific Northwest SWE has declined since monitoring became widespread in the late 1940s, with 30-60% losses at many *individual* monitoring sites in the Cascades (Mote 2003, Mote et al. 2005). When looking at the period 1950-1997, the overall observed decline in April 1 SWE for the Cascades is -29% (Mote et al. 2005). Relative losses are greatest in lower and mid-elevations where mid-winter temperatures are warmer; higher elevation sites where average mid-winter temperatures are still well below freezing (even with 20th century warming) don’t show any declines in SWE. See the plots yourself. An examination of SWE trends for more recent years (e.g., beginning about 1975 or later) appears to show a small increase in SWE for the Cascades, consistent with Albright’s finding about the average of the last 10 years, though SWE in the last five to seven years has been at least 20% below the long-term mean. This leveling of the trend appears to be associated with increased precipitation in the late 1990s, especially the near-record wet winter of 1998-99, and appears to have temporarily offset the persistent declines produced at low elevations by warming. Trends over intervals as short as 30 years are rarely significant, given the shorter time frame and the higher precipitation and snowpack variability experienced in the PNW since the mid-1970s. In other words, the apparent leveling of trends appears to be the result of large natural variability in precipitation masking the declines driven by temperature.
• Data availability.
  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. “A substantial collection of snow course data records with a reasonably representative and stable distribution with altitude exists since about 1945,” notes Prof. Hartmann.
• The role of natural variability.
  Natural variability has played – and will continue to play – an important role in determining year-to-year and decade-to-decade variability in SWE. Mote 2006 found that natural variability as represented by the North Pacific Index (NPI) explains about 50% of the trends in Pacific Northwest SWE since mid-century (and less from earlier starting points). The remaining portion of the trend “clearly includes the influence of the monotonic warming observed throughout the West, which is largely unrelated to Pacific climate variability and may well represent human influence on climate” (p.6219). Natural variability has also played a role in 20th century Pacific Northwest temperature trends, explaining perhaps one-third of November-March warming in the region since 1920 (Mote et al. 2005, p.47).As noted above, natural variability will continue to be a factor in 21st century snowpack accumulation. The Pacific Northwest will have good snowpack years in the coming decades as well as poor snowpack years even as the long-term temperature trends continue. This natural variability can hide long-term trends over short periods of time. Additionally, the potential for increases in precipitation as a result of climate change may make it difficult to see distinct trends in the near-term.
• Future impacts.
  The warming projected for the 21st century is expected to have a significant negative impact on snowpack, particularly mid-elevation snowpack, even if increased precipitation from natural variability and/or climate change is enough to “hold off” the impacts of warmer temperatures on snowpack in the near term. Changes in 21st century precipitation are less certain than temperature, however. Given that approximately 50% of the snowpack in the Cascades sits below 4200 feet, where spring snowpack is very sensitive to small increases in average temperature, preparing for climate change impacts is critical.

Related links

• Summary statement by Prof. Dennis Hartmann, Chairman of the U.W. Dept. of Atmospheric Sciences.
• N.W. Temperature, Precipitation, & SWE Trend Analysis
• Climate Impacts Group (CIG)