Has Pacific Northwest snowpack declined? Yes.
eric @ 2:38 am
There has been a bit of a flap here at the University of Washington over the state of the snowpack in United States Pacific Northwest region. The Seattle city mayor, Greg Nickels (a well known advocate for city-based CO2 reduction initiatives) wrote in an Op-Ed piece in the Seattle Times that
The average snowpack in the Cascades has declined 50 percent since 1950 and will be cut in half again in 30 years if we don't start addressing the problems of climate change now. That snow not only provides our drinking water, it powers the hydroelectric dams that keep our lights on.
The number "50 percent decline" apparently comes from a statement in an Oregon State University report in 2004 signed by many Northwest Scientists. This is not actually the best estimate for average snowpack decline, according to published work by Phil Mote, of Climate Impacts Group at the University of Washington. Mote reviewed the Oregon report back in 2004 and pointed out that the 50% figure was erroneous. Mote's 2003 paper in Geophysical Research Letters, highlighted in Science a few years ago, cited losses "as great as 60%" in some locations. Subsequent work (Mote et al., 2005) attempted to quantify change in total snowpack for the Cascades, and arrived at 15-30% for the period 1950-1997. This remains the best estimate, even when including the time period up to the present. Furthermore, this number represents measurements at many different elevations. If high elevation stations are excluded, the declines are generally larger, as would be expected if the chief culprit is increasing temperature, rather than declining snowfall.
So why the fuss? Well, the media can't seem to get enough when there is apparent controversy over something. Both the local and national media were thus quick to jump on the "story" when another UW staff member, Mark Albright, sought to correct the 50% number in a series of emails to the Mayor's office, among other places. Unfortunately, Albright didn't simply provide the corrected information, but set up a web site entitled "The Myth of the Vanishing Cascade Mountain Snowpack", with the very clear message that snowpack has not declined at all. Beyond the appearance of a controversy, this makes for a good newspaper story because it appears to cast doubt on Mote’s credibility. This is rather important given that Mote is Washington’s “State Climatologist” and is also the lead author on the chapter on the cryosphere in the upcoming IPCC Fourth Assessment report.
So has Pacific Northwest snowpack declined? Emphatically yes. I say “emphatically yes” for three reasons. First, because Albright illustrates the supposed lack of a trend by comparing specific periods (e.g. 1940-1946 vs. 1997-2006), in which snowpack has increased in some locations. This is not very informative, because both the spatial and temporal variability is large, and any question of decline can only be correctly addressed using all the data together, and over a statistically significant time period (30 years or more would be preferred). According to a summary statement prepared by Dennis Hartman to try to clarify the situation for the media and government, the decline is quite evident when the analysis is done correctly. (Hartmann is currently Chair of the Atmospheric Sciences Department at the University of Washington).
Second, “yes”, because lost in the media focus on controversy over the exact number is the fact that even a 15% decline would be huge. You can get this smaller number by starting in 1940 rather than 1950. This isn’t really justified because low elevations are substantially undersampled before 1945. Using a starting point of 1950 gives you about 30%. Either way, these are big numbers, and represent anything but a trivial change for water resource planners, backcountry skiers, and fisherman (not to mention fish). (For those readers that might wish to look at the data themselves, the State Climatologist's Office has a rather nifty mapping and trend-analysis tool, here: climate.washington.edu.
Third, “emphatically" because the snowpack will very likely continue to decline in the future. In his summary statement, Hartmann notes that “temperatures in the … Cascades will increase in the future as a result of global warming … and it is expected that this, by itself, should result in further decreases of snow … particularly at lower elevations.” Hartmann points out that it is less certain how precipitation will change in the future, and this could conceivably balance some or all of the increased spring time snow melt due to increasing temperatures. However, this is a pretty weak statement. It generally takes a very large increase in snowfall to offset small changes in temperature. Furthermore, precipitation is highly seasonal in this region, and an overall increase in temperature will, at a minimum, cause earlier snowmelt, even in the unlikely event that annually averaged total snowpack remains the same. This means less available water in summer, when fish, farmers, and hydroelectric dams most need it.
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