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Editor's Notes

• #2: Narrative: Across the West, observations show a general trend towards rising temperatures. Over recent decades, there has been a 1-2% decline in snow per decade, with greater impacts seen at lower elevations, which tend to be more sensitive to temperature-driven changes than colder, higher elevation regions. The authors of one climate study found that human-driven effects are responsible for half of the observed changes in snowpack over the western U.S. during the period 1950-1999. Source: Pierce et al, 2008. “Attribution of Declining Western U.S. Snowpack to Human Effects.” American Meteorological Society, 21, 6425-6444. URL: http://journals.ametsoc.org/doi/abs/10.1175/2008JCLI2405.1Note: Although SWE/P shows an upward trend in the Southern Sierras, the trend is not statistically significant at the 5% significance level.
• #3: Narrative: Currently, the most significant loss of snowpack occurs in April and May, but is nearly absent during February, when temperatures are usually far below freezing level. Given projections of increased temperatures in the coming decades, many scientists predict that we will see an earlier onset of snowmelt and a reduction in late season snowpack. Source: Kapnick, S. and Hall (2012). “Cause of changes in western North American snowpack.” Climate Dynamics 38:1885-1899. URL:http://link.springer.com/article/10.1007%2Fs00382-011-1089-y
• #4: Narrative: The most significant loss of snowpack has been observed during the mid to late snow season. This figure shows trends in changes in monthly snow water equivalent values for February through May. As you can see, most stations accumulate snow from February to April and then lose snow during April to May. With rising temperatures in the near-term, locations at lower elevations that are more sensitive to temperature change may experience earlier snowpack loss. Source: Kapnick, S. and Hall (2012). “Cause of changes in western North American snowpack.” Climate Dynamics 38:1885-1899. URL:http://link.springer.com/article/10.1007%2Fs00382-011-1089-y
• #5: Narrative: Although there is considerable uncertainty in estimating near-term regional climate change, scientists have used models which predict a decrease in spring snowpack and a transition to more precipitation in the form of rain across the West. The figures above show the mean winter temperature according to various models, both during the 1960-1999 time period and the projected change during 2006-2039. The largest increases in temperature influence are projected in the Sierra Nevadas in California, the Cascades in the Pacific Northwest, and northern Rocky Mountains in Idaho and Montana. Source: Ashfaq et al, (2013). “Near-term acceleration of hydroclimatic change in the western U.S.” Journal of Geophysical Research: Atmospheres, Vol. 118, 10,676-10,693. URL: http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50816/abstract
• #6: Narrative: This figure shows a transect of snow course locations in California, Washington, Oregon, and British Columbia. The trendline shows a general decrease in 1 April snow water equivalent (SWE) as elevation declines. An important implication of this figure is that spring SWE will grow increasingly sensitive to temperature. As temperature follows an upward trend in the coming decades, the warming effect is poised to impact the snowpack even in higher elevations. Source: Mote, P. (2006). “Climate-Driven Variability and Trends in Mountain Snowpack in Western North America.” American Meteorological Society, 19. URL: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/27724/MotePhilipW.CEOAS.ClimateDrivenVariability.pdf?sequence=1