Snowpack is declining. One of the most pronounced responses to warmer winter temperatures in the Pacific Northwest has been the loss of spring snowpack (Mote et al. 2005; Casola et al. 2007; Pederson et al. 2013; Sproles et al. 2013). As temperatures rise, the likelihood of winter precipitation falling as rain rather than snow increases. This is especially true in the Pacific Northwest where a significant amount of mountainous areas with snow accumulation are at relatively low elevation, and winter temperatures are near freezing (Nolin and Daly 2006). Small increases in average winter temperatures can therefore lead to increased rains, reduced snowpack, and earlier snowmelt. The loss of spring snowpack in the Pacific Northwest has been significant, with most of the weather stations showing a decrease. Data recorded each April 1 show a decline of 25 percent snowpack over the past 40 to 70 years (Mote et al. 2005). The fact that the declines are greatest at low-elevation sites and that the trend has occurred in the absence of significant decreases in winter precipitation implicates temperatures rather than precipitation as the cause of the trend.
Similarly, in a study focused on the Rocky Mountains, Pederson et al. (2013) found that after 1980, snow cover at low to middle elevations has dropped by approximately 20% (see figure 2), partly explaining earlier and reduced streamflow, and both longer and more active fire seasons. The study finds that this is predominately due to warmer springs (Feb-March) and finds that the post-1980 period is a “turning point” where temperature influences snowpack accumulation more than precipitation for large-scale snowpack patterns. (The past millennium’s snowpack before 1980 tracked regional precipitation climate variability, for example those caused by the El Nino Southern Oscillation and Pacific Decadal Oscillation.)
Hamlet and Lettenmaier (2007) modeled changes in the current and future peak snowpack versus October-to-March precipitation for watersheds in the Columbia Basin. Generally, a large shift is projected in the form of winter precipitation from snow to rain, especially in lower elevation basins. As these changes occur, there will likely be a tendency for higher winter flows, an increased risk of flooding, earlier snowmelt and runoff peaks, and lower summer streamflows.
Casola et al. (2009) found similar results when evaluating the impact of global warming upon the Cascades portion of the Puget Sound drainage basin. These researchers evaluated four analytical and modeling methods to determine the temperature sensitivity of snowpack. Results project a 20% reduction in snowpack (mean April 1 snow water equivalent) for each degree Celsius (1.8°F) of warming in the absence of indirect effects, and a 16% reduction in snowpack taking into account a projected warming-induced increase in precipitation. A regional study in the McKenzie River Basin in the Cascades Mountains of Oregon, shows a higher rate with a 2°C (3.6°F) increase in temperature shifting peak snowpack 12 days earlier and decreasing basin-wide volumetric snow water storage by 56% (Sproles et al. 2012).