Disappearing Snow: A Sign of Global Warming?

The phenomenon of melting glaciers and shrinking Arctic ice sheets offers dramatic evidence of global warming, but is actually the product of centuries of climate adjustments, and therefore does not provide a real time picture of what has occurred on the planet over the last century. A more useful method for examining the effect of greenhouse gas accumulation is to measure snow cover from season to season.

Since 1922, spring snow cover in the northern hemisphere of the world has decreased by about 8%. The authors of a study in Hydrology and Earth System Sciences studied snowfall records for the Cascade Mountains of Washington state, in order to establish a trend in snow cover over time. What they found was that the snow water equivalent (SWE) declined by about 15% to 35% between the middle of the twentieth century and 2006, and that warming was the cause.

Summers can be dry in mountainous regions of the western United States. For that reason, water supply projection is important for agricultural purposes. Water stored during the winter as snow, can be critical for later in the year, when things get dry. For this reason, snow survey records have been kept in the region at various locations for many years. They record the 1 April snow water equivalent (SWE), or the amount of water in a particular quantity of snow. The earliest records date back to1928, with additional locations added over time. These recorded observations provide a record of climate changes, and an opportunity to analyze trends in snow cover over time.

The study examined long-term snow survey records from 49 locations. They used computer simulations to estimate temperature cycles and the amount of incoming solar radiation. According to the authors, even small temperature differences can melt snow, and increase absorption of the sun's heat, for even greater warming. They attempted to differentiate between the influences of temperature and levels of precipitation from year to year, on snow cover, in order to establish a trend over time.

The results of the study showed a linear decline in SWE from about mid-century on. Larger decreases were observed at lower elevations. Although regional warming was clearly a factor in the decline, a quantifiable relationship to greenhouse gas emissions could not be established. However, using a statistical method for determining global CO₂ levels, the authors were able to relate decreasing snow cover to temperature changes, rather than variations in precipitation from year to year, an indication that greenhouse emissions are a key factor.

The authors came to three important conclusions. First, snow cover has declined in the region due to rising temperatures. Second, the year-to-year fluctuations in precipitation levels are not a factor, but have made the job of interpreting the data more difficult. Finally, they concluded that declining snow cover can be strongly associated with results that would be expected from rising levels of greenhouse gases. For those who still wonder whether global warming is real, results like these are difficult to ignore. Melting glaciers may seem remote, but disappearing snow, well that just hits a little closer to home.

Reference

Mote, P., Hamlet, A., Salathe, E.

Has spring snowpack declined in the Washington Cascades?

Hydrology and Earth System Sciences