Snow Melting Earlier

Getting Ready for Spring (3):

The previous blogs started a discussion on spring coming earlier (Getting Ready for Spring (1) Getting Ready for Spring (2)). One of natural places to expect warming to be measured is at the seasonal transitions, and one measure of change would be changes in snow cover. A figure showed in the previous blog was from a single station, Barrow, Alaska. At Barrow, surface observations show that there is a greater than 10 day shift in the melt date over the 60 years of the record. The last 25 years of this record show enormous variability. There is a random aspect to this variability, and after averaging a strong trend is found. The attribution of this trend requires consideration of many other sources of information. The variability and the trends at a single station could be due to many factors.

Figure 1 shows snow trends from visible satellite observations from 1978 to 2006. This is for March-April-May.



Figure 1: Spring duration of snow cover: This map shows trends derived from visible satellite data, 1978-2006. Image by Richard Armstrong and Mary Jo Brodzik, National Snow and Ice Data Center, University of Colorado, Boulder. From the National Snow and Ice Data Center.


The blue shows areas where the duration of snow cover has decreased; the red is where the duration of snow cover has increased. In northern Alaska and the western United States the duration has decreased up to 3 days. Over much of Eurasia the snow cover has decreased. In the eastern part of North America there has been a small increase; in some of the high mountains of Asia there have been increases on the order of 3 days. Stephen Dery and Ross Brown in Geophysical Research Letters calculated snow trends for each week of the year for the Northern Hemisphere from 1972-2006. (Full paper from Stephen Dery's Web page) In both North America and Eurasia negative trends are calculated, with the trends in North America being larger. The behavior of the snow in North America and Eurasia vary together. The largest decrease of snow cover occurs in spring. There is some decrease in winter, and there is some increase of snow cover in fall. This accumulation of information across large expanses of land, and from different sources indicates systematic changes.

Phil Mote in a 2006 paper in the Journal of Climate examined the melting the western United State more fully. Mote used observations compiled at the United States Department of Agriculture. His analysis separates the changes that are correlated with indices that represent variability in the North Pacific Ocean. About half of the snow trend can be correlated with this variability, and half is related to a warming climate in the U.S. West. Further, Mote analyzed the behavior of precipitation in the west and the snow pack at relatively low altitudes in the Sierra Nevada and the Cascades – this is the transient snow zone. Aside from a general reduction in snow coverage, there is a change in how snow accumulates. Rather than a steady buildup, there is accumulation and melt, accumulation and melt. This profoundly changes stream flow and water availability.

Changes in snow are not an easy signal to extract. Snow and atmospheric warming form a complicated relationship. (Jeff Masters blog on snowy winters.) Snow observations from across the northern hemisphere paint a picture of decreasing snow cover. The decreases are largest in the spring, which is getting earlier.

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