October 18, 2006

Tread Carefully on Snow Cover

Filed under: Climate Changes, Precipitation

The depth and extent of Earth’s snow cover have been championed as tools for measuring climate variability and change since the early 1970s when satellite-derived measurements of large-scale snow became reliable. The scientific literature of recent years is well stocked with research articles that document various trends toward decreasing snow cover across portions of the Northern Hemisphere. This corresponds nicely with the atmospheric warming that has occurred across the hemisphere, and intuition leads to the conclusion that snow cover reduction is a response to global warming. Armed with this idea, it’s off to the global warming debate we go…on the wings of a common mistake. A deeper intuition suggests that precipitation is also a necessary ingredient of snow cover, and therefore variability in snow cover is not solely dependent on temperature.

One can argue that across Earth’s cryosphere, or its frozen realm, precipitation can be more completely monitored with snow cover than can atmospheric temperature. Consider this: decreases in snow cover across regions with seasons that are persistently below freezing have nothing to do with air temperature and they have everything to do with precipitation. This fact is well demonstrated in a recent article appearing in the Journal of Climate: “Snow cover distribution, variability, and response to climate change in western China” authored by Qin Dahe of the Cold and Arid Regions Environmental and Engineering Research Institute in Lanzhou, China, and two of his colleagues. The impetus for their work is twofold: (1) snow cover is a vital water resource in western China, and (2) “The majority of the climatic community is convinced of a pronounced reduction in seasonal snow cover in response to CO2-induced global warming.” In partial response to the second issue, Dahe et al. point out that “Up to now global snow cover monitoring has not found any convincing evidence of the trend in snow cover variations on global scale.”

The group of researchers examined spatial and temporal variability in snow cover over a 35 degree longitude by 23 degree latitude area in western China from 1951 through 1997 using several data sources. Snow depth data for the period 1978 through 1987 were obtained from the Scanning Multichannel Microwave Radiometer six-day snow depth charts of the National Aeronautics and Space Administration (NASA). Snow cover extent data for the period 1972 through 1989 were assembled from the National Oceanic and Atmospheric Administration’s (NOAA) satellite derived weekly snow cover extent charts. Lastly, in-situ data for snow depth and number of days with snow cover across the period 1951 through 1997 were obtained for a network of 106 stations distributed across western China. Stations were chosen to provide a representative distribution across the spatial extent and elevation ranges of the region.

Analysis of the data led Dahe et al. to conclude that through the 47-year study period “a general and uniform positive trend of snow cover” occurred over western China. Within one-half of their study area, northwestern China, the snow season temperature “fluctuated in a fairly similar manner as that of the global temperature” over the period of study. However, within this region “the long-term variability of snow cover is marked by a stochastic oscillation superimposed on a small increasing trend over the past 47 years.” Over the southern half of their study area, the Qinghai-Xizang (Tibet) Plateau, “the annual cumulative daily snow depth increased by 2.3% per year.” That annual rate of increase in snow depth equates to approximately a 108% increase over the period of study.

The grand conclusion of Dahe et al. is that over the period of study, “Western China did not experience a continual decrease in annual snow storage and early disappearance of spring snow cover, even during the great warming periods of the 1980s and 1990s.” Relative to the regional climate, the group reports that the increasing trend of snow cover across western China “is in good agreement with the snowfall positive trend, but is in contradiction to the regional warming.”

As diagnostics of the global climate, snow depth and extent across large regions can be useful tools. However, they come with a warning label containing two caveats. First, stated well by Dahe et al. is that “a persistent and misleading assertion is that global warming would decrease snow cover because snow cover has a strong negative relationship to temperature.” They illustrate the error in this assumption through their finding that a trend in snow cover is “dependent on the snowfall trend rather than on the temperature trend, if winter temperature is well below freezing.” Second, inherent difficulties and inconsistencies in measuring snow cover through remote sensing and in-situ means makes a clear establishment of historical trends more difficult than with many other climate variables.

Dahe et al. label the snow cover-warming debate a “controversial issue,” as the greater climate community is convinced of a snow cover reduction in association with global warming, but there are “important regional exceptions” along with many studies that “have shown that higher snowfall is a characteristic of a warming climate in cold regions.” Seemingly, snow cover is a slipperier platform for debate than what many would like to believe.

Reference:

Dahe, Q., Shiyin, L., and Peiji, L. 2006. Snow cover distribution, variability, and response to climate change in western China. Journal of Climate, 19, 1820-1833.




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