October 31, 2006

New Northern Eurasian Snow Cover Data Not Cooperating

Filed under: Arctic, Polar

The cryosphere, or Earth’s frozen realm, is widely thought of as a wonderful monitor of climate change. The idea that warming reduces the area extent and/or persistence of frozen ground, snow cover, sea ice, and glaciers seems rather straightforward. Using the cryosphere as a natural barometer of global warming appears to be ideal considering the fact that the high latitude regions of the Northern Hemisphere are associated with some of the largest increase in surface air temperature both observed during the 20th century and predicted for the 21st century. Reduction of the cryosphere should be a train run amok, as frozen surfaces positively feed back to the overlying atmosphere such that without refrigeration from beneath, the overlying atmosphere should become even warmer.

Information about changes in the cryosphere has become widely available over the past two decades through satellite-derived data sets. However, in-situ observations of the cryosphere, or direct measurements from on the ground, are valuable because they can be used for validation of remote sensing products. But more importantly, in-situ observations provide a much longer record than satellite data sets, making them better monitors of the long-term climate. A newly assessed “state of the ground” data set for northern Eurasia has hit the streets, and the in-situ data offer an interesting caveat to the global warming debate.

In a recent issue of the Journal of Climate (October, 2006), a group of scientists, most of which are associated with the United States’ National Climatic Data Center (NCDC), published a research article in which they detail their analysis of a rarely used data set describing the land surface of the cold northern regions of Eurasia. The work by Groisman et al. examined twice daily or daily observations at over 1800 stations within the boundaries of the former Soviet Union as part of the Global Synoptic Data Network (GSDN). The data set includes non-instrumental observations of ground conditions for the past 70 years, including soil wetness, snow cover, and the degree to which the ground is frozen. The group of authors assessed seasonal climatology and linear trends in three ground characteristics: (1) the duration of the period when soil is unfrozen and not covered by snow, (2) presence of snow on the ground, and (3) changes between frozen/unfrozen ground during the day. Groisman et al. contextualized their findings to the high latitude warming of the 20th century.

The group reports that across northern Eurasia, trends of increasing regional surface air temperature and warming of the ground, including decreasing snow cover, were most intense during the period 1956-1991. This means that these trends did NOT continue along the same trajectory over the last 13 years of the record (1992-2004). Groisman et al. also report that surface air temperature for the periods 1881-2004 and 1956-2004 show statistically significant increases of 1.3oC and 1.4oC respectively. However, for the period 1936-2004, “no systematic increases in the Arctic can be reported due to the warm first two decades of this period.” In response to a decrease in snow cover extent in the data set during the periods 1956-1991/2004, Groisman et al. conclude that: “Generally speaking, the above means that the relationship that could be implied from ’simultaneous’ linear trends of the temperature in high latitudes and snow cover extent over northern Eurasia for the 1956–1991 (2004) periods cannot be based on the data for the 1936–2004 period.” In other words, the greatest trend in warming and its relationship with a decline in the cryosphere of northern Eurasia occurred during the period 1956-1991, as the very warm period 1936-1955 weakens the temperature trend, and the trend did not continue at the same rate after 1991.

The work of Groisman and his co-authors shows that elements of the cryosphere across northern Eurasia did decline over the past 50 years (1956-2004). However, the work clearly shows that over the last decade the declines were diminished, and over the greater period of the past 69 years (1936-2004) the length of time with snow on the ground across northern Eurasia actually increased (Figure 1). The increases were on the order of 3-5 percent, or 5-12 days per year. Groisman et al. state that “this is in agreement with other findings” and that it “cannot be associated with ‘Arctic warming’,” as there was no warming apparent during the period.

Figure 1. Annual snow cover duration over the former Soviet Union (dashed lines) and Russia (solid lines) as derived from satellite (red lines) and in situ observations (black lines). Separately, the Eurasian snow cover extent (106 km2) is shown for comparison (blue lines, right y-axes).

The findings of this work are important because (1) much has been made of Earth’s cryosphere serving as a monitor of climate change and (2) much has been made of the idea that the high latitude regions of the Northern Hemisphere have been associated with the largest increase in surface air temperature during the 20th century and will be for the 21st century. The work wonderfully illustrates the different pictures that can be painted when examining certain time intervals. Global warming alarmists would love to take a few minutes of your time to tell you about the great warming period of 1956-1991 across northern Eurasia – a time when data from the cryosphere cooperated. Groisman et al. have certainly helped us to see the rest of the story.


Groisman, P.Y., R.W. Knight, V.N. Razuvaev, O.N. Bulygina, and T.R. Karl, 2006. State of the ground: Climatology and changes during the past 69 years over northern Eurasia for a rarely used measure of snow cover and frozen land. Journal of Climate, 19, 4933-4955.

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