Frauenfeld, O.W. and Zhang, T. 2011. An observational 71-year history of seasonally frozen ground changes in the Eurasian high latitudes. Environmental Research Letters 6: 10.1088/1748-9326/6/4/044024.
The active layer depth of seasonally-frozen ground is the maximum annual depth of thaw over permafrost; and it is thus a reliable indicator of climate change (Nelson et al., 1997; Brown et al., 2000; Osterkamp, 2007). Therefore, in the words of the authors, "long-term increases in active layer depth indicate that permafrost is degrading from the top down," while in a warming climate they indicate that "less and less land area experiences seasonal freezing, and the depth of the frost penetration also decreases over time."
What was done
Focusing on the massive seasonally-frozen ground region of Eurasia, Frauenfeld and Zhang employed a database of soil temperature profiles obtained at 423 stations to estimate the maximum annual soil freezing depth at 387 sites, which they did for the period 1930-2000, with an extension to 2008 based on the close correlation between maximum soil freezing depth and cumulative degree-days of below 0°C over the course of each year's cold season.
What was learned
From 1930 to the late 1960s, there was no significant trend in seasonal freeze depth. Thereafter, however, seasonal freeze depths decreased significantly until the early 1990s, indicative of significant warming. But "from that point forward, likely through at least 2008," in the words of the two researchers, "no change is evident," indicative of a stagnation in Eurasian temperatures.
What it means
Except for warming during the 1970s and 80s, northern Eurasian temperatures appear to have remained fairly stable. And of that warming, Frauenfeld and Zhang state that "the strong decrease in seasonal freeze depths during the 1970s to 1990s was likely the result of strong atmospheric forcing from the North Atlantic Oscillation during that time period." Thus, their work provides little to no evidence for any significant warming of this massive portion of earth's land mass over the past two decades, and absolutely no evidence for recent CO2-induced warming.
Brown, J., Hinkel, K.M. and Nelson, F.E. 2000. The circumpolar active layer monitoring (CALM) program: research designs and initial results. Polar Geography 24: 165-258.
Nelson, F.E., Shiklomanov, N.I., Mueller, G.R., Hinkel, K.M., Walker, D.A. and Bockheim, J.G. 1997. Estimating active-layer thickness over a large region: Kuparuk river basin Alaska USA. Arctic and Alpine Research 4: 367-378.Reviewed 29 February 2012