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Soil Temperature Records: Can They Be Used to Reconstruct Accurate Air Temperature Histories?
Zhang, T., Barry, R.G., Gilichinsky, D., Bykhovets, S.S., Sorokovikov, V.A. and Ye, J. 2001. An amplified signal of climatic change in soil temperatures during the last century at Irkutsk, Russia. Climatic Change 49: 41-76.

What was done
The authors examined records of soil temperature at various depths, along with several climatic indices, including air temperature, precipitation, snowfall and snow thickness data, at Irkutsk, Russia over the period 1898 to 1995.

What was learned
The relationship between air temperature and soil temperature was found to be so complex that, in the words of the authors, "changes in air temperature alone cannot explain the changes in soil temperatures in this region." Among the list of complex findings was the observation that summer soil temperatures cooled by up to 4C while summer air temperatures experienced a slight increase; while in the winter, air temperatures increased between 4 to 6C, while soil temperatures rose even higher, by as much as 9C. Possible explanations for these summer and winter trends include (1) an increase in summer rainfall and (2) an increase in early winter snowfall coupled with an earlier increase in spring snowmelt, respectively.

What it means
According to the authors, "when changes in soil temperature are used as evidence of climatic warming, caution is required because changes in soil temperature are a combined product of changes in air temperature and precipitation, especially snowfall and snow cover on ground." Furthermore, they stress that "present findings of the surface warming of permafrost at high latitudes and ground warming at a certain depth below ground surface elsewhere in the world could be fortuitous and may be misleading since air temperature alone cannot account for such a ground warming." Thus, the potential exists for serious flaws to manifest themselves in temperature histories derived from analyses of soil temperature data.