Zhang et al. (2001) examined records of soil temperature depth profiles, along with several climatic indices, including air temperature, precipitation, snowfall and snow thickness, at Irkutsk, Russia over the period 1898 to 1995. They found a relationship between air temperature and soil temperature that was 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 4°C while summer air temperatures experienced a slight increase; while in the winter, air temperatures increased between 4 to 6°C, while soil temperatures rose even higher, by as much as 9°C. 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.
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. However, useful climatic information can be obtained through direct comparisons of both surface and soil temperature histories, as in the case of Changnon (1999).
Rather than deriving a surface air temperature history from a soil temperature record, Changnon compared a soil temperature series from a totally rural setting in central Illinois between 1889 and 1952 with a contemporary series of air temperature measurements made in an adjacent growing community, as well as similar data obtained from other nearby small towns, in an effort to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed by the IPCC to be free of urban-induced warming. Results indicated that soil temperatures obtained in the totally rural setting experienced a temperature increase of 0.4°C from the decade of 1901-1910 to that of 1941-1950. This warming was 0.2°C less than the 0.6°C warming determined for the same time period from data of the U.S. Historical Climate Network, which is supposedly corrected for urban heating effects. It is also 0.2°C less than the 0.6°C warming determined for this time period by eleven benchmark stations in Illinois with the highest quality long-term temperature data, all of which are located in communities with populations of less than 6,000 people as of 1990. And it is 0.17°C less than the 0.57°C warming derived from data obtained from the three benchmark stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people.
The results of Changnon’s study led him to conclude that "both sets of surface air temperature data for Illinois believed to have the best data quality with little or no urban effects may contain urban influences causing increases of 0.2°C from 1901 to 1950." He further notes - in a grand understatement - that "this could be significant because the IPCC (1995) indicated that the global mean temperature increased 0.3°C from 1890 to 1950." Thus, truly rural soil temperature series may provide valuable information to scientists attempting to separate the influence of urban warming from the global surface air temperature record.
References
Changnon, S.A. 1999. A rare long record of deep soil temperatures defines temporal temperature changes and an urban heat island. Climatic Change 42: 531-538.
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.