An amplified signal of climatic change in soil temperatures during the last century at Irkutsk, Russia

Climatic changes at the Earth's surface propagate slowly downward into the ground and modify the ambient ground thermal regime. However, causes of soi l temperature changes in the upper few meters are not well documented. One major obstacle to understanding the linkage between the soil thermal regime and climatic change is the lack of long-term observations of soil temperat ures and related climatic variables. Such measurements were made throughout the former Soviet Union with some records beginning at the end of the 19th century. In this paper, we use records from Irkutsk, Russia, to demonstrat e how the soil temperature responded to climatic changes over the last cent ury. Both air temperature and precipitation at Irkutsk increased from the l ate 1890s to the 1990s. Changes in air temperature mainly occurred in winte r, while changes in precipitation happened mainly during summer. There was an anti-correlation between mean annual air temperature and annual total pr ecipitation, i.e., more (less) precipitation during cold (warm) years. Ther e were no significant trends of changes in the first day of snow on the gro und in autumn, but snow steadily disappeared earlier in spring, resulting i n a reduction of the snow cover duration. A grass-covered soil experiences seasonal freezing for more than nine months each year and the long-term ave rage maximum depth of seasonally frozen soils was about 177 cm with a range from 91 cm to 260 cm. The relatively lower soil temperature at shallow dep ths appears to represent the so-called 'thermal offset' in seasonally froze n soils. Changes in mean annual air temperature and soil temperature at 40 cm depth were about the same magnitude (2.0 degreesC to 2.5 degreesC) over the common period of record, but the patterns of change were substantially different. Mean annual air temperature increased slightly until the 1960s, while mean annual soil temperature increased steadily throughout the entire period. This leads to the conclusion that changes in air temperature alone cannot explain the changes in soil temperatures at this station. Soil temp erature actually decreased during summer months by up to 4 degreesC, while air temperature increased slightly. This cooling in the soil may be explain ed by changes in rainfall and hence soil moisture during summer due to the effect of a soil moisture feedback mechanism. While air temperature increas ed about 4 degreesC to 6 degreesC during winter, soil temperature increased by up to 9 degreesC. An increase in snowfall during early winter (October and November) and early snowmelt in spring may play a major role in the inc rease of soil temperatures through the effects of insulation and albedo cha nges. Due to its relatively higher thermal conductivity compared to unfroze n soils, seasonally frozen ground may enhance the soil cooling, especially in autumn and winter when thermal gradient is negative.