Most climate records and climate change scenarios projected by general circulation models are for atmospheric conditions. However, permafrost distribution as well as ecological and biogeochemical processes at high latitudes is mainly controlled by soil thermal conditions, which may be affected by atmospheric climate change. In this paper, the changes in soil temperature during the twentieth century in Canada were simulated at 0.5¿ latitude/longitude spatial resolution using a process-based model. The results show that the mean annual soil temperature differed from the mean annual air temperature by -2¿ to 7¿C, with a national average of 2.5¿C. Soil temperature generally responded to the forcing of air temperature but in complex ways. The changes in annual mean soil temperature during the twentieth century differed from that of air temperature by -3¿ to 3¿C from place to place, and the difference was more significant in winter and spring. On average, for the whole of Canada the annual mean soil temperature at 20 cm depth increased by 0.6¿C, while the annual mean air temperature increased by 1.0¿C. Three mechanisms were investigated to explain this differentiation: air temperature change, which altered the thickness and duration of snow cover, thereby altering the response of soil temperature; seasonal differences in changes of air temperature; and changes in precipitation. The first two mechanisms generally buffer the response of soil temperature to changes in air temperature, while the effect of precipitation is significant and varies with time and space. This complex response of soil temperature to changes in air temperature and precipitation would have significant implications for the impacts of climate change.