Snow effect on North American ground temperatures, 1950-2002

Changes in snow's influence on surface ground temperature (SGT) could create a bias in the borehole temperature record of climate change. Using a snow-ground thermal model which predicts changes in the mean annual offset between SGT and surface air temperature (SAT), we calculate the response of SGT to changes in seasonal snow cover in North America from 1950 to 2002, the period for which comprehensive snow and air observations exist across the region. Daily snow and SAT observations come from the U. S. Historical Climatology Network, the Canadian Daily Climatic Dataset, and a set of National Weather Service cooperative stations in Alaska. For the period 1961-1990 the mean snow onset date in North America is 15 December, with mean snow cover duration of 81 days. There are no significant trends in either onset or duration from 1950 to 2002. Winter season air temperature, however, has warmed during this period, particularly from 1970 to 2002. The effect of the combination of a relatively stationary snow season with winter season SAT warming has been to diminish the mean annual SGT-SAT offset by -0.05 K/decade over the past 30 years. This effect is most pronounced between 50 degrees and 75 degrees N in west central North America, coincident with the location of greatest winter season warming since 1970. Although comprehensive snow cover data do not exist prior to 1950, this analysis quantifies the changes in snow cover required to account for the difference between borehole temperature and multiproxy climate reconstructions.