Ground and air temperature tracking: Applications in climate change

Borehole temperature-depth profiles contain a record of surface ground temperature (SGT) changes with time and complement surface air temperature (SAT) analysis to infer climate change over multiple centuries. Ground temperatures are generally warmer than air temperatures due to solar radiation effects in the summer and the insulating effect of snow cover during the winter. The low thermal diffusivity of snow damps surface temperature variations; snow effectively acts as an insulator of the ground during the coldest part of the year. A numerical model of snow-ground thermal interactions is developed to investigate the effect of seasonal snow cover on annual ground temperatures. Using the model, we calculate the response of SGT to changes in seasonal snow cover in North America from 1950--2002, the period for which comprehensive snow and air observations exist across the region. There are no significant trends in either onset or duration from 1950--2002. Winter season air temperature has warmed during this period, particularly from 1970--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. In addition to snow cover, incident solar radiation can have a pronounced effect on ground temperatures. Using data from Emigrant Pass Observatory in northwestern Utah, we investigate the impact that radiation and snow have had on SGT at this site during the period 1993--2004. Solar radiation is observed to exert a linear influence on the SGT-SAT difference with a trend of 1.21 K per 100 Wm-2 and can account for the 2.47 K average offset in SGT-SAT. Snow cover exerts only a minor influence on the annual tracking of SGT and SAT at the site.