Ground warming and continental energy storage in the Northern Hemisphere during the Common Era: Impact of borehole depth.

Abstract:

Estimates of ground surface temperature changes and continental energy
storage from geothermal data have become well accepted indicators of past climatic changes since industrialisation. Borehole reconstructions of these quantities over the Common Era have been used, inter alia, for the validation of general circulation models and to help quantify the overall energy budget of the global climate system. Recent analyses of geothermal data have been performed with data obtained from the borehole climatology data base of the International Heat Flow Commission that contains subsurface temperature profiles for over 600 sites spread across the Northern Hemisphere, with the majority of maximum borehole depths varying between 200 m to 600 m. Due to the nature of heat conduction, different depth ranges contain the record of past and persistent changes in the energy balance between the lower atmosphere and the ground for different time periods.
 Here we examine the dependency of estimated ground surface temperature histories and subsurface heat content changes on the maximum depth of the borehole temperature profile. We find that previous results remain unchanged for the period since industrialisation, but differ during earlier periods over the last millennium. We suggest that large-scale temperature reconstructions from terrestrial boreholes should be derived from datasets that comprise boreholes truncated at similar thermal depths. Analyses that do not control for the impacts of the depth of truncation are otherwise subject to difficult-to-estimate biases in temperature change estimates during the early and middle part of the last millennium.