Geothermics of Climate Change

Temperature profiles in boreholes contain valuable information about surface temperature changes over the past five centuries. Borehole temperatures are particularly valuable because they are (1) a direct rather than proxy measure of temperature change, (2) need no calibration to the surface air temperature (SAT) record, (3) respond to annual rather than seasonal temperatures on the surface, and (4) capture long period (decade to century) surface temperature changes. Work at the University of Utah has validated using borehole temperatures to study climate change by systematically answering several questions: How well does ground temperature track air temperature?; What is the quantitative “snow effect” on ground temperature?; How might one combine borehole temperatures with SAT data?; and, Can we really isolate the transient Earth temperature field? Several hundred boreholes spread over all continents are available for analysis. They confirm widespread, but not uniform, surface warming of 1.1 +/-0.2 oC generally consistent with the instrumental record; some areas at high latitudes show much greater warming. Whereas early multi-proxy reconstructions departed significantly from the borehole temperature reconstructions, more recent multi-proxy analyses designed to extract longer period information are now in essential agreement with the borehole studies to about 1600 AD. We now need to translate the greater certainty with which we know past climate change into creative approaches to meet the challenges of future climate change.