Climate change inferred from analysis of borehole temperatures: An example from western Utah

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Abstract
Temperature‐depth profiles measured in a suite of boreholes in western Utah are used to infer climate change in the region over the past century and to document how effectively the solid earth records secular changes in surface air temperature. The data for this analysis consist of (1) high‐resolution temperature logs from six sites where terrain, hydrologic, and cultural disturbances to the temperature field are minimal, (2) surface air temperature records for the period 1891–1990 from seven meteorological stations geographically interspersed with the borehole sites, and (3) ground and air temperature records available from four weather stations. Deviations from linear temperature‐depth profiles in the boreholes, interpreted in terms of a linear change of surface temperature with time, suggest changes of −0.8°C to +0.6°C (average +0.3°C) in surface temperature for western Utah over the last several decades. These changes are consistent in trend but smaller in amplitude than the 100‐year linear trends in the surface air temperature data (average +0.8°C) for the same region. If the last 100 years of surface temperature change is assumed to be given by a nearby meteorological station record, then borehole temperatures yield additional information about the mean temperature prior to 1891. For three western Utah borehole sites the pre observational means are within ±0.3°C of the mean air temperatures for this century, indicating that up to 50% of the temperature increase seen in the 100 year record constitutes recovery from a cold period toward the end of the last century. The veracity with which our observed borehole temperature profiles match synthetic temperature‐depth profiles computed from air temperature records leaves little doubt that the solid earth is a valuable recorder of climatic change.