Borehole temperature-depth profiles potentially contain a rich source of information about the Earth’s changing surface temperature.  In the absence of moving fluids, changes in surface ground temperature (SGT) diffuse slowly downward by heat conduction and are later manifested as departures from the Earth’s background temperature regime.  Because the thermal diffusivity of rock is relatively low, temperature changes that occurred 500 years ago are recoverable in boreholes that are several hundred meters deep with high accuracy temperature measurements.  This method of climate reconstruction has several advantages including (1) bypassing empirical temperature calibrations inherent with proxy records of climate change; (2) extending the record of climate change to remote widely dispersed continental sites where meteorological records of climate change are not available; and (3) establishing a surface baseline temperature prior to the industrial revolution when anthropogenic greenhouse gases started their unprecedented growth.  However questions about links between surface air and ground temperature, and noise in subsurface temperature records need to be addressed to improve this record of climate change.

We show that changes in surface air temperature (SAT) and subsurface temperature profiles are strongly coupled by investigating links between borehole temperatures in northwestern Utah that have been repeatedly measured over a time span of 30 years and nearby surface air temperature records over the same time span.  Over this 30-year time span transient temperatures diffuse to approximately 100 m below the ground surface; below this depth transients are within observational noise.  Computing differences between temperature logs isolates transient variations in ground temperature that can be ascribed to changes in GST. The SAT records are used as forcing functions at the ground surface to compute the subsurface temperature field.  SAT data fit both the amplitude and pattern of transient temperature observations fitting the borehole temperature profiles to within 0.03° C or better. This experiment confirms strong coupling between surface temperature change and borehole temperature transients supporting the use of borehole temperatures to complement SAT and multiproxy reconstructions of climate change.

We also investigated broader climatic variables and the subsurface temperature record using data collected at meteorological stations in the Oregon Cascades operated by the Corvallis, Oregon office of the Environmental Protection Agency. The meteorological stations are paired sites in both mature and clear-cut forest locations. We used these data to model changes in ground temperature and water content using the Noah Land Surface Model (LSM) from the National Center for Environmental Prediction and compared modeled results with subsurface observations. A comparison of meteorologic data at the site Soapgrass Mountain in the Oregon Cascades from the forested and clear-cut site leads to the following conclusions: (1) The magnitude of subsurface temperature is largely a function of solar radiation and winter snow cover. Differences in solar radiation and snow cover between the forested and clear-cut sites lead to proportional changes in the magnitude of subsurface temperature between these sites; (2) Changes in subsurface temperatures generally track changes in air temperature; (3) Precipitation has the greatest impact on soil moisture. Changes in soil moisture can be directly correlated to precipitation events. However, there is no apparent change in the magnitude or variation of subsurface temperature related to precipitation.

We also show predictions of temperature change in the next century informed by temperature changes in the past. For illustrative and edu...

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