A quality assured comparison of the surface air temperature and ground surface temperature histories from two North American mid-continent boreholes

Ground surface temperature histories (GSTH) obtained from boreholes have recently been proposed as a method of paleoclimate reconstruction. Temperature-depth (T-z) profiles act as a low-pass filter that filters high-frequency ground surface temperature (GST) noise, but records longer term trends in the GST. Several studies suggest that recent warming trends derived from geothermal methods agree closely with warming trends measured in the historical instrumented record. Forcings other than the surface climate regime, however, can also produce anomalies in T-z profiles. Previous investigations have generally assumed that the GST history (GSTH) closely tracks the SAT history (SATH). Efforts to document the relation between the GSTH and SATH have generally used an ensemble approach in which the climate records from large numbers of boreholes and meteorological stations within a region have been analyzed and compared.

We use a quality assured approach to compare the GSTH and SATH from two boreholes in the mid-continent of North America that were expressly drilled for paleoclimate reconstruction purposes. Boreholes were drilled and logged at Gunter, TX and Wall, SD in sites within native grassland environments that were optimal with respect to conditions for paleoclimate reconstruction. The Wall site experienced seasonally frozen soils and permanent winter snow cover, while the Gunter site was in a more temperate climate that received only negligible snow cover. A geostatistical analysis of the mean annual air temperature and annual precipitation fields was performed within each region to select the Historical Climatology Network station (HCN) for each comparison, and to document the coefficients of determination (R^2) between the borehole sites and the HCN stations. Cottonwood, SD and Greenville, TX were selected for the quality assured comparison.

The GSTH at Wall, SD indicates a warming of +1.75°C over the past 100 years. The GSTH at Gunter, TX shows a warming of +0.50°C over the past century. In contrast, the SATH record at Cottonwood, SD reveals a warming of +0.94°C over the past century, with statistically significant warming trends during the winter and spring. Greenville, TX, however, exhibits a cooling trend of -1.75°C over the past 100 years, with statistically significant cooling trends during the winter, spring and fall. Annual and seasonal trends in other climatic variables were also examined to more clearly document the climate change record over the past century, and to serve as a basis for assessing the comparison of the GSTH and SATH results. Results suggest that boreholes are unique paleoclimate indicators. Although changes in T-z profiles are a direct thermophysical consequence of changes in the GST, researchers cannot assume any inferred relationship between the GSTH and SATH records. Improved understanding of the physical relationship between T-z profiles and the surface microclimate in a more diverse range of climatic and lithologic environments is needed.