TESTING THE ASSUMPTIONS OF BOREHOLE PALEOCLIMATE RECONSTRUCTIONS

Paleoclimate reconstructions using borehole T-z profiles require the assumption that the surface air temperature correlates 1:1 with the ground temperature. We are testing this assumption in several ways. First, we compare climate driven T-z models with T-z data collected at several intervals during the past two decades. Three boreholes in North Dakota, four in South Dakota, three in Nebraska and three in Saskatchewan were logged at multiple times between 1978 and 2003. These boreholes are located within several tens of kilometers of NCDC or Environment Canada weather stations, thus they offer and excellent opportunity for comparative study. We compared individual climate station data and ensembles of climate station data to the borehole records, all of which show ground surface warming between logging intervals. In this aspect of our study, monthly mean-air-temperatures obtained from NCDC weather stations within 100 km of the boreholes were used as model-forcing signals at each borehole site (Figure 1). Two ensemble signals, one for weather stations within a 50 km radius of the borehole and one within a 100 km radius of the borehole were used in the test and found to agree closely with the borehole data. Radiative heating and heat exchange between the ground and the air directly control the ground surface temperature, and a time-series of borehole T-z measurements spanning time periods when solar radiation, soil and air temperatures have been recorded will enable comparison of the thermal energy stored in the ground to these quantities. If coherence between energy storage, solar radiation, GST, SAT and multi-proxy temperature data can be discerned for a one or two decade, synthesis of these data over the past several centuries may enable us to separately determine anthropogenic and natural forcing of the climate change.