1. Annual air temperatures from the US HCN for the north central United States show a high degree of interannual variability and overall trends of warming from 1895 until the mid 1930’s followed by cooling from the late -1930’s to the early 1970’sa and warming from 1970 to present. The climate divisions shown are those that contain borehole sites. A Test of Borehole Paleoclimatology William D. Gosnold, Jr., Xiquan Dong, Julie Popham A critical comparison in this study is that of the multi-proxy record with the borehole record. Here we compare the synthetic T-z profiles for four proxy records with a synthetic borehole profile based on the GST record of Huang and Pollack (2000). We also compare the 1895-2000 climate data for the North American mid-continent with borehole records from the north central US. A critical point is that both the borehole data and the climate data show that warming during the past century has increased in a pattern predicted by GCM models based on increases in greenhouse gases. That is warming increases with latitude in the mid continent region. PP52A-0659 Climate Change in the Recent Past: Integrating Meteorological, Proxy, Borehole, and Modeled Climate Reconstructions It has been argued that differences between multi-proxy reconstructions of temperature change and inversions of borehole temperatures are largely due to the fact that snow cover decouples air and ground temperatures resulting in biased trends in the GST recorded in boreholes [Mann et al., 2003; Mann and Schmidt, 2003; Schmidt and Mann, 2004]. Chapman et al., [2004] counter that this interpretation of the differences are based in “selective and inappropriate presentation of model results by Mann and Schmidt,” and provide examples of good correlations between SAT and GST.” We address the argument by assessing the accuracy of direct coupling between ground surface temperatures (GST) and surface air temperatures (SAT) in regions affected by seasonal snow cover in two critical tests: (1) Comparison of borehole T-z profiles to synthetic T-z profiles generated from century-long daily, monthly, and annual SAT data from the north central US and Canada. (2) Comparison of changes in borehole T-z profiles over two or more decades to changes in SAT recorded at automated weather stations near the borehole sites. The upper section of this poster focuses on the first test and the lower section of the poster focuses on the second test. The results from Alberta and Saskatchewan were presented by Jacek Majorowicz this morning in the oral session of PP19. This research is supported by National Science Foundation Award ATM - 0318384 Fig. 1 (below). Borehole sites included in this study are shown as red circles and climate stations are shown as green squares. Boreholes in Kansas, Nebraska, South Dakota, North Dakota, Manitoba, and Ontario were drilled specifically for heat flow measurements. Sites in Saskatchewan and Alberta are holes of opportunity that were drilled for mineral or oil exploration. Climate stations are part of the US Historical Climatology Network and climate data were obtained from National Climatic Data Center, High Plains Regional Climate Center, and Environment Canada. A critical aspect of the heat flow boreholes in the US is that they were sited specifically to be free from microclimate effects, land use change, and terrain effects that could contaminate the temperature- depth profiles. Conclusions: 1.Observed changes in multiple T-z profiles at sites which experience seasonal snow cover in the Northern US and Canada match closely with SAT data. 2. Synthetic T-z profiles derived from SAT data from the US and Canada match closely with observed T-z profiles. 3. Contamination of the climate signal in borehole data due to terrain effects and changes in microclimate and land use at borehole sites can be recognized and avoided. 4. Both the borehole data and the climate data show that warming during the past century has increased in a pattern predicted by GCM models based on increases in greenhouse gases. Multiple T-z profiles recorded between 1990 and 2002 compared to synthetic T-z profiles using ensembles of climate data from the automated arrays depicted in Figure 1 show that the borehole profiles changed in close agreement with the SAT. The two examples shown above are two of a total of 51 that have been examined to date. Of this number only three show disagreement with predicted changes. In each case, the reason for disagreement was found to be change in the microclimate of either the borehole or the climate station. In addition to the sites in South Dakota, multiple T-z logs from a site in New Hampshire show excellent correlation in overall between observed T-z profiles and synthetic profiles based on climate data. New Hampshire site US_NH-72 shows a signal opposite to predictions and to observations at USNHa. The disagreement is caused by terrain effects at that site. See poster by Heinle and Gosnold. The plots above compare synthetic T-z profiles generated by climate data from 1895 to the date of borehole logs with borehole T-z profiles. Background heat flow was removed from the borehole profiles so the curves reflect only the climate signal and variation in thermal conductivity. Sites in North Dakota and South Dakota penetrated relatively homogeneous bedrock (Pierre Shale) and have lesser noise due to variability in thermal conductivity. Interestingly, the best agreement between GST and SAT is found in North Dakota and South Dakota, i.e., regions with significant seasonal snow cover. The plots above show synthetic T-z profiles at 10-year intervals for the period of record generated by the SAT data in the top panels. Two-dimensional finite-difference conductive models used monthly temperatures from the period of record to generate the synthetic profiles. Linear least-squares fits to SAT time series were used to translate the SAT data so that the initial temperatures, i.e., the pre-observational mean, were 0 °C. References: Chapman, D. S., M. G. Bartlett, and R. N. Harris (2004), Comment on ‘‘Ground vs. surface air temperature trends: Implications for borehole surface temperature reconstructions’’ by M. E. Mann and G. Schmidt, Geophys. Res. Lett., 31,L07205, doi:10.1029/2003GL019054. Huang, S., H. N. Pollack, and P. Y. Shen (2000), Temperature trends over the past five centuries reconstructed from borehole temperatures, Nature, 403, 756–758. Mann, M. E., and G. Schmidt (2003), Ground vs. surface air temperature trends: Implications for borehole surface temperature reconstructions, Geophys. Res. Lett., 30(12), 1607, doi:10.1029/2003GL017170. Mann, M. E., R. S. Bradley, and M. K. Hughes (1998), Global-scale temperature patterns and climate forcing over the past six centuries, Nature, 392, 779– 787.