GEOTHERMICS AND CLIMATE-CHANGE - 2 - JOINT ANALYSIS OF BOREHOLE TEMPERATURE AND METEOROLOGICAL DATA

Long-period ground surface temperature variations contained in borehol e temperature-depth profiles form a complementary climate change recor d to high-frequency, but noisy surface air temperature (SAT) records a t weather stations. We illustrate the benefits of jointly analyzing ge othermal and meteorological data for two regions in Utah where both hi gh-quality temperature-depth measurements and century long SAT records exist. Transient temperature-depth profiles constructed from SAT time series reproduce in considerable detail borehole transient temperatur e-depth profiles. Typical rms differences between these transient temp erature profiles are less than 13 mK. The analysis yields a preobserva tional mean (POM) temperature, a parameter describing the long-term me an surface temperature prior to the onset of SAT measurements (i.e., p rior to the 20th century). The average POM for these two regions is 0. 6 degrees +/- 0.2 degrees C cooler than the 1951-1970 average SAT, sug gesting that 20th century warming represents a real and significant de parture from 19th century surface temperature values. In certain cases , borehole temperature profiles might be used as an independent check on long-wavelength adjustments made to SAT data.