Long-period ground surface temperature variations contained in borehole temperature-depth profiles form a complementary climate change record to high-frequency, but noisy surface air temperature (SAT) records at weather stations. We illustrate the benefits of jointly analyzing geothermal and meteorological data for two regions in Utah where both high-quality temperature-depth measurements and contury long SAT records exist. Transient temperature-depth profiles constructed from SAT time series reproduce in considerable detail borehole transient temperature-depth profiles. Typical rms differences between these transient temperature profiles are less than 13 mK. The analysis yields a preobservational mean (POM) temperature, a parameter describing the long-term mean surface temperature prior to the onset of SAT measurements (i.e., prior to the 20th century). The average POM for these two regions is 0.6¿¿0.2 ¿C cooler than the 1951--1970 average SAT, suggesting that 20th century warming represents a real and significant departure 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. ¿ 1998 American Geophysical Union