Geothermics and climate change 2. Joint analysis of borehole temperature and meteorological data

Abstract

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 century 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.