Heat flow and inferred ground surface temperature history at Tynong North, southeastern Australia

Abstract

ABSTRACTBorehole temperature data have the potential to record historical variations in ground and air surface temperature, yet very few reliable, purpose-drilled, boreholes are available to explore such impacts, particularly in the southern hemisphere. The 400-m deep Tynong-1 borehole, approximately 65 km ESE of Melbourne, Australia, was drilled specifically to determine conductive heat flow and provides a unique dataset for evaluating ground surface temperature history in southeastern Australia. Steady-state conductive heat flow of 87 ± 1 mW m−2 was determined in the deeper borehole sections, with measured temperature profiles clearly demonstrating a progressive divergence of the observed temperature profile from the equilibrium model in the upper ∼150 m of the hole. We applied a Bayesian method employing a reverse jump Markov chain Monte Carlo search algorithm to explore the origins of this variation. Our results indicate a 2°C increase in ground surface temperature since 1800, after at least 500 years of relatively stable ground surface temperature. The inversion results are consistent with the trend of surface air temperature recorded in southeast Victoria by historical meteorological data since 1950. The inferred increase in ground surface temperature evident prior to 1950 is likely a cumulative effect of land clearing and a rise in surface air temperature.