The climate record inverted from borehole temperatures in Slovenia

Abstract

Temperature profiles from nine boreholes in Slovenia, located mostly in the northeastern part of the country, have been studied for evidence of past climatic changes. The reconstruction of the ground surface temperature (GST) history, based on 1-D theory of heat conduction, was accomplished using the functional space inversion (FSI) method. Because of the paucity of quality thermal conductivity data and because the nature of heat conduction is such that information about GST changes decay with the passage of time, the a-priori standard deviations of the thermal conductivity model (1 W m −1 K −1) and measured temperature (0.2 K) were chosen large enough, so that the GST histories older than 200 years are considerably smoothed and essentially merge with the long-term mean. The average GST history based on consistent results from seven boreholes starts to rise at the end of the last century. The amplitude of the warming is about 0.6 to 0.7 K in the past 100 years. Two boreholes were rejected. The long-term mean GSTs lie between 9.4 and 11.1°C. The mean annual air temperatures from the Ljubljana meteorological station are in good agreement with the average GST solution. The GST history of the past 20–30 thousand years (ka) was reconstructed from the almost 2-km-deep Ljutomer temperature profile. The history evidences climate variations during the Würm (Weichselian) ice age: it places the glacial minimum of about 3°C at 13–14 ka ago and the postglacial maximum of 10.5°C roughly at 2–3 ka ago.