On the use of geothermal data for climate studies: a case study from Portugal

Portugal using geothermal data has started in 1996. Reconstruction of ground surface temperature (GST) history from temperature logs measured in a 200 m deep borehole (TGQC-1), located near the town of Evora in Portugal, indicates warming of 1K since the second half of the nineteenth century to the middle of the 90s of the twentieth century, increasing considerably in the last 10 years. Results of the reconstruction (based on the functional space inversion (FSI) method) are compared with air temperatures recorded at the Lisbon meteorological station since 1856. The series display a warming trend with the amplitude about 1K for the same period. The coupling of the air and ground temperature changes and their downward propagation by heat conduction was confirmed by repeated logging in November 2003, 6.7 years after obtaining the first temperature log.