Ground surface temperature history at a single site in southern Portugal reconstructed from borehole temperatures

Abstract

Reliable reconstruction of ground surface temperature (GST) history from borehole temperatures can often be biased by convective heat transport, unrecognized thermal conductivity variations, topography and vegetation changes near borehole locations. To a great extent, all these factors are negligible in the case of a 200-m-deep borehole located near the town of Evora, in southern Portugal. This allows confidence in the interpretation of the borehole temperature versus depth ( T– z) profile in terms of the GST changes during the last 150–200 years. To obtain estimates of the GST history, the functional space inversion method was used, which allows incorporation of uncertainties in the data as a priori standard deviations. The method yielded a GST history that indicates warming of about 1 K since the second half of the last century to the middle of the 1990s, with an increase in the last 10–15 years. The results agree with the surface air temperatures (SAT) recorded at the Lisbon meteorological station since 1856, which display a warming trend with an amplitude of about 1 K for the same period. The reduced temperature of the studied T– z profile was used to extend the observed SAT series to times before the instrumental period by estimating the long-term pre-observational mean (POM). The shape of the reduced temperature curve is best fit by POM values that are a few 10ths of a degree Celsius higher than 15.6°C, which is the mean of the SAT series in the period 1856–1900.