Preliminary ground surface temperature history in mainland Portugal reconstructed from borehole temperature logs

From a set of twenty temperature logs, measured at fourteen sites in mainla nd Portugal, only seven were found suitable for estimating the ground surfa ce temperature (GST) histories. Seven of the rejected logs are noisy, proba bly due to convection in the borehole, and six display groundwater signatur e. The boreholes were logged in the time period 1989-1991 and the depth ran ge of the logs varies between 155 and 485 m. In order to reconstruct the su rface temperature past changes the functional space inversion method was us ed, To avoid the reconstruction of spurious GST variations from non-climati c features of the profiles, the inversions were carried out for large aprio ri thermal conductivity and temperature standard deviations (2 W m(-1)K(-1) , 0.1 K). Despite a scatter in the results, most of the inversions reveal w arming in the last 100-150 years. It turned out, however, that a joint inve rsion was necessary to suppress noise and extract the signal common to all profiles. The joint GST history shows warming by 0.5-0.6 K since the second half of the 18th century with a maximum in the 40's, followed by 0.2 K coo ling till the beginning of the 80's. The sign and timing of the GST changes are consistent with the air temperature record of a meteorological station in Lisbon. However, their amplitude is half the average of the air tempera ture variation. (C) 1999 Elsevier Science B.V. All rights reserved.