Temperature signal in the underground for climate history reconstruction in Italy

Underground temperature data from height boreholes logged between 1981 and 2000 were studied to infer the climate change in central-northern Italy during the last 250 years. The ground surface temperature history was reconstructed by using the functional space inversion method. A different inverse approach was also used for two temperature sets to obtain the fine details of the most recent surface temperature change. The results were compared with the air temperature recorded since the beginning of the 19th century at meteorological observatories. The analysis puts into evidence that the trend of the temperature change in the western side of the Apennines chain differs from that of the eastern side. Since 1750 the western side shows temperature lower than that of the 1990s, with minimum values in the period 1930-1960, followed by an almost linear increase in the ground surface temperature. Along the eastern side the temperature is always larger than that inferred for the 1970s, with maximum values in the period 1920-1940, which is followed by a sharp temperature decrease. Only since 1970-1980 a local warming phase has started. By combining borehole temperature logs with meteorological surface air temperature records, the pre-observational mean temperature was calculated. The results corroborate the difference of the climatic histories in both sides of the Apennines concerning the ground surface temperatures. It also appears that the recent climatic changes have partly a local origin and can obscure the changes forced by the regional surface air temperature influence.