Climate change of the last 2000 years inferred from borehole temperatures: data from Finland

Temperature–depth profiles measured in three boreholes in the Finnish part of the Baltic shield are used to infer climate pattern in the region over the last 2000 years. The reconstructed ground surface temperature (GST) variations for this period have not exceeded the range of 2.0–2.5 K. Obtained GST history includes a long cold period before year 1000 A.D., followed by gradually warming conditions which culminated around 1500 A.D., and also by several less pronounced recent climatic excursions. The study confirmed certain incoherence of the climatic history in Finland with the results obtained for other parts of Europe. Combination of borehole temperature profiles with the meteorological surface air temperatures (SAT) provided a 19th century baseline temperature—that is, 0.42±0.13 K below the 1961–1993 mean temperature for the region. Our calculations yield a noise reduction in the estimate of the 20th century warming trend and evidence an accelerated warming in the last decade.