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The Pre-Observational Mean Temperature of Tachlovice, Czech Republic
Reference
Bodri, L. and Cermak, V.  2005.  Borehole temperatures, climate change and the pre-observational surface air temperature mean: Allowance for hydraulic conditions.  Global and Planetary Change 45: 265-276.

What was done
Borehole temperatures have been increasingly used to obtain proxy climate signals at various locations across the surface of the earth.  However, the techniques employed by scientists reconstructing pre-observational (pre-instrumental era) mean temperatures from these data suffer from certain limitations, one of the major ones being the presence of underground fluids that can distort the true climatic signal (Lewis and Wang, 1992).  In the present study, Bodri and Cermak develop a corrective measure that accounts for vertical conductive and advective heat transport in a 1-D horizontally-layered stratum, as opposed to the current purely conductive approach, which they apply to four borehole records drilled near Tachlovice, Czech Republic.

What was learned
Analyses of the four borehole temperature logs revealed that the new conductive/advective approach was far superior to the purely conductive approach, explaining 83-95% of the temperature signal where the purely conductive model could explain no more than 27-58%.  In addition, the purely conductive approach was found to significantly underestimate the pre-observational mean temperature by 0.3 to 0.5C.  This underestimate produces a significant overestimate of the degree of warming experienced from the pre-observational period to the present.  What is more, the two scientists report that both pre-observational mean temperature values for 18th-century Bohemia (the one derived from the conductive/advective approach and the one derived from the purely conductive approach) "exceed the annual temperatures characteristic for the 19th/20th centuries" and "may indicate that the warming has still not achieved its earlier (late 18th century) level."

What it means
The results of this study indicate a significant warming bias is likely present in borehole temperature reconstructions based solely on a purely conductive approach.  Unless accounted for and removed, this bias can result in incorrect interpretations of 20th-century warming, which, as in the case of Tachlovice, need not be a manifestation of unprecedented global warming, but, in the words of Bodri and Cermak, merely "a recovery to previous warmer conditions after a noted cold period."

Reference
Lewis, T.J. and Wand, K.  1992.  Influence of terrain on bedrock temperatures.  Global and Planetary Change 98: 87-100.

Reviewed 10 August 2005