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On the Use of Borehole Temperatures to Deduce Earth's Climatic History
Gonzalez-Rouco, F., von Storch, H. and Zorita, E.  2003.  Deep soil temperature as proxy for surface air-temperature in a coupled model simulation of the last thousand years.  Geophysical Research Letters 30: 10.1029/2003GL018264.

In the words of the authors, "borehole temperature profiles (Huang et al., 2000) are perhaps the only direct measurements of past temperatures, in contrast to the analysis of other climate proxies that have to be interpreted in terms of climate anomalies through transfer functions."  However, as they note, "the validity of the interpretation of borehole temperature profiles has been questioned (Mann and Schmidt, 2003)."

What was done
Gonzalez-Rouco et al. investigated the relationship between simulated surface air temperature (SAT) and terrestrial deep soil temperature (TDST) in a climate simulation of the last millennium with the state-of-the-art coupled climate model (ECHO-G) described by Legutke and Voss (1999), which was driven by historical forcing provided by "solar variability, atmospheric greenhouse gas concentrations, and radiative effects of stratospheric volcanic aerosols, in the period 1000-1990 AD, derived from the estimations provided by Crowley (2000)."

What was learned
The authors report finding that "at long timescales, annual TDST is a good proxy for annual SAT, and their variations are almost indistinguishable from each other" in a significant rebuff to the challenge of Mann and Schmidt (2003).  Also in contradiction of the many Mann-inspired claims that late 20th century warming has led to temperatures that are unprecedented over the past millennium, they report that "the simulated annual global SAT shows a period of temperatures roughly as warm as today around 1100 A.D. (the Medieval Optimum), a subsequent cooling trend until around 1850 A.D. (the Little Ice Age) punctuated by deeper temperature minima at around 1450 A.D., 1700 A.D., and 1820 A.D., coincidental with known minima of the solar output or periods of more frequent volcanic eruptions (the Spoerer, Maunder and Dalton minima, respectively)."  In addition, they note that "recent reconstructions on extratropical tree-ring chronologies [such as Esper et al. (2002), see our Editorial of 27 Mar 2002] indicate a better agreement with the borehole based reconstructions."

What it means
As more and more data are collected and careful analyses are conducted, it is becoming ever more clear that (1) there has been nothing unusual about the temperature history of the past quarter-century and (2) current temperatures do not materially exceed those of the Medieval Warm Period.  If the warming of the past hundred or so years has seemed unusually steep, it is simply because the Little Ice Age was so cold, thanks largely to historical variations in solar activity.

Crowley, T.J.  2000.  Causes of climate change over the last 1000 years.  Science 289: 270-277.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Huang, S., Pollack, H.N. and Shen, P.-Y.  2000.  Temperature trends over the past five centuries reconstructed from borehole temperatures.  Nature 403: 756-758.

Legutke, S. and Voss, R.  1999.  The Hamburg Atmosphere-Ocean Coupled Circulation Model ECHO-G.  Technical Report No. 18.  German Climate Computing Center, Hamburg, Germany.

Mann, M.E. and Schmidt, G.A.  2003.  Ground vs. surface air temperature trends: Implications for borehole surface temperature reconstructions.  Geophysical Research Letters 30: 10.1029/2003GL017170.

Reviewed 17 December 2003