Background
The authors note that recent studies have questioned whether processes operating at the earth's surface bias the geothermal (borehole) method of reconstructing long-term surface temperature histories, "thereby rendering them unsuitable for climatic reconstruction," as claimed by Mann and Schmidt (2003) and Mann et al. (2003).  Hence, they conducted an experiment to address this question.

What was done
Beltrami et al. analyzed high-quality borehole temperature data measured in four boreholes in quasi-steady state from a small region in northern Quebec, to ascertain whether records of surface air temperature collected nearby reproduce the subsurface temperature anomalies observed in the area.

What was learned
This exercise revealed, in the words of the authors, that "subsurface temperatures are consistent with nearly 70 years of surface air temperature data even though snow cover effects and soil moisture phase changes are present."

What it means
Beltrami et al. say their results "show that the ground tracks the variations of the surface air temperature," that "borehole data can be considered as a robust and independent indicator of past climatic conditions," and that "borehole records are robust long-term paleoclimatological indicators."

References
Mann, M.E., Rutherford, S., Bradley, R.S., Hughes, M.K. and Keimig, F.T.  2003.  Optimal surface temperature reconstructions using terrestrial borehole data.  Journal of Geophysical Research 108: 10.1029/2002JD002532.

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.