Mid-Latitude (30 degrees-60 degrees N) climatic warming inferred by combining borehole temperatures with surface air temperatures

We construct a mid-latitude (30 degrees -60 degrees N) reduced temperature- depth profile from a global borehole temperature database compiled for clim ate reconstruction. This reduced temperature profile is interpreted in term s of past surface ground temperature change and indicates warming on the or der of 1 degreesC over the past 100 to 200 years. The combination of an ini tial temperature (the primary free parameter) with the last 140 years of gr idded surface air temperature (SAT) data yields a synthetic temperature pro file that is an excellent fit to observations, accounting for 99% of the ob served variance and a RMS misfit of only 12 mK. The good correlation sugges ts that this reduced temperature profile shares much information with the m ean SAT record over large areas and long time-scales. Our analysis indicate s 0.7 degrees +/- 0.1 degreesC of ground warming between pre-industrial tim e and the 1961-1990 mean SAT.