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

We construct a mid-latitude (30–60 N) reduced temperature-depth profile from a global borehole temperature database compiled for climate reconstruction. This reduced temperature profile is interpreted in terms of past surface ground temperature change and indicates warming on the order of 1C over the past 100 to 200 years. The combination of an initial temperature (the primary free parameter) with the last 140 years of gridded surface air temperature (SAT) data yields a synthetic temperature profile that is an excellent fit to observations, accounting for 99% of the observed variance and a RMS misfit of only 12 mK. The good correlation suggests that this reduced temperature profile shares much information with the mean SAT record over large areas and long time-scales. Our analysis indicates 0.7±0.1C of ground warming between pre-industrial time and the 1961-1990 mean SAT.