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

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