Ground Temperatures Since the Year 1500

Ground Temperatures Since the year 1500 can be read back by examining the temperatures in deep boreholes. Temperatures in the Earth's crust are determined by a combination of surface climate effects and internal heat flow. The general trend is a linear rise in temperature with depth, but this is modulated by heat perturbations which act in a nonlinear way; typically perturbations penetrate about 20 meters of depth per year or about 150 m in 100 years. Hugo Beltrami (St. Francis Xavier University in Nova Scotia) has examined temperature-depth profiles from 826 places around the world. Taking into account the known temperature anomalies, he is able to work out the average surface energy flux and temperature for many localities and for the world as a whole back for a period of 500 years. Beltrami (902-867-2326) finds that global average surface temperature has increased by 0.45 K in the last 200 years. During this time, however, some places have experienced more dramatic average temperature swings: for example, parts of Africa show a cooling while northern Canada is warmer (3-4 K) during the same period. (Geophysical Research Letters, vol 29, 23, 2111; also see http://geophysics.stfx.ca/public/index.html)