When the earth's surface temperature changes, the subsurface responds at magnitude and rate that depends on depth. In the top few centimeters, the daily rise and fall of surface temperature appears as a weak rise and fall approximately following the timing of the surface temperature. At a depth of 1 meter, the daily cycle is not observed, but a weak annual cycle of maximum in summer and minimum in winter can be observed. At a depth of several meters, even the annual cycle is obliterated.
A long slow change in surface temperature will slowly penetrate the earth to greater and greater depths. Measurement of earth temperatures as a function of distance from the surface then should reveal a record of surface warming, with deeper temperatures reflecting temperatures of the more distant past.
Henry N. Pollack, Shaopeng Huang, and Po-Yu Shen (Science 282, 279-281, 9 October 1998) report analysis of temperature measurements from 358 boreholes of depth ranging from 200 - 600 meters in southern Africa, eastern North America, central Europe, and Australia. Results indicate that the surface temperature of Earth has warmed about 0.5ºC in the 20th century and that the 20th century is the warmest of the last five centuries. These data for the 20th century are in agreement with climatological measurements taken at land and ocean observing locations over the last 100 years. According to the borehole data, the earth's mean surface temperature has increased about 1ºC in the past five centuries. These data confirm that the 20th century has experienced an unusual amount of warming compared to the last 500 years.
