E. Linacre and B. Geerts
Temperature generally increases with depth in the ground, and the rate at which it increases (the geothermal gradient) depends on local geological characteristics, such as the composition of the rock strata and the local thickness of the crust. Details of the temperature profile are also affected by the history of surface temperatures, since soil temperature anomalies slowly propagate into the ground, depending on the heat conductivity and thermal capacity of the layers. Under suitable conditions the geological factors affecting the geothermal gradient can be isolated, so the history of past surface temperatures can be inferred from small temperature anomalies along the length of a borehole (as mentioned in Table 15.3 in the textbook)
Analyses of underground temperature measurements from 358 boreholes in eastern North America, central Europe, southern Africa, and Australia indicate that the average surface temperature of the Earth has increased by about 0.5 K in the 20th century and that the 20th century has been the warmest of the past five centuries (1). The subsurface temperatures also indicate that the Earth's mean surface temperature has increased by about 1.0 K over the past five centuries. The warming is roughly consistent with surface-based estimates.
Data from 29 boreholes in the Great Plains of North America (mostly 100 - 200 metres deep) show a warming trend during the past century, particularly at the higher latitudes, i.e. 0.4 K at 41°N but 2.0 K at 50°N (2). The observed trend of surface air temperatures for the past century in the Great Plains is 0.5 K at 40°N and 1.6 K at 50°N. The trends of the two data sources are reasonably consistent. It should be noted that borehole temperature anomalies do depend on factors other than the history of surface air temperatures. For instance, at higher latitude the ground-surface temperature changes may be larger because of the non-linear behaviour of water at 0°C: the number of days with a frozen soil may have dropped significantly, and liquid water transfers heat more efficiently than ice.
Measurements down 12 boreholes in the Czech Republic (3) have been taken to greater depths, indicating much earlier surface temperatures. The results show fluctuations of about 1 K, with a minimum in 500 AD, maximum in 1200 AD and a minimum in 1800AD, approximately. The latter variations match similar variations of air temperatures in mid-England.