Evaluation of past climate using borehole temperature measurements
A change of temperature at the surface of the earth produces a perturbation which diffuses slowly into the earth. It is thus possible to deduce the record of past climatic changes from the analysis of temperatures measured within boreholes. This technique was tested at two boreholes (Kaikoura, South Island, 215m deep and Raetihi, North Island, 240m deep). Past surface temperatures were reconstructed over the last 150 years, enabling a comparison to be made with the air temperatures measured at nearby climate stations of the New Zealand Meteorological Service. Past surface temperatures from Kaikoura show the same trends as the climate records of the New Zealand Meteorological Service. They show that a warming of 0.8°C has occurred this century, and that a cool period in the 1940s was followed by a warmer period in the 1950s, which was followed by a cool period in the 1960s & 1970s, and a warmer period in the 1980s. Past surface temperatures from Raetihi show that a warming of 4°C has occurred this century, most of which is attributed to deforestation.