Borehole temperatures and climate change: ground temperature change in south India over the past two centuries

Variations in surface g round temperature (SGT) at the Earth's surface diffuse downward in a predictable way causing systematic perturbations to the subsurface temperature field. The pioneering study of Lachenbruch and Marsh all [1986] in Alaska demonstrated that present-day borehole temperature-depth profiles have the potential to reveal a surface ground temperature history over past several decades to a few centuries. Through the process of heat diffusion the Earth acts as a low-pass filter and a recorder of past surface temperature variations. Borehole temperature-depth profiles thus serve not only to complement the meteorologic record of climate change, but also provide important constraints on temperature trends prior to the occurrence of a global instrumental meteorological record (i.e., ^ 1 8 6 0 A.D.) and in areas where there is a paucity o f instrumentally recorded data..