Geothermal climate change observatory in south India 1: Borehole temperatures and inferred surface temperature histories

Temperature–depth profiles measured in boreholes contain records of changes in surface ground temperature over the past few centuries. We have recently set up a geothermal climate change observatory at the Choutuppal campus of National Geophysical Research Institute (17.29 °N, 78.92 °E) to measure subsurface temperature changes on annual to decadal timescales and quantify how well they track surface temperature changes. The site is located about 60 km to the east of Hyderabad in south India, in a designated reserved forest land and far from potential urban heat islands. In April 2009, two boreholes were drilled to depths of 210 m and 21 m respectively after careful site selection to minimize perturbations to the subsurface temperatures on account of groundwater flow in the borehole, large thermal conductivity contrasts and rugged topography. Temperature measurements in the two holes are being carried out periodically. Analysis of equilibrium temperature–depth profile in the 210 m deep borehole reveals at least two ongoing events that started during the past Century: (i) surface ground warming of 0.5 ± 0.1 °C over the past 92 ± 7 years, and (ii) a more recent cooling of ∼1 °C over the past ∼39 years, probably representing local changes to surface vegetation caused by the presence of a thicker grass cover throughout the year inside the campus since 1967 AD compared to the short cropping of grass outside it. The inferred surface ground warming is consistent with estimates from temperature measurements in three other boreholes (170–300 m deep) distributed in a 10 × 5 km2 area in the vicinity of the observatory (mean: 0.5 ± 0.1 °C over the past 93 ± 21 years), and is characteristic of the Interior Peninsula region of south India.