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

Abstract

Seventy‐five borehole temperature‐depth profiles in south India, located between 8° and 15°N, are analyzed to infer past changes in surface ground temperature. Solutions for a linear surface temperature change indicate average warming of about 0.9 ± 0.3°C over the past 127 ± 25 years at the 95% level of confidence for the entire data set, albeit with considerable geographic variability. Some sites in a restricted region exhibit surface ground temperature cooling during the last 50 to 100 years while a number of other borehole sites show large surface warming amplitudes in the range 1–3°C with onset times during the last few decades to less than a Century. Such rapid changes may represent effects of local land use changes superimposed on the long‐term climate change. Results of borehole analysis do not support a latitude effect in climate change. A set of 28 meteorological surface air temperature (SAT) records, distributed in the three major climatic provinces (Interior Peninsula, West Coast and East Coast) in south India yield an average warming trend of 0.6 ± 0.2°C/100 years over the period 1901–2006 for which records exist. Combined analysis of borehole temperatures and SAT data yields a long‐term, pre‐observational mean temperature (baseline) 0.6 ± 0.1°C lower than the 1961–1990 mean SAT. With an additional 0.35°C of warming beyond the 1961–1990 mean, the total warming from the ∼1800 baseline is 0.95°C. Given multiple uncertainties, we consider the 0.9°C of warming from borehole temperature inversion and 0.95°C of warming from the hybrid borehole temperature‐SAT analysis to be consistent if significant warming occurred in the 19th Century, prior to the onset of SAT records. The present data set together with the set of 70 temperature profiles in India analyzed earlier constitute an extensive documentation of climatic warming for the low latitude region 0°–20° N that was previously under‐sampled in global geothermal climate change studies.