Borehole temperatures reveal details of 20th century warming at Bruce Plateau, Antarctic Peninsula

Two ice core boreholes of 143.18 m and 447.73 m (bedrock) were drilled during the 2009–2010 austral summer on the Bruce Plateau at a location named LARISSA Site Beta (66°02' S, 64°04' W, 1975.5 m a.s.l.). Both boreholes were logged with thermistors shortly after drilling. The shallow borehole was instrumented for 4 months with a series of resistance thermometers with satellite uplink. Surface temperature proxy data derived from an inversion of the borehole temperature profiles are compared to available multi-decadal records from weather stations and ice cores located along a latitudinal transect of the Antarctic Peninsula to West Antarctica. The LARISSA Site Beta profiles show temperatures decreasing from the surface downward through the upper third of the ice, and warming thereafter to the bed. The average temperature for the most recent year is −14.78°C (measured at 15 m depth, abbreviated <i>T</i><sub>15</sub>). A minimum temperature of −15.8°C is measured at 173 m depth, and basal temperature is estimated to be −10.2°C. Current mean annual temperature and the gradient in the lower part of the measured temperature profile have a best fit with an accumulation rate of 1.9×10<sup>3</sup> kg m<sup>−2</sup> a<sup>−1</sup> and basal heat flux (<i>q</i>) of 88 mW m<sup>−2</sup>, if steady-state conditions are assumed. However, the mid-level temperature variations show that recent temperature has varied significantly. Reconstructed surface temperatures (<i>T</i><sub>s</sub>=<i>T</i><sub>15</sub>) over the last 200 yr are derived by an inversion technique (Tikhonov and Samarskii, 1990). From this, we find that cold temperatures (minimum <i>T</i><sub>s</sub>=−16.2°C) prevailed from ~1920 to ~1940, followed by a gradual rise of temperature to −14.2°C around 1995, then cooling over the following decade and warming in the last few years. The coldest period was preceded by a relatively warm 19th century at <i>T</i><sub>15</sub>≥−15°C. To facilitate regional comparisons of the surface temperature history, we use our <i>T</i><sub>15</sub> data and nearby weather station records to refine estimates of lapse rates (altitudinal, adjusted for latitude: Γ<sub>a(l)</sub>). Good temporal and spatial consistency of Γ<sub>a(l)</sub> over the last 35 yr are observed, implying that the climate trends observed here are regional and consistent over a broad altitude range