Japan warming inferred by combining borehole temperatures with surface air and mean sea surface temperatures

Past ground surface temperature (GST) changes have been reconstructed from borehole temperature-depth profiles in many areas over the world, but there are still significant uncertainties in understanding regional responses. We reconstruct the past GST changes in southwestern Japan over the past century from borehole temperature-depth profiles using the Bayesian least-squares method to invert borehole temperatures to produce the histories. This analysis reveals that the average reconstructed GST shows temperature increases of about 1.0^oC during the past century. This is consistent with nearby meteorological annual surface air temperature (SAT) and area-averaged annual mean sea surface temperature (SST) reported by the Japan Meteorological Agency. The consistency suggests that GST from borehole temperature-depth profiles shares information with the mean SAT and SST records over long time scales. Reconstructed GST changes using other temperature profiles at different parts of in and around Japan showed that the amplitude of the temperature increase varies by site. This suggests that further research with better spatial resolution is required.