Reconstruction of the 500-year ground surface temperature history of northern Awaji Island, southwest Japan, using a layered thermal property model

Changes in the ground surface temperature (GST), propagating underground, can be recorded as thermal perturbations to the background thermal field. This paper presents a forward model of conductive propagation of GST in a layered material model with uniform thermal properties in each layer and a series of step functions as GST history. This model, which is expressed using the same mathematical form of that for a uniform thermal property model with a series of step functions as GST history, calculates subsurface temperature perturbations that originate from the GST change by superimposing numerically solved solutions of the model with surface boundary condition of a unit function. Using this model, we reconstruct the recent 500-year GST history from borehole temperature data in northern Awaji Island, southwest Japan, by Bayesian inversion. The reconstructed GST history shows the onset of warming in the mid-18th century to the early 19th century and an increase of 1.1-1.3 K up to the mid-20th century. From the middle to late 20th century, the GST decreased by about 0.2 K. The GST change in the 20th century fits the trend of mean annual surface air temperature records in Kobe, opposite the coast of northern Awaji Island. The GST history in northern Awaji Island differs from that in Ulsan, in the southeastern Republic of Korea, which is located at the same latitude as northern Awaji Island. Differences of the GST histories of these regions most likely reflect differences in sea surface temperatures in these regions.