The history of the surface temperature at Ajameti/Georgia as extracted from long-term temperature records in the subsurface

Abstract

The transient temperature component was determined from long-term highly resolved temperature records in a borehole at Ajameti near Kutaisi in Western Georgia during the period between July 2017 and September 2018. Temperatures were recorded at depths of 100, 175 and 250 m with a resolution below the Millikelvin range and a recorded measurement frequency of 3 per hour, resulting in 72 individual measurements daily. At the depth of 100 m, a linear temperature increase of 0.0036 K/year was observed during those 15 months of measurement. At both larger depths, a precise linear trend could not be estimated. Additional impacts of water flow in the subsurface, penetrating from the surface or ascending from deeper layers during the time of measurements, superpose the transient component. The linear trend at 100 m can be understood as an increasing surface temperature at a rate of 0.015 K/year since 80 to 90 years. Two models agree with the data, i.e. a fast rise of the temperature as well as a continuous increase at a temperature diffusivity of κ= 0.9*10^-6m²/s of the subsurface. The result coincides with the history of the Ajameti village which was founded in 1935, a period in which the settlement trees were probably cut to obtain a larger area of cultivated land, continuously increasing the surface temperature.