International Journal of Terrestrial Heat Flow and Applied Geothermics VOL. 4, NO. 1 (2021); P. 119-126. ISSN: 2595-4180 DOI: https://doi.org/10.31214/ijthfa.v4i1.60 http://ijthfa.com/ 119 Keywords Ground warming, Borehole temperature log, Paleoclimatology, Land use, Geophysical inversion. Received: December 08, 2020 Accepted: January 24, 2021 Published: April 01, 2021 Influence of past vegetation changes on estimates of ground surface temperature histories GSTH obtained by inversion of borehole temperature logs: Example from the Western Canadian Sedimentary Basin Jacek Majorowicz 1 , Jan Šafanda 2 1 Department of Physics, University of Alberta, Edmonton, Canada. 2 Institute of Geophysics, Czech Academy of Sciences, Boční II, Praga - Spořilov, Czech Republic. Email address majorowi@ualberta.ca Abstract Functional space inversions (FSI) of precise temperature logs from 43 wells, located in low conductivity clastic sediments of the Western Canadian Sedimentary Basin, (WCSB), reveal evidence of extensive, recent ground surface temperature (GST) warming. Simultaneous inversion of log data acquired during the period of 1987-2005, as well as averaging of the individual site reconstructions of subsurface temperature signals, indicate evidence of high magnitude of warming of about 2°C (with standard deviations of 0.7°C). Magnitudes of such warning events exceeds 3-4 times that of globally averaged continental GST’s for the 20th century and is significantly higher than that of changes in surface air temperatures (SAT) based on instrumental records in the WCSB. Within this region, GST warming in the 20th century could have been at least partially caused by changes in vegetation cover. The temporary or permanent removal of vegetation, through deforestation, forest fires, and grassland conversion for agriculture occurred in the relatively young provinces of WCSB, during centennial long settlement and development programs. This might have significantly changed the surface properties of the area, since changes in surface albedo affects the radiation budget, while changes in the thermal, moisture and aerodynamic characteristics affect the energy balance. The results of our modelling for typical range of bedrock thermal diffusivities and assumed surface warming history for studied areas in WCSB show that a possible jump in ground surface temperature can easily be interpreted in the FSI results as a gradual warming event of large amplitude and attributed to SAT. 1. Introduction Temperature log data for boreholes in the Alaskan permafrost region has been interpreted as evidences indicative of ground surface warming (Lachenbruch and Marshall, 1986; Lachenbruch et al., 1988). Similar conclusions were reached for areas to the south of continuous permafrost in Canada (Cermak, 1971; Jessop, 1990a, b; Wang, 1992; Mareschal and Beltrami, 1992; Lewis, 1992; Lewis, 1998; Lewis and Wang, 1992; Majorowicz, 1993; Majorowicz et al., 2002a,b; Majorowicz et al., 2012; Majorowicz et al., 2014) and elsewhere in the world (Cermak et al., 1992, 2000; Deming, 1995; Pollack and Huang, 2000; Pollack et al, 2000; Harris and Chapman, 2002; Huang 2006, Bodri and Cermak, 2007; Putnam and Chapman, 1996; Šafanda et al, 2003; Hamza and Vieira, 2011). Data sets reported by Huang and Pollack, 1998 for the NOAA 2019/IHFC IASPEI continental well temperature data for borehole temperature inversion compilation of GSTs also reveal similar trends. It has been argued that GST warming derived from FSI (Shen and Beck, 1991) inversions of temperature logs in boreholes in Western Canada, has been indicative of climate changes. But deforestation has been an ongoing activity especially in previous century. Hence the observed signal is largely affected not just by climatic warming but also by permanent step changes in ground surface temperatures, arising from land surface changes of the past (Majorowicz, 1993; Skinner and Majorowicz, 1999; Majorowicz and Skinner, 1997a, b). Similar observations were made in other places (Blackwell et al., 1980; Cermak et a.l, 1992; Lewis and Wang, 1992; Lewis 1998, and Lewis and Skinner, 2003). The effect has been an improved understanding of the subsurface