Temperature-depth data from six boreholes in western Utah and nine boreholes in southeastern Utah are reanalyzed for evidence of ground surface temperature (GST) histories. We invert the temperature-depth data using the functional space inverse algorithm of Shen and Beck <1991, 1992> which we prefer over previous inversions of these data because of its greater sophistication and flexibility in suppressing noise. GST histories for western and southeastern Utah are generally consistent and suggest that temperatures in the mid-1800s are, on average, cooler than previous centuries, followed by about 0.6 ¿C of warming in this century. Attention is given to the temporal resolution of our GST solutions showing the time-smearing effects of heat conduction on the solutions. GST solutions represent an average ground temperature over a time window that expands as we look farther into the past. The size of the time window is a function of measurement and geologic noise and limits the ultimate resolution of GST reconstructions. ¿ 1998 American Geophysical Union