Climate change on the Colorado Plateau of eastern Utah inferred from borehole temperatures

Abstract

Temperature profiles from boreholes on the Colorado Plateau of southeastern Utah have been examined for evidence of climate change. Because these boreholes penetrate layered sedimentary rocks with different thermal conductivities, Bullard plots (temperature versus integrated thermal resistance) are used to estimate background heat flow and surface temperature intercepts. Reduced temperatures, which represent departures from a constant heat flow condition, are inverted for a surface ground temperature history at each borehole site using a singular value decomposition algorithm. Singular value cutoffs are selected by analyzing the spectral energy and the standard deviation of the model fit to the data as a function of the number of eigenvalues; solutions are constructed from areas of large spectral energy and a cutoff where additional eigenvalues fail to improve the solution significantly. The solution is parameterized in terms of 13 time steps increasing in duration and going back 400 years. Eight of nine borehole sites indicate between 0.4 and 0.8°C (±0.2° C) warming over the past 200 years with some evidence for accelerated warming in this century; one borehole indicates local cooling over the same time period. The amplitude of the warming inferred from borehole temperatures is less than that deduced from analysis of 100‐year surface air temperature records at four of the five weather stations surrounding the borehole sites.