Borehole climatology: contributions to the understanding of climate change

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Data from borehole temperature profiles (BTPs) have been one source of information that has significantly contributed to our understanding of centennial temperature changes. Climate reconstructions based on BTPs assume that surface air temperature changes are coupled to ground surface temperature changes and propagate to the subsurface by thermal conduction. In other words, if we assume the Earth's upper crust is at thermal equilibrium, then the temperature distribution in the upper few kilometers is determined by the long-term (>1000 years) surface temperature and the internal heat flow (constant at the 100-105 year scale). Under these conditions, for known thermal properties of the rocks, the temperature increases linearly with depth. If the temperature at the Earth's surface changes, then a quantity of heat will be gained or lost by the ground; these changes in the energy balance at the surface propagate and are recorded underground as perturbations to the equilibrium thermal regime. The downward propagation of the climate disturbance depends on the thermal diffusivity of the rock (ca. 10(-6) m2/s) so that the first few hundred meters of the surface crust store the integrated thermal signature of the last millennium. Due to the nature of heat conduction the amplitude of surface disturbances is exponentially attenuated and their phase shifted with depth as a function of their time scales. The process operates as a low pass filter such that the low-frequency variations propagate deeper than those with higher frequency. Typically, perturbations penetrate about 20 m in a year, 150 m in 100 years and about 500 m in a millennium, such that recent energy balance changes at the surface remain recorded in the shallow subsurface. Analysis of these underground perturbations provides the base of the borehole approach to retrieve past temperature changes at the ground surface. This proposal deals with the most important issues facing this discipline and proposes to find solutions to this issues and at the same time will train future scientists through research on these key areas of borehole climatology.