Objectives of this chapter

In this chapter we show how borehole temperature profiles can be used to infer past climate variations, and discuss the usefulness and limits of such methods. We also discuss the thermal conditions in ice sheets and show the importance of boundary conditions to calculate temperature profiles in the ice.

The record of past climate in temperature profiles

Time variations of the boundary condition at the Earth's surface affect subsurface temperatures with two important consequences: (1) the perturbations to steady-state temperature profiles may systematically affect the heat flux estimates, particularly in regions that were glaciated; (2) with careful measurements, these perturbations can be detected and interpreted to infer past variations in the surface boundary conditions. As early as 1923, temperature profiles from deep holes in the United States were used to infer the timing of the glacial retreat 10,000 years ago. The first heat flux estimates from Great Britain were corrected to account for the effect of the last glaciation. Birch (1948) proposed adjustments to account for the effect of the last glaciation on heat flux estimates in previously glaciated areas. The main obstacle for such corrections is that we still do not know what the temperature was at the base of the ice sheets. When the glacial retreat started, the temperature in the bedrock beneath the glacier was not in equilibrium. Although warming after the last glacial retreat 10,000 years ago is the dominating component of the temperature perturbation, the entire history of glacial retreats and advances must be included to calculate present perturbations to the temperature profiles (Figure 12.1).