Borehole Climate Studies

We can use subsurface temperatures to determine how the surface temperature of the Earth changes with time. Imagine we take a frozen rock an stick it in a hot oven: it will take time for the heat to propagate into the ice-cold rock. Below is a simple plot showing how temperature in a 160 degree oven might propagate into a rock with an initial temp = 0 C with time (assuming 1-D diffusive heat-flow).

Like the rock, the Earth responds similarly......

The image below shows the temperature measured in a borehole in Helena, Montana. The green line shows temperatures measured in the subsurface in 1975. The red line shows the temperature in 2014 at the same well (and using the same probe). The blue lines shows a best-fit forward model solution for the surface warming between 1975-2014 ( indicating an average rate of surface warming of 0.013 deg. C/yr--it's unlikely that this warming is linear however). We find similar results in several wells not only in Montana and the Rocky Mountain West, but in many other boreholes on Earth. We can use such data to determine warming rates in areas with limited historical temperature data. We can also use it to make predictions about future temperature change, freeze-line elevation/migration patterns, snowpack changes, and rates of permafrost melting in the next several years (see, for example, this & this).