Warming of the Continents in the Anthropocene

The Intergovernmental Panel on Climate Change (IPCC) reports a very high confidence that anthropogenic activities have warmed the climate system. A significant component of this warming climate system is the continental lithosphere where measurements of temperature-depth profiles from boreholes contain valuable information about the changing surface temperature and provide a direct method for reconstructing surface temperature variations over the past several centuries. The borehole record indicates that ground temperatures began notably heating in the mid-1830's with a total increase approaching 1 °C at the ground surface, comparable to the atmospheric temperature rise over the same time. This temperature change resulted in an increase in the heat content of the outermost part of the continental lithosphere on order of 1.55 x 1022 J, equivalent to an additional heat flux of 15 mW m-2 over the last 170 years. By utilizing the physics of heat diffusion and the theory of borehole thermometry, we have modeled the future temperature state of the continental lithosphere using three emission scenarios (A2, A1B, and B1) from the IPCC AR4, along with a minimum commitment scenario. Our results indicate the heat content increase for the next century will be between 3.11 (for the minimum commitment scenario) and 9.42 x 1022 J (for scenario A2) with the three-scenario mean at nearly 8 x 1022 J. These heat content increases are equal to an additional heat flux of almost 60 mW m-2 at the ground surface over the next 100 years. While this change in heat content is less than estimates for the ocean, it nevertheless is an important component of the climate system, and can be directly linked to the anthropogenic influence on climate change.