Long-term evolution of the global carbon cycle: historic minimum of global surface temperature at present

We present a minimal model for the global carbon cycle of the Earth containing the reservoirs mantle, ocean floor, continental crust, continental biosphere, and the kerogen, as well as the aggregated reservoir ocean and atmosphere. This model is coupled to a parameterised mantle convection model for describing the thermal and degassing history of the Earth. In this study the evolution of the mean global surface temperature, the biomass, and reservoir sizes over the whole history and future of the Earth under a maturing Sun is investigated. We obtain reasonable values for the present distribution of carbon in the surface reservoirs of the Earth and find that the parameterisation of the hydrothermal flux and the evolution of the ocean pH in the past has a strong influence on the atmospheric carbon reservoir and surface temperature. The different parameterisations give a rather hot as well as a freezing climate on the early Earth (Hadean and early Archaean). Nevertheless, we find a pronounced global minimum of mean surface temperature at the present state at 4.6 Gyr. In the long-term future the external forcing by increasing insolation dominates and the biosphere extincts in about 1.2 Ga. Our study has the implication that the Earth system is just before the point of evolution where this external forcing takes over the main influence from geodynamic effects acting in the past.