Modelling the global carbon cycle for the past and future evolution of the earth system

The Earth may be described as a global system consisting of the components solid Earth, hydrosphere, atmosphere, and biosphere. This system evolves under the external influence of increasing solar luminosity. In spite of this changing external forcing, the Earth's climate has been stabilized by negative feedbacks against global freezing in the past (faint young Sun paradox). The future long-term trend of further increasing solar luminosity will cause a further atmospheric CO, decrease. Atmospheric CO, will fall below the critical level for photosynthesis and the plant based biosphere will die but. In the present paper we propose a modelling study of the evolution of the carbon cycle from the Archaean to the planetary future. Our model is based on a paper published previously by Caldeira and Kasting [Caldeira, K., Kasting, J.F,, 1992. The life span of the biosphere revisited. Nature 360, 721-723]. The difference of the current study with respect to this work resides in the forcing function used for the silicate weathering rate. While Caldeira and Kasting used a constant weathering rate over time, we calculate the time evolution of this rate by assuming a balance between the weathering flux and the CO, release flux by volcanism and metamorphism. We use the geodynamics theory to couple the two internal forcing functions continental area (for weathering) and spreading (for CO, release flux) which were generally considered as independent in previous models. This coupling introduces an additional feedback in the system. We find a warmer climate in the past and a shortening of the life span of the biosphere up to some hundred million years. (C) 1999 Elsevier Science B.V. All rights reserved.