S. Franck, A. Block, W. von Bloh,
C. Bounama, H. J. Schellnhuber and Y. Svirezhev
Potsdam Institute for Climate Impact Research (PIK)
P.O. Box 601203, D-14412 Potsdam, Germany
The long-term co-evolution of the geosphere-biosphere complex from the
Proterozoic up to 1.5 billion years into the planet's future is investigated
using a conceptual Earth system model including the basic geodynamic processes. The model
focuses on the global carbon cycle as mediated by life and driven by increasing
solar luminosity and plate tectonics. The main CO
sink, the weathering
of silicates, is calculated as a function of biologic activity, global run-off
and continental growth. The main CO source, tectonic processes dominated by sea-floor
spreading, is determined using a novel semi-empirical scheme. Thus a geodynamic
extension of previous geostatic approaches can be achieved.
As a major result of extensive numerical investigations, the ``terrestrial life corridor'', i.e. the biogeophysical domain supporting a photosynthesis-based ecosphere in the planetary past and in the future, can be identified. Our findings imply, in particular, that the remaining life span of the biosphere is considerably shorter (by a few hundred million years) than the value computed with geostatic models by Caldeira and Kasting[1], and other groups. The ``habitable-zone concept'' is also revisited, revealing the band of orbital distances from the Sun warranting Earth-like conditions. It turns out that this habitable zone collapses completely in some 1.4 billion years from now as a consequence of geodynamics.