S. Franck, A. Block, W. von Bloh, C. Bounama, H.-J. Schellnhuber, and Y. M. Svirezhev
Tellus 52B, no. 1, 94-107 (2000)
Potsdam Institute for Climate Impact Research (PIK),
Telegrafenberg, P.O. Box 60 12 03,
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.
focusses on the global carbon cycle as mediated by life and driven by increasing
solar luminosity and plate tectonics. The main CO2 sink, the weathering
of silicates, is calculated as a function of biologic activity, global run-off
and continental growth. The main CO2 source, tectonic processes dominated by
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 (1992), 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.
Full text of article incl. figures as a HTML-document.
Figure: Evolution of the habitable zone (HZ) for GSM-asymptotic (red line) and
GDM-asymptotic (green line). Note that for GSM-asymptotic the HZ has a slightly
width and shifts outward from the Sun. The HZ for our favoured model,
GDM-asymptotic, is both shifting and narrowing over geologic time, terminating
life definitely at 1.4 Ga. The
optimum position for an Earth-like planet would be at Ropt=1.08 AU. In
this case the life span of the biosphere would realize the maximum life span,
i.e. the above-mentioned 1.4 Ga. This figure is the main finding of our investigation.