We present a new conceptual Earth system model to investigate the long-term
co-evolution of geosphere and biosphere from the geological past upto 1.5
billion years into the planet's future. The model is based on the global ca
rbon cycle as mediated by life and driven by increasing solar luminosity an
d plate tectonics. As a major result of our investigations we calculate the
"terrestrial life corridor", i.e. the biogeophysical domain supporting a p
hotosynthesis-based ecosphere during planetary history and future. Furtherm
ore, we calculate the behavior of our virtual Earth system at various dista
nces from the Sun, using different insolations. In this way, we can find th
e habitable zone as the band of orbital distances from the Sun within which
an Earth-like planet might enjoy moderate surface temperatures and CO2-par
tial pressures needed for advanced life forms. We calculate an optimum posi
tion at 1.08 astronomical units for an Earth-like planet at which the biosp
here would realize the maximum life span. According to our results, an Eart
h-like planet at Martian distance would have been habitable up to about 500
Ma ago while the position of Venus was always outside the habitable zone.
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