The fact that there has existed fluid water at the Earth's surface since at least 3.8 Ga ago [3, 4] is one of the main indicators of our planet's strong ability to self-regulate against external driving forces. Although the solar luminosity was significantly lower in the past, the dead end of an ice planet has been avoided. One of the main reasons for this may be the intrinsic stabilization of the coupled geosphere-biosphere system through the carbonate-silicate cycle. In spite of this surprisingly resilient character, short-term perturbations, such as a sudden increase of the carbon content of the atmosphere, may ultimately destabilize the dynamic regulation properties of the Earth system in a way that paves the way towards the fate of planet Venus. A better understanding of the critical limits of the fundamental self-stabilization mechanisms is therefore crucial, especially, with respect to the unintentional global experiments humankind is presently conducting via modification of the composition of the atmosphere or fragmentation of terrestrial vegetation cover. There is increasing evidence that the main feed-back properties of the ecosphere depend not only on the capacity of its subsystems (i.e., atmosphere, hydrosphere, terrestrial and marine biosphere, pedosphere, cryosphere and lithosphere). Instead, the subsystems seem to be linked with each other in such a way that the entire system behaves as a dynamic totality, consisting of strongly interacting processes of high complexity (see, e.g.,[5]).
The analysis of this ``linkage'', mainly realized through the global biogeochemical cycles, is particularly important for our ability to assess the stability of the climate system with respect to civilisatory perturbations like fossil fuel combustion. The synergetic operation of the planetary biogeochemical cycles controls a variety of Earth-system properties as a result of geosphere-biosphere co-evolution over some four billion years. The main factors governing the further development are insolation, plate tectonics, photosynthesis, and atmosphere-hydrosphere interaction. As a consequence, the long-term planetary evolution defines the dynamic context in which short-term physiogenic or anthropogenic perturbations unfold. The Earth system may ignore, attenuate or amplify such disturbances. Therefore, if we wish to assess the impacts of civilization-driven global change, it is mandatory to gain an understanding of how the ecosphere functions at geologic time scales: the pertinent insights will allow to define quasi-stationary boundary conditions as a systems-analytic background for studying processes at smaller temporal and spatial scales.
One of the main feedback mechanisms in the long-term evolution of the Earth system
is the global carbon cycle. The principal features of this cycle are important
both for the past and the future of our planet. A full explanation and re-construction of the
quaternary glaciation episodes, for example, seems to demand a thorough understanding of the
carbon dynamics as mediated by life.
``Sub specie aeternitatis'', even more important questions concern the role of
carbon dynamics in the determination of the overall life span of the
biosphere [43, 1] (see [8] for a recent review).
In order to elucidate that role, the
properties of all planetary CO
sources and sinks as a function of
surface temperature, biological activity, weathering rates, sea-floor spreading,
plate subduction and continental growth have to be explored in some depth.
In this paper, we will explicitly take into account geodynamic properties,
by way of contrast to former investigations into this issue.
The main sources of carbon are emissions
from spreading zones and volcanic activities. The main sink involves the removal of CO
from the atmosphere by weathering processes. In extension
to most of the previous studies on the carbon cycle, which refer only to present-day tectonic
activities and
present-day continental area, we use a general model for the thermal and degassing evolution
of
the Earth to calculate the rate of sea-floor spreading and the growth of the continental area.
As a result of our calculations, we obtain a significantly revised ``terrestrial life corridor'' for
the Earth system and a new determination of the
so-called
``habitable zone'', i.e., the orbital region within which a planet like the Earth might enjoy
moderate surface
temperatures and atmospheric CO concentrations suitable for photosynthesis.