Erdgeschichtliche Treibhaus-Episoden

Fluchtpunkt Mutter Erde: die Gaia-Hypothese als Leitbild

In view of the man-induced transformations of the global environment science and society are forced to question the present-day human-nature relationship and to search for solutions to the increasing ecological problems. “A new look at life on Earth” resulting in a potentially new relationship between man and nature was proposed by James Lovelock in his Gaia hypothesis. He recognized, how life on Earth is coupled with an atmosphere which is anomalous in its chemical composition if compared with the flanking planets, Mars and Venus. The deep chemical disequilibrium of the atmosphere cannot be explained by physics and chemistry alone.

An understanding of biota and its properties is necessary.

Lovelock postulates that the biosphere, defined as the part of the atmosphere, lithosphere and hydrosphere, where life exists, maintained climate at an optimum for itself through geological time.

However, Gaia is just a new name for an old and commonly forgotten view of our planet. The importance of biotic action for the evolution of climate throughout earth history was recognized at the turn of this century by Chamberlin and Salisbury.

At a time of severe perturbation of the biosphere through human action, it is the merit of Gaia, that it has brought back to us how the living and the non-living part of the biosphere are interacting.

Questions on the evolution of the biosphere through geological time have to be reformulated.

The investigation of past perturbations of the biosphere, of response mechanisms of biota and physical environment provides insight into the mode of functioning of the biosphere. An excellent example on perturbations of the atmosphere's energy budget and response mechanisms of the biosphere in search of stabilization of the environmental conditions is given by the Cretaceous climate history. Increased transfer of volcanic CO₂ resulted in perturbation of the dynamics of the Cretaceous biosphere.

The biosphere responded with an acceleration of biogeochemical cycling reflecting the altered atmospheric energy budget and the related greenhouse climate. Through the acceleration of biogeochemical cycles negative feedback loops were initiated within the biosphere. These feedback mechanisms favoured an accelerated storage of CO₂ in the sedimentary carbon reservoir. Excessive storage of CO₂ may even have initiated ice age interludes contrasting with preceding greenhouse conditions as indicated by high-amplitude fluctuations in the Early Cretaceous carbon isotope record. The history of the Early Cretaceous biosphere may serve as an example of a self-organizing biosphere in attempt to maintain environmental conditions on an optimal level for life. Despite natural perturbations, the biosphere could maintain conditions favourable for life throughout earth history.

In addition, earth history teaches us, that variable atmospheric energy conditions forced the biosphere to function within a broad range of dynamic equilibrium levels.

If the present-day perturbation of the biosphere results in a new mode of operation of the biosphere which is still favourable for mankind remains questionable.