Friday, March 16, 2007

Biological feedbacks cause and effect

Interesting developments in Astrobiology

In the presence of a biosphere, burial of organic carbon also contributes to atmospheric carbon sequestration in addition tosilicate weathering. The biosphere also effectively circulates carbon between its organic and inorganic states and through this process has been able to completely change the composition of the atmosphere. The early biosphere may have produced the methane greenhouse during the Archean and early Proterozoic and later, through oxygenic photosynthesis, has depleted the methane and regulated the ratios of atmospheric CO2 and O2, thus having a significant impact on the greenhouse effect of the atmosphere.

Thus the cooling of the Earth interior has slowed down mantle convection which has contributed to surface cooling due to its influence on the greenhouse effect. The brightening Sun has had an opposite effect to increase the effective temperature by 7 K from the Paleoproterozoic to the Neoproterozoic. The combined secular trend of the mean surface temperature is hard to predict and need not be large. However, both mechanisms led to the reduction of the importance of the greenhouse effect. This increased spatial and temporal temperature gradients, with corresponding increases in wind speeds and ocean currents. The climate has transformed from an early rather featureless and homogeneous greenhouse-dominated system to one containing important temperature gradients and thus exhibiting latitudinal and seasonal effects and a rich set of nonlinearities and feedbacks, among them the albedo feedback.This development would have also taken place without life, but biological effects have substantially modified it.


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