Summary of Main Conclusions

The salient theses of this book can be summed up as follows:

1. The net effect of land biota is to significantly intensify chemical weathering rates of minerals that lead to the removal of carbon dioxide from the atmosphere/ocean system, thus leading to a lower atmospheric pCO2 and surface temperature of the Earth than would be the case for an abiotic state. The progressive increase in the biotic enhancement of weathering over geologic time has cooled the surface, given the abiotic constraints of the carbonate-silicate geochemical cycle.

2. The cooling of the Earth's surface, particularly in the first two thirds ofits history, has permitted the emergence of major organismal groups (cy-anobacteria, eucaryotes, Metazoa) at such times when the surface temperature corresponded to their biochemically/biophysically determined upper temperature limit for growth.

3. Thus, the biotic and abiotic components of the biosphere are linked in a geophysiological feedback loop, which itself changes its characteristics (evolves) over geologic time.

Are these theses at worst fruitful errors? Only time will tell. Our earlier work has already played a role in stimulating a research program that should provide critical empirical and theoretical tests. To this end, I propose several research directions to extend this now embryonic research program.

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