Non-steady state carbonate recycling and implications for
the evolution of atmospheric PCO2
The search is for feedbacks or other mechanisms that will act to stabilize the atmospheric composition in the habitable range that appears to have obtained without significant interruption
for at least the past 700 m.y. and probably much longer.
There is no doubt, given the fossil and isotopic records, that there has been a steady deterioration in Earth surface temperatures over the last 60 m.y., from essentially global tropical conditions with ocean bottom water temperatures of approx. 12C to the present ice age conditions . In the contemporary Earth surface environment, CO2 is close to being a limiting nutrient for terrestrial vegetation, especially C3 plants (trees, shrubs, etc.). Strongly enhanced growth is observed in greenhouse experiments with artificially increased PCO2 , and greatly reduced growth results at PCO2 150 ppmV (c.f. preindustrial interglacial value is V280 ppmV). C4 plants, mainly grasses, are much more resilient to both low PCO2 (limit V20 ppmV) and low PH2O. Between 8 and 5 Ma, a drastic change in the relative preponderance of C3 and C4 plants occurred in response to decreasing PCO2 .