The rate of CO
2 degassing to the atmosphere-ocean system via volcanism has probably changed greatly during the history of the Earth. Change of the CO
2 degassing rate results in change of the atmospheric CO
2 level and so the climate, through the geochemical cycle of carbon at the surface of the Earth. If the CO
2 degassing may cease or weaken suddenly, a carbon geochemical cycle model coupled with ocean chemistry and the climate model predicts that the Earth's surface environment should cool very rapidly on the order of 10
5 yr, and, at last, fall into the globally ice covered state. The time required for this is estimated to be on the order of 10
6 years throughout the Earth's history, irrespective of large change of the atmospheric CO
2 level with time. However, absence of geological evidence for such an exstreme climate in the past suggests that the CO
2 degassing process via volcanism should have been almost contiuous, at least, on the order of >10
5-10
6 yr, which indicates continuity of plate tectonics on the Earth.
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