AbstractEarth’s climate is considered to be stable on the order of > 106years, owing to a negative feedback mechanism in a carbon cycle system. However, any decrease in net input flux of CO2to the atmosphere-ocean system (i.e., volcanic-metamorphic CO2flux minus excess organic carbon burial flux) lowers the surface temperature and would eventually initiate global glaciation. TheFD-FOBdiagram (FD: the total CO2degassing flux,FOB: the organic carbon burial flux) is proposed as a measure of the susceptibility of the Earth to global glaciations. By using this diagram with the carbon fluxes estimated from a carbon cycle model during the Phanerozoic, the net input flux of CO2is found to have been very close to the critical condition for a global glaciation at the Late Paleozoic. During the Proterozoic, a carbon isotope mass balance model with this diagram shows that global glaciations occurred probably due to a decrease in the CO2degassing in addition to an increase in the organic carbon burial. Because the Sun becomes brighter as it evolves, the critical level of atmospheric CO2pressure to cause global glaciation will be lower than the critical CO2pressure for photosynthesis of C4 plants within 500 million years. At this point, the net input flux of CO2will be too large to cause global glaciations. Continuous volcanic-metamorphic activities (i.e., plate tectonics) may be one of the necessary conditions for the Earth and Earth-like planets in extrasolar planetary systems to keep liquid water and life over the timescales of planetary evolution.