Atmospheric carbon dioxide and the long-term control of the Earth's climate

Abstract. A CO₂-weathering model has been used to explore the possible evolution of the Earth's climate as the Sun steadily brightened throughout geologic time. The results of the model calculations can be described in terms of three, qualitatively different, "Megaclimates". Mega-climate 1 resulted from a period of rapid outgassing in the early Archean, with high, but declining, temperatures caused by the small weathering rates on a largely water-covered planet. Mega-climate 2 began about 3 Gyear ago as major continental land masses developed, increasing the weathering rate in the early Proterozoic and thereby depleting the atmospheric CO₂ concentration. This process produced the first Precambrian glaciations about 2.3 Gyear ago. During Mega-climate 2, evolutionary biological processes increased the surface weatherability in incremental steps and plate tectonics modulated the CO₂ outgassing rate with an estimated period of 150 Myear (approximately one-half the period for the formation and breakup of super continents). Throughout Mega-climate 2 the surface temperature was controlled by variations in the atmospheric CO₂ level allowing transitions between glacial and non-glacial conditions. The results of the model for Mega-climate 2 are in agreement with the occurrence (and absence) of glaciations in the geologic record. Extending the model to the future suggests that CO₂ control of the Earth's temperature will no longer be able to compensate for a solar flux that continues to increase. The present level of atmospheric CO₂ is so small that further reduction in CO₂ cannot prevent the Earth from experiencing Mega-climate 3 with steadily increasing surface temperatures caused by the continued brightening of the Sun. During Mega-climate 3, the main danger to the biosphere would come not from an increasing temperature but from a decreasing (rather than an increasing) CO₂ level which could, in time, fall below 0.5 PAL, causing serious damage to the biosphere. Fortunately, the rates of change due to solar brightening are slow enough that Mega-climate 3 appears to pose no threat to the biosphere for the next 0.5-2 Gyear.