Greenhouse warming by CH4 in the atmosphere of early Earth

Earth appears to have been warm during its early history despite the faintn ess of the young Sun. Greenhouse warming by gaseous CO2 and H2O by itself i s in conflict with constraints on atmospheric CO2 levels derived from paleo sols for early Earth. Here we explore whether greenhouse warning by methane could have been important. We find that a CH4 mixing ratio of 10(-4)(100 p pmv) or more in Earth's early atmosphere would provide agreement with the p aleosol data from 2.8 Ga. Such a CH4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an impo rtant component of the biota at that time. Elimination of the methane compo nent of the greenhouse by oxidation of the atmosphere at about 2.3 - 2.4 Ca could have triggered the Earth's first widespread glaciation.