KEYNOTE: METHANE GREENHOUSES AND ANTI-GREENHOUSES ON THE EARLY EARTH

Earth's climate prior to 2.5 Ga seems to have been, if anything, warmer than today, despite the faintness of the young Sun. Higher concentrations of greenhouse gases, either CO₂ or CH₄ (or both), are the most likely solution to this problem. CO₂ probably dominated early on, but once life had evolved CH₄ may well have become highly abundant. CH4 is produced by anaerobic bacteria (methanogens) that are thought to have evolved early in Earth history. A biological methane flux comparable to today's flux, ~500 Tg CH₄/yr, could have been generated by methanogens living in an anaerobic early ocean and marine sediments. This flux should have increased once oxygenic photosynthesis evolved because of increased production and recycling of organic matter. An Archean methane flux of this magnitude could have generated atmospheric CH₄ concentrations in excess of 1000 ppmv. This, in turn, could have provided 30 degrees or more of greenhouse warming—enough to have kept the early Earth warm even if atmospheric CO₂ concentrations were no higher than today. The recent calculations by Tian et al. (Science Express, 2005), which show that hydrogen escape may have been slower than previously thought, only amplify the amount of CH₄ predicted in the early atmosphere. CH₄/CO₂ ratios greater than unity could have led to formation of organic haze and an accompanying anti-greenhouse effect that could have stabilized the Archean climate. The rise in O₂ at ~2.3 Ga brought an end to the methane greenhouse and may have triggered the Huronian glaciations.

Although methane concentrations declined with the rise of O₂, they may still have remained much higher than today throughout much of the Proterozoic. High methane production rates in marine sediments underlying a sulfidic Proterozoic deep ocean could have generated methane fluxes several times higher than today. The response of atmospheric CH₄ to its input flux is nonlinear, so Proterozoic CH₄ concentrations of 50-100 ppmv are not implausible. A rise in either atmospheric O₂ or oceanic sulfate near the end of the Proterozoic could have caused CH₄ concentrations to decrease a second time and may have triggered the “Snowball Earth” glaciations that took place at that time.