ATMOSPHERE × CLIMATE
The Methane Greenhouse
~3.5 to 2.4 billion years ago
The Earth's atmosphere contained no oxygen and much methane made by microbes, a gas that traps heat and kept the planet warm under a fainter Sun, for a billion years, from about 3.5 to 2.4 billion years ago.
The air of the early Earth contained no oxygen and a great deal of methane, produced by microbes, for a billion years. Methane traps heat far more effectively than carbon dioxide, molecule for molecule. A gas that traps heat warms the planet beneath it, an effect called a greenhouse. This methane greenhouse is the usual answer to the faint young Sun paradox. The Sun then shone about a fifth more weakly than today, yet liquid water persisted at the surface, so something must have compensated. An organic haze drifted high in the atmosphere. It was made of orange particles, formed when ultraviolet light breaks methane apart and the fragments recombine. The haze thickened and cleared as the amount of methane rose and fell.
Why it matters
The Sun did not warm the Earth enough, yet the Earth kept its water liquid for a billion years. The explanation most often proposed is biological. Microbes made the methane that trapped the heat, so on this explanation living things were keeping their planet warm.
Dating & uncertainty
Researchers do not dispute that the Sun emitted 20 to 25 per cent less light than today. They do not dispute either that liquid water was present throughout the billion years, as the rocks show. What compensated for the weaker Sun is modelled and not measured. Methane, carbon dioxide and the organic haze each have advocates, and the result of a model changes with the proportions given to each.
Earth history as one day
05:29:5218 h 30 min before midnight
if Earth’s 4.54 billion years were compressed into a single day