Faint young Sun paradox

The faint young Sun paradox is the conflict between the Sun’s lower luminosity billions of years ago and geological evidence that early Earth had liquid water and a temperate climate.

Carbon dioxide: A colorless gas that absorbs infrared radiation and is a major greenhouse gas in Earth’s atmosphere. Higher atmospheric concentrations could have compensated for the young Sun’s weaker output.
Archean: The eon from about 4.0 to 2.5 billion years ago, when early life and continents developed. Much of the paradox concerns evidence that Archean Earth was not globally frozen.
Carl Sagan: An American astronomer and science communicator who studied planetary atmospheres and the possibility of extraterrestrial life. Sagan and George Mullen framed the early-Earth climate puzzle in a 1972 paper.
Ammonia: A nitrogen- and hydrogen-containing gas that absorbs infrared radiation and can act as a greenhouse gas. Sagan and Mullen proposed abundant ammonia, but ultraviolet sunlight rapidly destroys it.
Archean atmospheric oxygen: The concentration and evolution of oxygen in Earth’s atmosphere during the Archean eon. Oxygen levels affect the survival of methane and other candidate greenhouse gases.
Methane: A simple hydrocarbon gas that absorbs infrared radiation and can warm a planet’s atmosphere. Methane produced by early microbes may have added warming alongside carbon dioxide.
Huronian glaciation: A series of glaciations between roughly 2.4 and 2.1 billion years ago, among the earliest well-documented on Earth. Its timing raises the possibility that early Earth sometimes crossed into severe global cooling.
George Mullen: An American planetary scientist who studied early Earth atmospheres and planetary climate. His work with Carl Sagan proposed ammonia as a possible greenhouse solution.
Nitrous oxide: A nitrogen oxide gas that absorbs infrared radiation and contributes to atmospheric warming. It has been proposed as an additional greenhouse gas in an oxygen-poor early atmosphere.
Great Oxidation Event: The major rise of atmospheric oxygen around 2.4 billion years ago. The resulting chemical changes may have reduced methane and contributed to glaciation.

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