What is the Faint Young Sun Paradox?
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The Faint Young Sun Paradox refers to the discrepancy between the Sun’s faintness in its early years and the evidence for a relatively warm early Earth, which suggests that other factors, like greenhouse gases, must have kept the planet warm.
The faint young Sun paradox is the apparent contradiction between the Sun’s lower luminosity in the past and the evidence that Earth’s early climate was warm enough to support liquid water and life:
Lower luminosity
The Sun was dimmer when Earth was young, generating 20–30% less energy than it does today.
Warm climate
Geological evidence shows that Earth’s early climate was at least as warm as today, and that liquid water was abundant on the surface.
Carl Sagan and George Mullen identified the paradox in 1972. Some possible explanations for the paradox include:
Stronger greenhouse effect
A stronger greenhouse effect than previously thought might be needed to reconcile the low level of CO2 with a temperate climate.
Different greenhouse gases
Ethane and carbonyl sulfide might have been more efficient greenhouse gases in the young Earth’s atmosphere than methane.