A recent paper published in Geophysical Research Letters finds that the ‘Faint young Sun problem‘ has become “more severe” because to solve the problem using conventional greenhouse theory would require CO2 to comprise 0.4 bar or about 40% of the young Earth atmosphere, far greater than CO2 partial pressures today [0.014 bar or 28 times less] or those estimated for the young Earth [0.06 bar]. According to the authors,
“Our results suggest that currently favored greenhouse [gas] solutions could be in conflict with constraints emerging for the middle and late Archean [young Earth].”
However, if one instead simply assumes CO2 concentrations had no significant influence on Earth temperature, the faint young Sun problem can be resolved with the basic thermodynamics of the lapse rate, as shown in this post today by Dr. Claes Johnson, Professor of Applied Mathematics, KTH:
The faint young Sun paradox describes the apparent contradiction between observations of liquidwater early in the Earth’s history and the astrophysical expectation that the Sun‘s output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan and George Mullen in 1972.
Abstract from Geophysical Research Letters:
During the Archean (3.8–2.5 billion years ago), the Sun was up to 25% less luminous than today, yet there is strong evidence that the Earth’s ocean surface was not completely frozen. The most obvious solutions to this ‘faint young Sun problem’ demand high concentrations of greenhouse gases such as carbon dioxide. Here we present the first comprehensive 3-dimensional simulations of the Archean climate that include processes as the sea-ice albedo feedback and the higher rotation rate of the Earth. These effects lead to CO2 partial pressures required to prevent the Earth from freezing that are significantly higher than previously thought. For the early Archean, we find a critical CO2 partial pressure of 0.4 bar in contrast to 0.06 bar estimated in previous studies with 1-dimensional radiative-convective models. Our results suggest that currently favored greenhouse solutions could be in conflict with constraints emerging for the middle and late Archean.