This early formation has been difficult to explain because of a problem known as the faint young Sun paradox.

An alternative model of solar evolution may explain the faint young Sun paradox.

In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice free earth and the faint young sun paradox.

The fact that this is not in line with the - compared to today 30% lower - solar radiance of the early sun has been described as the faint young Sun paradox.

However, there is evidence for the presence of water on the early Earth, in the Hadean and Archean eons, leading to what is known as the faint young Sun paradox.

This is known as the faint young sun paradox and is usually explained by invoking much larger greenhouse gas concentrations in Earth's early history, though such proposals are poorly constrained by existing experimental evidence.

Svensmark claimed that this mechanism was responsible for virtually every climatic event in Earth history, from ice ages to the Faint Young Sun paradox to Snowball Earth to our current warming trend.

Astronomers think that the sun had about 70-75% of the present luminosity, yet temperatures appear to have been near modern levels even within 500 Ma of Earth's formation, which is puzzling (the faint young sun paradox).

Pierrehumbert's central research interest is how climate works as a system and developing idealized mathematical models to be used to address questions of climate science such as how the earth kept from freezing over: the faint young sun paradox.

The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water 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.