Figure 1 During the Archean eon, from 3.8 to 2.5 billion years ago, life on Earth was thriving for the first time, growing in a world with much less land and a faster planetary rotation than today. At the same time, the energy flowing to the early Earth from the Sun was just three quarters of what it is now. Despite the drastically lower levels of solar irradiance, previous research has suggested that the Archean Earth was not a planet encased in ice but instead remained a watery world. To explain this seeming inconsistency, a dilemma known as the “faint young Sun paradox,” researchers have suggested that the planetary greenhouse effect must have been much more potent than today. Previous research suggested that atmospheric carbon dioxide levels would need to have had a partial pressure of approximately 0.06 bar, equivalent to an atmospheric concentration 200 times that of the pre-Industrial modern era.