No climate paradox under the faint early Sun

The cool of the Sun The faint early or 'young' Sun paradox, raised by Carl Sagan and George Mullen in 1972, points out that solar luminosity during the Archaean was about 70% of today's, so it would — the theory goes — have been too cold for liquid oceans to survive on Earth. Yet the geological record shows that liquid water was present. This is usually explained as the consequence of a greenhouse effect due to a high concentration of atmospheric carbon dioxide and/or methane. Minik Rosing et al. suggest that there is no need to invoke greenhouse warming — and no climate paradox. They demonstrate that the mineralogy of Archaean sediments is inconsistent with high greenhouse gas concentrations and the metabolic constraints of the methanogens of the time. They hypothesize that the low albedo of the early Earth, with little in the way of continents, and a preponderance of dark heat-absorbing ocean, together with a lack of biologically induced cloud condensation nuclei, were sufficient to maintain temperatures above freezing.