On the tail of my post about planetary snowballs, the Faint Young Sun Paradox has raised its head again. Three to four billion years ago the Sun was only about 70% as luminous as it is today - it was young and its core temperature was a little lower, so the output of radiation was somewhat less. This has long been a vexing issue for the early Earth. The geological record firmly points to a warm world with plenty of nice liquid water, but if the Sun was heating the planet so much less then something must have offset this to prevent a global freeze. A long postulated solution, and a seemingly good one, has been that the young Earth must have had more atmospheric carbon dioxide, and possibly methane to boost the greenhouse effect and keep things warm.
However, a new study of ancient marine sediments (banded iron formations) by Rosing et al. (and a great discussion by Jim Kasting) adds significantly to other geological evidence that atmospheric carbon dioxide in that young Earth was at about the same level it is today. So how do we solve the paradox ?
Rosing et al. bring up an idea that has been around before - that the young Earth just absorbed more of the sunlight hitting it, by virtue of being less reflective. One way to do this is to alter the amount and type of cloud cover. Because we think a lot of modern clouds are 'seeded' by the muck that life itself (not just us) dumps into the atmosphere, then a young Earth - with a different biosphere - could have have less reflective cloud cover. This is by no means a done deal, but it's certainly interesting to reconsider, and the paradox of the faint young Sun continues to intrigue.