Just up on arxiv.org this week appeared Warming the early Earth - CO2 considered, by von Paris et al.. Just a preprint, and i'm working through its 53 pages now, put its likely to put the cat among the pidgeons:

Abstract: Despite a fainter Sun, the surface of the early Earth was mostly ice-free. Proposed solutions to this so-called "faint young Sun problem" have usually involved higher amounts of greenhouse gases than present in the modern-day atmosphere. However, geological evidence seemed to indicate that the atmospheric CO2 concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO2 necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies. For the late Archaean and early Proterozoic period of the Earth, we calculate that CO2 partial pressures of only about 2.9 mb are required to keep its surface from freezing which is compatible with the amount inferred from sediment studies. This conclusion was not significantly changed when we varied model parameters such as relative humidity or surface albedo, obtaining CO2 partial pressures for the late Archaean between 1.5 and 5.5 mb. Thus, the contradiction between sediment data and model results disappears.

The suspected composition of the eary Earth (the Archean, when life is believed to have started, 3.8 to 2.5 billion years ago), has alternated between CO2 and other reducing gases, such as methane.

Originally CO2 was thought to be the main greenhouse gas, making life possible. However the high CO2 levels required were a problem; high CO2 levels would have created siderite (FeCO3) in the top layers of soil as iron interacted with CO2 in the oxygen-free air. Since then methane has top billing as the greenhouse gas responsible, with significant hydrogen levels a possibility. Over time, methane has come out on top. And thats what I wrote when I did my literature review for my PhD. Now I'm tidying it up a bit, and that section may be in for a rewrite.

But what does it mean for our Climate change models of today?