Just wanted to say thank you for writing articles that just reel me in and keep me hooked till the very last word. Even more you make me curious to find out more – for example I think I am going to be spending some time on the paleobiology site and learn some more about the different eras (proterozoic etc.)

May you continue to keep your readers hooked for many more years to come!

[CZ: Thanks for the encouraging words!]

However Ohmoto et al has been “convincingly challenged”.

But since Kasting did his grand work on early Earth atmospheres in the 80′s-90′s, there has been a lot of progress on GW prompted by AGW. Only ~ 3 – 5 parts/thousand CO2 is needed, so there is no conflict with the geological record.

“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.”

["Warming the early Earth - CO2 reconsidered", Paris et al, preprint arxiv:0804.4134v2, 2008].

Also, the geological record allows for a lot more CO2 now.

“Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O2
concentrations and pH conditions in well-aerated soils have favoured the formation of
ferric (Fe 3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H2 -rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today,
causing the rain and ocean waters to be more acidic than today.

We therefore conclude that CO2 alone (without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.”

["Evidence from massive siderite beds for a CO2-rich atmosphere before 1.8 billion years ago", Ohmoto et al, Nature 2004.]

[CZ: Thanks for these references. You might want to check out the 2011 review I linked to. Here's one relevant passage: "Geochemical data therefore suggest that CO2 partial pressures were likely smaller than a few hundred times pre-industrial levels in the late Archean and early Proterozoic, meaning that carbon dioxide alone would most likely have been unable to pro- vide enough warming during these times."]