> Go back 500 million years, and solar luminosity is only about 7% lower
> than now but CO2 was 20 times higher.
I think that we need to be careful when drawing conclusion from past
climate. At least for the last million years we've got very reliable
measurements for CO2 concentrations, but before that we are dealing
with proxy evidence. In addition, the overall forcing presumably
matters, not just the CO2 related forcing, and with regards to
aerosols or surface albedo we know awfully little.
Furthermore, a factor 20 for CO2 is not actually that far from a
quadrupling, as it might appear at first, because of the logarithmic
relationship. The forcing from a quadrupling is 8 W/m2, from a factor
16, 16 W/m2.
Let me say a bit more about surface albedo. All it takes for surface
albedo to have been lower 500 million years ago would for example be
for widespread algae to colour the ocean water slightly brighter. Or
cloudiness could have been different due to widespread sulphate
aerosol emissions from bacteria.
Granted this is all a bit handwavy, but when you talk about
vanishingly small probabilities, and base this on climate history, I
think the evidence needs to be pretty solid. Likewise, I think
Hansen's talk of certainty is pushing it, the same logarithmic
relationship applies there, so even if he's talking 2000 or 3000 ppm,
oil sands included, it's not that different from 1000 ppm.
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