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[bluewave]

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[bdgwx]

Using Gough's (1981) solar luminosity formula L(t) = L(t0)  / (1 + 0.4 * (1 - t/t0)) we can estimate that solar radiation was still L = 1 / (1 + 0.4 * (1 - 4.55/4.60)) = 99.5% of today's value. This yields a radiative forcing reduction of (1360 / 4) * 0.995 = -1.7 W/m2 relative to today.

Using Arrhenius' (1896) original radiative forcing formula ΔF = λ * ln(C/C0) and using 5.35 W/m2 as a more conservative estimate of CO2 sensitivity parameter (the IPCC used 6.3 at one time) we can see that 1000 ppm would produce an effect of ΔF = 5.35 * ln(1000/400) = +4.9 W/m2 relative to today.

So yeah, the net CO2 and solar radiative forcing 50 MYA relative to today would be around +3.2 W/m2 whereas just the CO2 forcing today at 1000 ppm would +4.9 W/m2 or about 1.5x times higher. It's obviously just a rough estimate I calculated and it doesn't factor in other climate forcing elements. The point...1000 ppm might be expected to have a bigger effect today all other things being equal because the sun is also brighter. Just food for thought...

[AfewUniversesBelowNormal]

agreed.