According to well understood physical parameters, the effectiveness of CO2 as a greenhouse gas diminishes logarithmically with increasing concentration and from the current level of ~390 ppmv, (parts per million by volume). Accordingly only ~5% of the effectiveness of CO2 as a greenhouse gas remains beyond the current level,
and claims that this is well known in the scientific community and references Section 6.3.4 of the IPCC’s Third Assessment Report. The WUWT post then goes on to calculate how the temperature changes with changing CO2 concentration.
0-100 ppmv: according to David Archibald / Modtran data ~2.22°C ~2.22°C
100-200 ppmv: plants die below this level of CO2 +~0.29°C ~2.51°C
200-300 ppmv: noted as the preindustrial CO2 level +~0.14°C ~2.65°C
300-400 ppmv: current level IPCC attributes all as Man-made +~0.06°C ~2.71°C
400-600 ppmv: business as usual till 2100 +~0.08°C ~2.79°C
This post is basically claiming that the surface temperature will only rise by 0.57oC if CO2 levels were to rise from 100 ppm to 600 ppm and by 0.14oC if CO2 levels were to go from 300 ppm to 600 ppm.
This seemed a little lower than I was expecting, so I went to the IPCC report and found the following relationship between CO2 concentration and radiative forcing:
ΔF = α ln(C/Co),
where ΔF is the change in forcing, α = 5.35, C is the new CO2 concentration, and Co is a reference CO2 concentration. If you assume C = 600 ppm and Co = 300 ppm then the above gives ΔF = 3.7 W m-2. This change in forcing should produce a 1oC change in surface temperature. There are two newer and more complicated equations (in the IPCC report) relating changes in CO2 to changes in radiative forcing, but they give the same kind of result. These are all calculations that do not include feedback and so they would be expected to be the minimum change in temperature, as feedback would tend to enhance the effect.
So, as far as I can tell, the IPCC does indeed acknowledge the logarithmic relationship between radiative forcing and CO2 concentration but suggests that – in the absence of feedback – a doubling of CO2 concentration would produce a 1oC rise in surface temperature. For some reason, the author of this WUWT post gets a change of 0.14oC for a doubling of CO2 concentration. Very odd.