Cool. Can we please have Okjokull again?
"The effectiveness of CO2 as a warming Greenhouse gas rapidly diminishes logarithmically as its concentration increases. The consequence of this logarithmic diminution mean that all further CO2 induced temperature increases can now only be absolutely marginal and that there is no chance of any further Catastrophic Global warming, whether effected by Man-kind or not."
ETA: The IPCC https://www.ipcc-data.org/guidelines/pages/reporting.html says this:
"Equilibrium GCM 2 x CO2 experiments commonly assume a radiative forcing equivalent to a doubling of CO2 concentration (for example from 300 ppmv to 600 ppmv). In fact the absolute concentrations are not especially important, as the temperature response to increasing CO2 concentration is logarithmic - a doubling from 500 to 1000 ppmv would have approximately the same climatic effect."
I would encourage folks to do some research on Anthony Watts; he is not communicating in good faith on this topic.