From The Royal Meteorological Society (in 2011):
If Earth’s atmosphere contained no GHGs, all the IR radiation would be propagated directly to space and would be ‘lost’. The world would be cooler than it is by around 33 degC, having an average temperature of around –18 °C instead of its present +15 °C, for reasons that will now be explained.
That is the most outrageous claim the Warmenists have ever advanced, it does not fit in with any of their other claims.
Let's allocate half that 33 degC to the effect of water vapour and half to CO2, 16C each. That means our current 400 ppm CO2 = extra 16C.
Assuming a linear relationship, when CO2 was only 300 ppm, the surface temperature would have been on average 4C lower, which is clearly wasn't, and to get the so-called 2C 'tipping point' increase, CO2 levels would only have to increase to 450 ppm (which they probably will fairly soon).
Assuming a logarithmic relationship, it would have been 0.8C cooler with 300 ppm*, which is quite possibly true, but to get to the so-called 2C 'tipping point' increase, CO2 levels would have to increase to 846ppm**, which they simply will not do.
* Log(300) ÷ log(400) x 16C = 15.2C
** 10^(log(400) x 18C/16C) = 846
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