The real question relates to Climate Sensitivity (CS) -- defined as the temperature rise associated with a doubling of CO2 (The definition varies slightly between different authors.)
IPCC initially claimed a very large CS. But after the first Assessment report of 1990, CS dropped from 4.5 to about 2.5OC. From then on, IPCC only considered the last part of the 20th century and no longer claimed the earlier warming (1910-40) to be manmade [see Slide-1].
S-1: (a) GAST (Global Ave Surface Temp) (b) Note that max US temp occurred in 1930s Source: GISS
In my view, CS may actually be close to zero. This means CO2 has very little influence on climate change -- probably because of negative feedback. There is still debate, however, about what kind of negative feedback to expect. Should it come from water vapor or from clouds?
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