further details

A 4% change in solar activity can be easily shown to have a forcing impact of around 9.5W/m2 ignoring feedbacks.

The forcing ignoring feedbacks due to changes in CO2 concentration from C₀ to C is given by

DF = 5.35 ln(C/C₀)

This works out that carbon dioxide levels then would have to be about 6 (exp 9.5/5.35) times higher than today to have the same effect as today.

If we now assume a climate sensitivity when the solar activity was 4% lower of say 2.5⁰C for a doubling of CO2 levels we can calculate the levels of CO2 to reach say 5⁰C higher. Due to the logarithmic effect of the forcing due to CO2 this will be a further 4 times higher. An overall 24 times higher CO2 concentration is consistent with the estimates we see 100’s of millions of years ago.