just to clarify. What I intended to convey was that there is quite a bit of uncertainty in the actual values of atmospheric CO2 calculated from proxy data. Consequently it is not at all certain that the present spike is actually very much greater than spikes in the past. This is what several of you have been alluding to. We do know with reasonable accuracy what the present CO2 concentration is, on average. But the proxy data is subject to large error.
Salby has looked at the proxy data and concluded that it wasn't properly corrected for CO2 diffusion. The atmospheric CO2 concentration in the past was higher than has been reported, and i am assuming higher than that shown in FC's graph.