cc: Sarah Raper date: Fri, 10 Feb 2006 14:35:22 +0000 from: Jonathan Gregory subject: Re: a thought on deltaQ2x to: Tim Osborn Dear Tim When I tried to do the extrapolation from constant 2xCO2 and 4xCO2 back to origin, it didn't really give sensible answers either. Possibly the forcings were too low then as well. This could happen if the effective climate sensitivity was changing, perhaps. In a standard "Gregory" plot of the HadCM3 4xCO2 run, for instance, climate sensitivity increases so the line of F vs T flattens out, and if you extrapolate back to the origin you consequently get a forcing which is too small. Maybe this kind of thing happens in all models. Given F = Q - lambda DT(t) = Q0(t) * f - lambda DT(t) where f is the forcing factor, what about doing a multiple linear regression of F on Q0(t) (the "profile") and DT? Of course it's probably rather degenerate. best wishes Jonathan