date: Fri, 10 Feb 2006 14:35:22 +0000

from: Jonathan Gregory <j.m.gregoryatXYZxyzding.ac.uk>

subject: Re: a thought on deltaQ2x

to: Tim Osborn <t.osbornatXYZxyz.ac.uk>

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

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