date: Fri, 14 Jan 2005 12:20:47 +0100
from: Stefan Rahmstorf <rahmstorfatXYZxyz-potsdam.de>
subject: Re: 6.5.8 revisions
to: David Rind <drindatXYZxyzs.nasa.gov>
thanks for the detailed response. I'll try to be brief.
On the orbital forcing you write:
The point here is that climate can be forced by other factors than simply a global,
annual average radiation change, which is the metric now being used.
I think we all agree on this point. My concern is only about how to present it in the
section. I think that giving a climate sensitivity wrt. global mean orbital forcing is
confusing to the uninitiated, e.g. your statement in the section:
This high climate sensitivity (2�C/ Wm^-2) is occurring in an atmospheric model
(ECHAM-1) whose sensitivity to doubled CO is about 0.6�C/Wm^-2.
I really think we should not give a number like 2�C/ Wm^-2 as "climate sensitivity" to
global-mean orbital forcing and contrast it to that to doubled CO2. It gives out the
message to people that climate sensitivity is all over the place and ill defined. That's
not the case. Climate sensitivity is a well-defined concept for a globally uniform forcing
like CO2 forcing, but nobody expects any clear relation between the global mean part of
orbital forcing and the climate response.
On uncoupled models:
I agree that for 2xCO2 runs, you will get very similar climate sensitivity with uncoupled
and coupled models, because there is no large change in ocean heat transport between
equilibrium 1x and 2x CO2 states (as confirmed by doing this in coupled models). The mixed
layer boundary condition used in the uncoupled models simply assumes a fixed, prescribed
ocean heat transport, which turns out to be a valid approximation in this case.
My concern was and is specific to the discussion for LGM climate, where this is not a valid
approximation, as we know both from proxy data and from model results that ocean
circulation and heat transport was very likely quite different in the LGM compared to
today. In our Nature 98 LGM simulation, we get 50% difference in the response of the
Northern Hemisphere mean temperature, between the uncoupled "mixed layer" experiment and
the one that includes the ocean model. 50% is a first-order difference, and hence I think
that all the evidence we have today, points to the "constant heat transport" approximation
breaking down when applied to the LGM. The IPCC report should not draw conclusions about
climate sensitivity from LGM experiments that have made this approximation, as I think
those would be hard to defend. I must say I'm starting to get a little concerned about the
chapter discussing 1980s papers for no other apparent reason then them being authored by
Rind, while leaving out important more recent, widely recognised advances in the field.
I attach the Schneider et al. paper I announced earlier, submitted to Science today and
arguable the most comprehense study on deriving climate sensitivity from LGM data
constraints that has been done so far.
On the directionality of the cimate sensitivity:
of course I understand the reasons, the ice feedback and water vapor feedback etc., I've
written about those myself in the past - again this is only a difference in how best to
present the same, undisputed facts. You make the argument that when going to a colder
climate, sensitivity is different from when going to a warmer climate. That is undisputed.
But that in my view has nothing to do with the "direction" of the experiment, but with the
fact that sensitivity in a colder climate is different from sensitivity in a warmer
climate. I explained with the ppm example because I thought that's simple. A
"directionality" would be, if going from 280 to 300 ppm would give a different equilibrium
response compared to going from 300 to 280. But that's not what you're talking about. Your
talking about going from 280 to 260 (say), as compared to going from 280 to 300. That of
course gives different results, because the difference 280-260 applies to a colder climate
than the difference 300-280 (no matter in which "direction" you derive this).
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