From	Stefan Rahmstorf	Date	Fri, 14 Jan 2005 12:20:47 +0100
To	David Rind
CC	Tim Osborn, Jonathan T. Overpeck, Keith Briffa, Eystein Jansen, fortunatjoos@email.arizona.edu
Subject	Re: 6.5.8 revisions
Hi David, 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[2] 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). Stefan -- Stefan Rahmstorf [1]www.ozean-klima.de [2]www.realclimate.org Attachment Converted: "c:\eudora\attach\schneider_jan05.pdf" References 1. http://www.ozean-klima.de/ 2. http://www.realclimate.org/