Wednesday, May 16, 2012


date: Wed, 12 Jan 2005 12:40:03 +0100
from: Stefan Rahmstorf <>
subject: Re: [Wg1-ar4-ch06] Bullet points for the executive summary for
to: David Rind <>

A response to David's comment on bullet point 6.2.3:
this is not a bullet point that refers to any particular model, but to
all efforts of modeling paleoclimate. Personally, I think that these
efforts show encouraging successes. Now, if other people think that the
efforts in modeling paleoclimate over the past years were mostly a
failure, we may need to revise this bullet point. This could turn out to
be a discussion over wether the glass is half full or half empty,
though, which we may not resolve.
The reason I wrote about "encouraging successes" is that many people
have the impression that climate models are generally unable to
reproduce past climate changes, and I think that's not true, I think we
are starting to get some encouraging successes, while we are of course
still in an early stage of this field.

On the different models there is indeed a disagreement between David and
myself, which partly reflects how in my perception the GCM community is
attempting to hold back the competition from other models and retain an
exclusive claim to the truth, so to speak. The term "first principle
models" used by David is a case in point, it is a term chosen to make
GCMs appear as "first principle", while other models are put into a
class below that. I don't think this kind of language is helpful or can
be justified in a scientific sense.

To give an example: arguably the most severe simplification in the
atmosphere model of CLIMBER2 is that synoptic eddies in the atmosphere
are not resolved, rather their effects on the large-scale circulation
are parameterised. To parameterise the effects of synoptic eddies is
standard practice in all ocean GCMs used in coupled climate models.
Hence, if using such a parameterisation of synoptic eddies makes a model
not "first principle" any more, then none of the existing coupled GCMs
is a "first principle" model either. What David calls "first principle"
models employ many parameterisations that are not first principle -
clouds, convection, you name it.

Clearly there is a whole spectrum of models, and they use different ways
of simulating and parameterising various mechanisms, which have to be
evaluated on a case by case basis, and some models are better for some
purposes than others. (Think of tsunami spreading models - the best
models for that will use the shallow water equations rather than the
primitive equations, and it would be silly to argue this is somehow bad
or not state-of-the-art because it is a "simplified" model.) I would
really object to any value-laden statements that suggest GCMs are
somehow fundamentally different from models that parameterise or
simplify one process or another.

Cheers, Stefan

Stefan Rahmstorf

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