From	David Rind	Date	Wed, 20 Jul 2005 12:11:20 -0400
To	Stefan Rahmstorf
CC	wg1-ar4-ch06@joss.ucar.edu
Subject	Re: [Wg1-ar4-ch06] Comments on Section 6.3
Dear Stefan, The distinction here is that GCMs attempt to calculate from first principles the zeroth and first order processes that dominate the problem they are studying, whereas EMICs parameterize many of those processes. The fact that EMICs can reproduce GCM results suggest that their parameterizations have been tuned to do so - but this does not in any way imply that if one alters the forcing or boundary conditions outside of a small range, or apply them to completely different problems, that the two types of models will react similarly. In fact, there is a history of this - the first "EMICs" had a very large sensitivity to a 2% solar insolation change; then they had to be re-tuned to prevent that from happening. EMICs are used for paleo-problems because of their ability to take large time-steps, but there is no free lunch - in doing so, they sacrifice calculating the fundamental physical processes the way the real world does it. GCMs have storms, they have real water vapor transports, they have winds calculated from solving the conservation of momentum equation, etc. etc. There is a quantum difference between the fundamental approaches - it is not a continuum, in which there are no real differences, everything is simply a matter of opinion, there is no such thing as truth - that's the argument that greenhouse skeptics use to try to make science go away. Because we can't use GCMs for long-time scale problems, we do the best we can - we use these heavily parameterized models. If we could use GCMs for those problems, EMICs could then be tuned to produce the GCM results on those time-scales as well. But in this case we have no way to validate the EMIC results - and since the first principles are not being used, we cannot know whether they represent a physically consistent solution or not. Therefore all they can do is suggest interactions among processes, a useful though not definitive addition to the field. David ps - concerning CLIMBER-2, I asked a number of leading climate scientists to read the model description paper. Peter Stone was the only person I asked who thought the model was at all useful for studying the types of problems we are discussing. And it was not only GCM scientists. If you want to hear further cogent arguments concerning its inapplicability, consider contacting Bill Rossow (the recent winner of a major honor as a leading climate scientist) but make sure your email program or telephone accepts unexpurgated text. At 4:22 PM +0200 7/20/05, Stefan Rahmstorf wrote: >Dear David, > >I take from your response that you consider all models that >parameterise an important first-order process "conceptual models". I >can live with that - but then there are only conceptual climate >models around. Any coupled climate GCM that I know of parameterises >oceanic convection (and in a very crude way), hence it is a >conceptual model in your terms, and there is no fundamental >distinction of category between your model and our model. > >To me the scientific question is not whether an important process is >parameterised (many are in GCMs) - it is how well this >parameterisation works, for the task at hand. We have tested the >feedbacks in great detail (e.g., the cloud, water vapour, lapse rate >and snow/ice albedo feedbacks for 2xCO2) in our model and they >perform quantitatively within the range simulated by various GCMs. >The same is true for many other diagnostics - the model has taken >part in model intercomparisons with GCMs and always falls within the >range of different GCMs, in a quantitative way. To repeat that >point, the quantitative differences between different GCMs are >larger than the typical difference between our model and a GCM. So I >see no basis for your claim that this model can only "suggest orders >of magnitude". That's just plain wrong from all the evidence that I >have seen (a lot). If you have concrete evidence to the contrary, >other than just knowing one person who happens to agree with you, >please come forward with it. > >Stefan > >-- >To reach me directly please use: rahmstorf@ozean-klima.de >(My former addresses @pik-potsdam.de are read by my assistant Brigitta.) > >Stefan Rahmstorf >www.ozean-klima.de >www.realclimate.org _______________________________________________ Wg1-ar4-ch06 mailing list Wg1-ar4-ch06@joss.ucar.edu http://www.joss.ucar.edu/mailman/listinfo/wg1-ar4-ch06