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I have trouble believing an author when he’s sloppy with terminology. Nir Shaviv defines GCM as "Global Circulation Model." I've studied GCMs for several years. I can only conclude one of three things: 1) he unintentionally used the wrong name implying a lack of due diligence, 2) he intentionally used the wrong name for reasons unknown, or 3) there really is a new class of model called "Global Circulation Model."
The paper he referenced--Cess et al. (1989)--has the following title: "Interpretation of Cloud-Climate Feedback as Produced by 14 Atmospheric General Circulation Models." (In this case, we are specifically talking about AGCMs.) This isn't even that latest word, because there's a later paper: Cess et al. (1996), "Cloud feedback in atmospheric general circulation models: An update." The more recent papers on the subject have apparently been overlooked.
The names of the models in his figure are all GENERAL circulation models (from the cited paper). By the way, the cloud feedback differences in the second paper aren’t as great.
Therefore, 1) it appears that there isn't a new class of climate model; 2) I wasted time trying to figure out what Nir Shaviv is really saying; and 3) his argument against GCMs doesn’t seem to carry as much weight.
I'm sure Shaviv was delighted when you e-mailed him to point out his error. Slips of the pen which we are all prone to make are extremely annoying. I bet you got a courteous reply though.
Alas, I'm not as masterful as you give me credit for, because I didn't send him an email. I'm glad to hear it was only a "slip of the pen."
Every doubling = exponential
Exponential, but not saturating
Many thanks Hans for the reference to Nir Shaviv's web site. Really clear and not at all difficult to understand.
I still feel the main point is being missed here. If you leave out the (admittedly complicated) problem of frequency response, a logarithmic response within an otherwise linear feedback loop ensures that there is ALWAYS a signal level at which the incremental loop gain is less than one. This means that runaway feedback instability cannot happen. That is why the concept of “saturating” is so important.
The Co2 spectrum can saturate (look at Venus), but just not for the optical thicknesses that are relevant for the near future and ice age past of the earth (150-1200 atm cm)
I begin to understand. We are divided by a common language. When a systems engineer speaks of saturation in an amplifier or material he means that the incremental gain decreases as the input signal increases, there is no implication of any physical mechanism analagous to a sponge taking up water. Saturation can be weak, strong or total. The logarithmic input/output characteristic is saturating because the incremental gain decreases monotonically with signal level. Over a wide range of sciences saturation is taken to mean the deviation from linearity, not just the flat bit (try googling, for instance, “onset of saturation”). It is important in the situation under discussion because runaway instability is impossible with a logarithmic characteristic in the feedback loop (unless of course there is a countervailing exponential characteristic).
As it happens, there is some relevance of the sponge analogy in this case, but it is modified by the competing process of spontaneous emission.