Fred,
People from the consensus side of the argument interpret Lindzen’s statement as an accusation of fraud, or something close to it. I do not read it that way.
Lindzen is actually quoted as saying,
“The higher sensitivity of existing models is made consistent with observed warming by invoking unknown additional negative forcings from aerosols and solar variability as arbitrary adjustments.”
You have paraphrased this as,
“…aerosol forcing is adjusted to make model projections match observed trends.”
If you look carefully, that’s not an accurate paraphrase.
I would compare Lindzen’s state with one of Kiehl’s statements,
“models with low climate sensitivity require a relatively higher total anthropogenic forcing than models with higher climate sensitivity.”
So what does Kiehl mean by “require”? I think it is either a physical requirement or an arbitrary requirement. No one is suggesting that there is a physical reason why forcing and sensitivity should compensate; so without such a reason you are left with only a few other possibilities – sheer chance, which would be extraordinary; and an unconscious tuning in response to expectations of the model developers – which is less extraordinary.
I also note you find the tuning argument implausible because climate sensitivity is an emergent property of the models. Sometimes the forcing is too. So, I would direct you to a paragraph from Huybers:
“Covariance could also arise through conditioning the models. A dice game illustrates how this might work. Assume two 6-sided dice that are fair so that no correlation is expected between the values obtained from successive throws. But if throws are only accepted when the
dice sum to 7, for example, then a perfect anticorrelation will exist between acceptable pairs (i.e., 1–6, 2–5, etc.). Now introduce a 12-sided die and require the three dice to sum to 14. An expected cross-correlation of 20.7 then exists between realizations of the 12-sided die and each of the 6-sided die, whereas the values of the two 6-sided dice have no expected correlation between them. The summation rule forces the 6-sided dice to compensate for the
greater range of the 12-sided die. This illustrates how placing constraints on the output of a system can introduce covariance between the individual components. Note that this covariance can be introduced, albeit not diagnosed, without ever actually observing the individual values.”
In the case of climate models, models may have only been accepted only when they reproduced aspects of the historical climate – in particular the surface temperature record. (Or, indeed, if their sensitivity lay outside the Charney range of 1.5 – 4.5 K.)
(By the way, I put my own view more fully at Michel Crucifix’s blog -
http://mcrucifix.blogspot.com.au/2012/02/ahem-few-clarifications.html.)