The issue at hand is whether the observational record could, in principle, be used to test a climate model’s assumption that the climate has high sensitivity to forcings such asCO2 increases. By using compensating parameters for forcing and sensitivity, climate modelers guarantee that such testing against the observational record cannot happen: models are effectively immunized from empirical challenge. More precisely, as Retto Knutti, a contributing author to the IPCC’s 2007 AR112 explained in a paper published after that report’s publication, “models with high sensitivity (strong feedbacks) avoid simulating too much warming by using a small net forcing (large negative aerosol forcing), and models with weak feedbacks can still simulate the observed warming with a larger forcing (weak aerosol forcing)…”113 Put slightly differently, the reason why the major climate models can all reproduce the late 20th century warming pretty well even though they don’t agree at all on the fundamental question of how climate responds to various forcings (the parameter S in equation (1)) is because they make whatever assumption about aerosols is necessary to adjust the radiative forcing ΔQ so as to be able to reproduce temperature changes ΔT observed during the late 20th century.114 But these assumptions are far from innocuous. As recent work has shown, if the (negative) aerosol forcing turns out to be much smaller than assumed, then the ensemble of GCM’s used by the IPCC would have to have a much larger climate sensitivity (with the mean moved up a full 2 degrees centigrade) in order to remain consistent with observations. On the other hand, if the negative aerosol forcing is even larger (more negative), then the ensemble GCM’s would fail on the other side, simulating too little warming. This “mismatch” between observed and simulated 20th century warming would mean that “current agreement between simulated and observed warming trends would be partly spurious, and indicate that we are missing something in the picture of causes and effects of large scale 20th century surface warming.”115
Hat tip to Graham Young from Online Opinion who says:
The arguments are not new, but put together with exhaustive documentation. Also puts the lie to the claim that there is nothing to challenge the IPCC in the peer reviewed literature.
Read it and get back to us 😉 Its a long PDF but most illuminating.