Kevin Trenberth, coordinating lead author of IPCC 4th Assessment Report, WG1 Chapter 3, says:
In fact there are no predictions by IPCC at all… they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents…
…none of the climate states in the models correspond even remotely to the current observed climate. In particular, the state of the oceans, sea ice, and soil moisture has no relationship to the observed state at any recent time in any of the IPCC models.
I postulate that regional climate change is impossible to deal with properly unless the models are initialized.
Therefore the problem of overcoming this shortcoming, and facing up to initializing climate models means not only obtaining sufficient reliable observations of all aspects of the climate system, but also overcoming model biases. So this is a major challenge.
My suspicion, as a statistician, is that they merely sought for doomsday scenarios even though the most loopy computer models couldn't get them there from current conditions. Then they fixed the initial conditions so that those outcomes were attainable.
Oh look, that's exactly what he says:
The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. There are a number of assumptions that go into these emissions scenarios. They are intended to cover a range of possible self consistent “story lines” that then provide decision makers with information about which paths might be more desirable. But they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents. There is no estimate, even probabilistically, as to the likelihood of any emissions scenario and no best guess.
Even if there were, the projections are based on model results that provide differences of the future climate relative to that today. None of the models used by IPCC are initialized to the observed state and none of the climate states in the models correspond even remotely to the current observed climate.
Why are we taking this stuff seriously, never mind skewing whole economies on the basis of it?