The US CCSP has a draft document out on its website; Climate Models – an Assessment of Strengths and Limitations for User Applications. Snappy title. The whole document runs to 225 pages. Feedback is still open, so if you are qualified, you can still input into the draft document.
Here is the executive summary, a more manageable 10 pages. For a non-scientist, it is useful to put into perspective the issues which do still matter about the skill of global climate models in forecasting future climate trends.
In a nutshell, the GCMs are imperfect in known ways. They have improved a lot in the last 25 years in some ways, but not in others. The uncertainty caused by aerosol and cloud dynamics/physics is still a defining limitation. Regional downscaling and statistical modelling have strengths and weaknesses. Some of the errors are sufficiently consistent to suggest that there are still elements effecting climate change which are not adequately represented in the models.
So? Three thoughts: in spite of all this, the models are probably good enough now to be confident that, in the ‘big picture’ and on climatically meaningful time-scales, they are pointing us in the right direction. Secondly; any products which agencies or organisation produce which derive conclusions from single-model runs should be viewed as speculative and highly conditional, and their merit evaluated accordingly. Thirdly; if somebody with a brain the size of a planet and a wallet to match could combine a major ensemble run of all the CMIP3 GCMs, several statistical models and a large tranche of closely-linked simplified models such as the ones used by ClimatePrediction.net, whilst at the same time succesfully incorporating the ‘missing link’ in the current models, it might be possible to end up with a genuinely skilful forecast of the twenty-first century’s climate at a resolution useful for adaptation planning to be undertaken.