She hasn’t done a blog post about it, nor has anyone else as far as I can see, so here’s a venue to discuss it.
The aim is “to describe the debate surrounding GCMs to an educated but nontechnical audience”.
Here are the key points from the summary:
- GCMs have not been subject to the rigorous verification and validation that is
the norm for engineering and regulatory science.
- There are valid concerns about a fundamental lack of predictability in the complex
nonlinear climate system.
- There are numerous arguments supporting the conclusion that climate models
are not fit for the purpose of identifying with high confidence the proportion
of the 20th century warming that was human-caused as opposed to natural.
- There is growing evidence that climate models predict too much warming from
increased atmospheric carbon dioxide.
- The climate model simulation results for the 21st century reported by the Intergovernmental
Panel on Climate Change (IPCC) do not include key elements
of climate variability, and hence are not useful as projections for how the 21st
century climate will actually evolve.
The first section explains what climate models are and how they work, emphasising that some physical processes are approximated and/or can’t be represented on the coarse grids used, and that this inevitably requires the modellers to make choices, which influence the results.
Part 2 discusses why some scientists have confidence in the models and some don’t. A key point here is that because of all the parameter choices, it’s fairly easy to tune the model to fit fairly well with whatever 20th century global temperature data set you like, but this does not mean that the model has any useful predictive skill.
Section 3 looks at climate model failings, such as the failure over more than 30 years to reduce the uncertainty in climate sensitivity. She also mentions the inevitable predictive failure of models due to the chaotic dynamics of the nonlinear fluid dynamics — a small change in the starting values leads to a large change in the future climate (the ‘sensitive dependence on initial conditions’ hallmark of chaos).
The last section builds on the unreliability of climate models for predicting what’s going to happen over the next century, commenting on the IPCC’s own acknowledgement that the models are running too hot, and noting a “troubling inconsistency” in AR5 WG1 in lowering near-term projections but leaving longer-term ones at a higher level.
As Bishop Hill always used to say, read the whole thing.