In the wake of David Rose's Mail on Sunday article yesterday, Piers Forster tweeted that he was unimpressed with the article. I asked him if he would be willing to set out why in a bit more detail and this is his response:
It's fine to say that current models overestimate the last decade of warming. They clearly do, and as I say in my quote I think we can rule out some high sensitivity values because of this. But to do a far comparison you need to remove the effects of variability. Note also that some model runs also get the temperature evolution pretty right - although the majority don't. Even with a suggested ECS of around 2.5 C or so we can end up with a very significant climate change by 2100 if we don't do something - therefore I think the tone of the article in terms of its implications for the IPCC, climate science and the climate itself are all wrong.
He also sent the full quote that he gave Rose for his article, which gives some context.
Basically, the climate sensitivity has always been very uncertain. > estimates have put it somewhere between 1 to 5 C for a doubling of CO2. The IPCC best estimate has been around 3C. The fact that global surface temps haven't risen in the last 15 years, combined with good knowledge of the forcing terms changing climate over the satellite era: greenhouse gases, volcanoes, solar changes and aerosol is beginning to make the high estimates unlikely. Given this, i would put the best estimate using this evidence around 2.5 C. There are still uncertainties though particularly in heat going into the ocean, but climate sensitivities above 3.5C or so don't seem to fit. Keep in mind that this is only one line of evidence for quantifying climate sensitivity. Other lines of evidence have been able to firm up the bottom end. We now have good observational evidence for a positive water vapour feedback and even clouds, which have always been the largest headache in climate change, are beginning to be understood and a positive cloud feedback is looking more likely. This line of evidence helps rule out climate sensitivities below 2C. So I see it very much as a positive story that careful science ( and time) is helping to reduce the most significant uncertainty in climate science.
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