To follow up on Michael’s post I thought I might just highlight this recent paper by Fasullo, Sanderson & Trenberth called Recent Progress in Constraining Climate Sensitivity With Model Ensembles.
It’s hard to summarise, as it’s essentially a summary itself. I think it’s open access, so you can read it yourselves if you wish. It mainly discusses different methods for constraining climate sensitivity using model ensembles. The two main methods are Perturbed Physics Ensembles (PPE) and Multi Model Ensembles (MME). A PPE is when you use a single GCM, but perturb the physics to see how that influences the resulting ECS. You can then weight the result on the basis of climatological skill. An MME is when you use multiple models and then try to identify emergent constraints (EC) that can then be used to constrain the resulting ECS.As I said above, it’s hard to really summarise this paper as it’s a summary itself. However, the top figure on the right shows an example of an EC – the strength of mixing in the lower troposphere over warm tropical oceans – that can be used to constrainn the ECS in an MME. They lower figure illustrates other possible emergent constraints and
suggest[s] an underestimation of ECS by models due to cryospheric and cloud feedbacks. While no single EC study should be regarded as definitive, the collective guidance of this literature broadly fails to support the hypothesis that model error is responsible for the divergence between GCMs and estimates of ECS based on simple models and the instrumental record …… Rather, it suggests the opposite, with the evidence showing that model error has more likely resulted in ECS underestimation.
The one thing that struck me about this work is possibly related to the point Michael was making in the previous post. Why has noone taken a GCM and shown that ECS could be much lower than we currently think? It seems that – in a sense – the PPE and MMEs are essentially trying to do something like this. What they seem to find, however, is that if you constrain your results according to climatological skill, it is difficult to get a result that is inconsistent with the IPCC range. Most of the results seem to put ECS at around 3oC – or slightly higher – with a range of around 1oC.