Realclimate has a new post, by Rasmus Benestad, that discusses predcitable and unpredictable behaviour. It focuses a little on Judith Curry’s recent report about climate models, that I discussed here. The Realclimate post is well worth reading, and I encourage you to do so, but there was one thing that I really liked and that I thought I would repeat here.
What’s quite often been discussed/mentioned here is that if one argues for a significant natural contribution to our long-term warming, then that’s potentially arguing for a high climate sensitivity. Any internally-driven long-term warming will require some kind of feedback in order for it to be sustained. However, such a physical process should respond to both internally-driven and externally-driven perturbations. Therefore arguing for a significant natural contribution to our observed warming AND arguing for a low climate sensitivity is potentially paradoxical.
I won’t say more and will simply repeat, below, the section from the Realclimate post, which explains it better than I can (credit: Realclimate/Rasmus Benestad).
A potential feedback paradox
Curry also introduces a potential paradox in her report when she emphasises natural variations. The magnitude of natural temperature variation are regulated by feedback processes and have physical causes. The climate sensitivity also involve such feedback processes.
Any feedback process based on temperature will act on both natural and forced changes in the temperature. If such feedbacks result in pronounced natural temperature variations, they also imply that the climate sensitivity is high.
Examples of such feedbacks include increased atmospheric humidity and reduced snow/ice cover. Processes involving clouds are more uncertain, but they too are likely to be affected by temperature (convection) and act to modify the climatic response.
It is possible to get enhanced variability on those timescales as a result of dynamical mechanisms without needing to appeal to higher climate sensitivity.
Nevertheless, the bottom line is that Curry must prove that the feedbacks involved in the natural variations are different to those affecting the climate sensitivity before she can conclude that natural variability dominates over a warming due to increasing greenhouse gases.