Ken Rice,
“again you seem to have chosen to entirely miss the point (this can’t be coincidence)”
Civil and on topic? Two snide comments in a single short line.
“That you can show that this is plausible/possible, does not mean that it is likely and certainly does not rule out that we could warm much more than you seem to be suggesting.”
I think you have this a bit backwards. Nic finds a model which best fits the historical data (using some AR5 best estimates) and then makes projections of warming for each emissions pathway. These turn out slower than the warming projected by CGM’s. It would be more accurate to just say Nic’s model suggests slower warming is more likely than faster warming.
Is faster warming possible? Sure, Nic could be wrong. It is fair to ask how the optimization of fit to the data in Nic’s model was done so that you can judge how tightly constrained the climate sensitivity is by the optimization process (and any assumptions which go into it), and so better judge the uncertainty in his projections. I don’t think it is fair to say his result is ‘not likely’ just because it disagrees with GCM estimates. Nic’s estimate for ECS lies within the AR5 “likely range” of 1.5C to 4.5C; it seems to me that actually showing his result is ‘not likely’ requires a lot more than pointing at other estimates which are higher.
The probability of much faster warming (eg. the GCM pooled estimate) is a different question, and one that requires critical evaluation of the assumptions (parameterizations) and uncertainties in each model. Of course, I expect the trajectory of warming over the next three decades will sort this all out, because there is a big difference between the GCM average warming (about 0.24C per decade IIRC) and Nic’s estimates (about 0.12C per decade), and because the emissions pathways have no significant impact on projected warming for the next few decades for either GCM’s or Nic’s model, so today’s “policy choices” will make little difference over that period. If warming follows a path of ~0.12C per decade, as Nic’s model projects, then I think relatively low climate sensitivity will be difficult to refute. If warming follows a path of ~0.24C (or more) per decade, then relatively high climate sensitivity will be difficult to refute.
