Met Office concedes the error
Sep 25, 2013
Bishop Hill in Climate: MetOffice, Climate: Models, Climate: sensitivity

Over the last day or so, Julia Slingo has sent a polite, but somewhat evasive response to Nic Lewis regarding his critique of the UKCP09 model. It can be seen here.

Nic Lewis's reaction is here. I don't think he is very impressed. The key exchange relates to the following paragraph in Slingo's paper:

Having said that, it is true that the relationship between historical aerosol forcing and equilibrium climate sensitivity (ECS) depicted in your Figure B1 is based only on the PPE. But we disagree with your assertion that the results from HadCM3 are fundamentally biased. It is certainly the case that versions of HadCM3 with low climate sensitivity and strongly negative aerosol forcing are incompatible with the broad range of observational constraints. But the key point is that the relationship between aerosol forcing and ECS is an emergent property of the detailed physical processes sampled in the PPE simulations.

To which Nic's response is this:

This is a key paragraph, which in effect concedes that my main criticism is valid.
I don't dispute the point that in HadCM3 – and very possibly other models – the relationship between aerosol forcing and ECS is an emergent property. That is precisely why HadCM3 is not suitable for a PPE study in which, supposedly, "uncertainty in the response of the climate system to CO2 forcing is comprehensively sampled".

In essence, Slingo seems to be admitting that the low climate sensitivity, low aerosol forcing scenario that the observations suggest exist in reality cannot be simulated by HadCM3, but effectively handwaves this problem away. As Nic puts it:


In effect, her view seems to be that it doesn't matter what the observations imply, because the models rule out the possibility of low aerosol forcing, low ECS combinations. I am reminded of a famous line by Bertold Brecht to the effect of: "The people have failed the government. The government must elect the new people." But the Met Office can no more replace the real climate system with one that agrees with the models than a communist government could replace the people with one that satisfied its ideology.

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