Nic Lewis has published a detailed comment on the Met Office’s report on climate sensitivity, which was itself very much a response to the Otto et al paper of which Nic was an author. The comment is here.
There is a great deal of interest, not least of which is the fact that the Met Office seems to have made a series of misrepresentations of Otto et al, as well as making several mistakes.
One of these though is astonishing. This concerns Harris et al 2013, a paper by a group of Met Office scientists. The paper is particularly important as it is the source of the official UK Climate Projections. It examines how the virtual climate inside the Met Office model responds to having key parameters tweaked. Different parameter combinations are then weighted in the final analysis depending on how well the resulting virtual climate matches observations.
Nic explains what he found:
As HadCM3's parameters are perturbed, the resulting changes in ECS and aerosol forcing are closely linked. When significantly lower values for ECS – as suggested by recent observational studies – are obtained, HadCM3's aerosol forcing takes on highly negative values. The observational data strongly contraindicate aerosol forcing being highly negative, so parameter combinations resulting in significantly reduced model ECS levels (and thus highly negative aerosol forcing) are heavily down-weighted. As a result, whatever the actual level of ECS, HadCM3-derived ECS estimates are bound to be high
Essentially, the observations would lead you to conclude either that a high climate sensitivity is being masked by a quite strong cooling effect from aerosols, or that there is lower climate sensitivity and that aerosol cooling is a much smaller effect too. However, the Harris et al study behaves in a very different way. If you tweak the settings on the model in a way that gives you relatively low climate sensitivity, the model ends up with a very strong aerosol cooling effect too. The relationship between aerosols and climate is the opposite way round to the observational studies!
The aerosol cooling that appears in the Met Office model when climate sensitivity is low is very strong, completely inconsistent with recent observationally-based estimates of aerosol forcing. In fact it is so strong that it would also have prevented most of the temperature rise seen since industrialisation. In other words, the virtual climate produced with these settings of the model doesn’t match the real one. This means any scenario in which climate sensitivity is low – as indeed the observational studies suggest it is – gets downweighted in the final analysis to the extent that it doesn’t really show up in the final results. The effect is essentially to rule out low climate sensitivity as a possibility.
I’ll say that again in a slightly different way. The Met Office’s model, one used generate the official climate projections, has big temperature rises built in a priori.
Wow.