Richard Rood's article about uncertainty in climate projections is a few weeks old now, but I came across it only today after someone tweeted a link to it. Rood is trying to make the case that:
the uncertainty in climate projections associated with the physical climate model is smaller than the uncertainty associated with the models of emission scenarios that are used to project carbon dioxide emissions.
His argument seems to rest partly on the fact that climate models include well-understood physical laws at their heart, while economic models are much more empirical. This argument seems to me to be somewhat spurious. The fact that an aeroplane includes a number of transistors, whose behaviour is well-understood, does not make it necessarily more likely to fly than one that doesn't.
He argues that the spread in the models would be much less if it were not for the different economic scenarios that feed them. This seems flawed to me. Rood argues that the spread in the models represents "simple estimate of uncertainty". I'm not sure this is right. To the extent that the models make the same erroneous assumptions and have the same unknown unknowns, surely the climate model uncertainty is much larger?