One of the most significant exchanges at Tuesday's parliamentary hearing was the admission by Brian Hoskins et al that the CMIP5 model ensemble did not incorporate the latest IPCC estimates of the effects of aerosol pollution on the climate. In rather simplified terms, the warming at the end of the twentieth century can be explained as a strong warming masked by strong aerosol cooling or by a relatively weaker warming masked by a relatively weaker aerosol cooling. Recent satellite observations are suggesting that the aerosol cooling is much weaker than previously thought and the corollary of this finding is that the climate sensitivity is necessarily lower.
Hoskins and his colleagues excused the IPCC's failure to update the models, noting that the new estimates of weaker aerosol forcing are relatively new. This may well be the case (although I understand that the evidence has been accumulating for some time), but the implications for UK policy are interesting regardless. Lord Deben's Climate Change Committee have just considered the findings on climate sensitivity in the Fifth Assessment Report and have concluded that no changes are required to the Fourth Carbon Budget. However, if we now know that the models have a warm bias, the findings of the CCC are inevitably undermined. Until we have new model runs incorporating the new aerosol forcings, what should policymakers do?