There has been much discussion recently on whether GCMs participating in intercomparisons, such as CMIP3 and CMIP5, are ‘independent’. But if they are not, how does this make a difference to the uncertainty in our projections for future climate?
The usual measure of spread of GCM projections, as estimated by the variance, assumes that the GCMs are independent. Jewson & Hawkins (2009) demonstrated that, assuming the same ‘correlation’ (
r) between all GCMs, the variance should be inflated by a factor of
1/(1-r) to account for the non-independence.
We do not yet have an estimate of the non-independence for future projections (suggestions welcome!) – as it is not simply the correlation between the temperature timeseries. However, a recent estimate of the non-independence of GCMs using the spatial patterns of biases compared to recent observations has suggested an
r of around
0.5. However, the IPCC AR4 ‘likely’ ranges for each scenario were given as the
mean-40% to mean+60% – which roughly equates to a
r of around
An estimate of the additional model response uncertainty produced by the non-indepedence, assuming
r = 0.5, is shown by the cyan region in Fig. 1 below, updating the figure from Hawkins & Sutton (2011). So… uncertainty in uncertainty.