However, for the 15-year trend interval corresponding to the latest observation period 1998-2012 , only 2% of the 62 CMIP5 and less than 1% of the 189 CMIP3 trend computations are as low as or lower than the observed trend. Applying the standard 5% statistical critical value(8), we conclude that the model projections are inconsistent with the recent observed global warming over the period 1998- 2012. (note, however, that the standard statistical-test terminology, although widely used, is not strictly appropriate in this case; see supplementary material(9). The inconsistency increases rapidly with increasing trend length. A continuation of the current observed global warming rate for a period of twenty years or longer would lie outside the ensemble of all model-simulated trends.Serendipity.
What do these inconsistencies imply for the utility of climate projections of anthropogenic climate change? Three possible explanations of the inconsistencies can be suggested: 1) the models underestimate the internal natural climate variability; 2) the climate models fail to include important external forcing processes in addition to anthropogenic forcing, or 3) the climate model sensitivities to external anthropogenic forcing is too high.
The first explanation is simple and plausible. Natural climate variability is an inevitable consequence of a slow system (climate) interacting with a fast system (weather)(10)
Note, though, that the quest for recalibrated (feasible) or more accurate out-of-sample (the greater challenge) climate models can go on completely independently of the quest for carbon taxes or emission markets or hydraulic fracking.