What Risbey et al. did was fairly simple. They used a moving 15-year window and evaluated multiple climate models based on each model's ability to match the actual El Niño/Southern Oscillation (ENSO) state over that time period. They took the models that best matched the actual ENSO state over that time period—regardless of how accurate anything else was about the selected models—calculated the predicted temperature trends from each selected model, and compared those predicted trends to the actual temperature trend over that same period. Then they shifted the window and repeated the exercise. One of the results was a graph (Figure 2) that looks similar to one that I created back in January to find the last time the Earth had a 15-year cooling trend.
|Figure 2 from Risbey et al. 2014 showing actual 15-year temperature trends versus raw IPCC models 15-year trends|
What their main results show is the reason their paper is "controversial" in the denial bubble. Risbey et al. showed that the mismatch between climate models and atmospheric temperatures in recent years is due to mainly to a mismatch between the predicted states of ENSO and the actual state of ENSO. Not because the models are inherently wrong. Not because scientists don't understand the climate system. Not because the physics are wrong or any other standard denier canard about climate models or climate scientists. It's because computer models are unable to predict, years in advance, exactly what one chaotic phenomenon will do. If there's a mismatch between that predicted input and the actual input, then the model temperature predictions will appear to be off. Once they used the match to ENSO as the selection criteria, the predicted temperature trends were very similar to the actual trends, as their Figure 3 shows.
|Figure 3 from Risbey et al. 2014 showing the match between model predictions and actual 15-year trends.|
To those who keep up with the scientific literature, this will not be a surprise. Others (i.e. Kosaka and Xie 2013) have shown before that the IPCC models accurately predict temperatures since 2000 if they are given actual ENSO values rather than predicted values. Risbey et al. just adds to the evidence that the climate models are accurate, that the main problem lies in predicting ENSO values rather than any inherent problem in the climate models. That should give us pause, as the effects of a chaotic oscillation will cancel out over time, leaving the trend unchanged. And we know where that trend is headed, regardless of the short-term effects of ENSO.
So, what to make of the kerfuffle roiling the denialsphere over Risbey et al.? Much of it is simple ignorance—they don't understand what Risbey et al. have done and what their paper shows. The rest is simply willful ignorance from those who should know better.