What if the IPCC had used different models?
The analysis above is just fine if we consider the IPCC projection to be based on the specific ensemble of models they actually used. However, when I presented a similar analysis for GISS, Nick Stokes objected that I did not consider how the results might have varied if the IPCC had used a different collection of models. Fortunately, it is very easy to extend the method to account for this. To account for this, I modified the standard error in the trend by taking the square-root of the sum of the square in the standard error in trends across all 22 models and the standard error in the difference in the mean. The latter is used to form the uncertainty intervals in the figure shown above. The standard error in trends across all 22 models was computed by finding the variance in the trends across all 22 models, subtracting an estimate of contribution of “weather” from this, and then dividing by 21, which is the number of degrees of freedom for the models and then taking the square root of
