Bob Tisdale has contributed a post to Watts Up With That in which he does a Model-Data analysis with trend maps. What he is discussing is a comparison of the GISSTEMP temperature anomalies with the CMIP5 model. The figure (credit: Bob Tisdale) that he uses is shown below. Personally, I think it looks quite impressive. However, I accept that simply looking at it isn’t a particularly scientific way of assessing the quality of the fit.
What Bob Tisdale did was to divide the time into 4 intervals which are labelled on the figure above. As already mentioned by HotWhopper, there seem no real justification for choosing these particular divisions and the dividing lines seem to lie on peaks or troughs. What Bob Tisdale then does is calculate the model trend and GISSTEMP trend for each interval. He then compares these trends and concludes that the comparison indicates that the model isn’t a particularly good fit to the data. Here’s where I have a problem. He does absolutely no error analysis. I find this a little ironic given that the prime reason for claiming that there has been a pause in global warming for the last 16 years is because the error in the temperature anomaly trend is large enough that we can’t rule out that the trend could be 0oC per decade.
Now, I can’t actually find errors for the CMIP5 trends, but using the Skeptical Science trend calculator I can determine the 2σ errors for the GISSTEMP trends. I’ve produced a table, below, that shows the 4 trends together with the 2σ errors on the GISSTEMP trends. Clearly, the two earlier intervals are still not consistent with each, but the two later intervals compare quite well. This does, however, ignore the errors in the CMIP5 trends. I don’t know how to work these out, but I did find the figure, below, that shows the CMIP3 and CMIP5 models, both with errors (blue and yellow regions), together with the observations. The model errors seem as though they would produce trend errors similar to (or even bigger than) than errors in the measured (GISSTEMP) trends. If so, the trends in all 4 intervals would be statistically consistent, although this might only be marginally so for the 2 earlier intervals. The main point, however, is that you can’t do this kind of analysis without considering the errors in your calculations. Ignoring the errors is very shoddy and given how skeptics regularly use the error in the trend to claim that warming has paused for the last 16 years, they can’t claim to not know about errors.