Addendum to “A Mistake with Repercussions”

Those experienced with coupled climate models know that the time scale to fully adjust after such a major drop in CO2 concentration is many hundreds of years, due to the slow response time of the oceans. Even if the Von Storch team had not expected this problem, they must have clearly seen it unfold: the trend in their model during the transition phase reveals that it was far from equilibrium, and they should at the very least have mentioned this problem as a caveat in their papers.

In the absence of this information, many colleagues were puzzled by the strong cooling trend throughout the pre-industrial era in this simulation, which made it an outlier compared to all other available simulations of the past millennium. Was this due to a particularly high climate sensitivity of this model? Or was this due to the forcing used? Correspondence with the Von Storch group brought no clarification. Finally, Osborn et al. (2006) identified this as due to a climate drift problem.

The latest publication by the Von Storch team (Gonzalez-Rouco et al. 2006) shows a second run with their model, where the medieval time is about 0.5 ºC colder and the subsequent cooling trend to the 19th Century is only about half as large. This shows that the medieval time was about 0.5 ºC too warm due the lingering anthropogenic warmth in their earlier model, and about half of the long-term climate cooling in their Science paper must have been due to the artificial drift. Several further papers in other journals are based on the ECHO-G simulation affected by the large drift, and their editors may now have to consider appropriate corrigenda.

4. Are the HadCM3 and ECHO-G results similar?

Von Storch et al. performed their pseudo-proxy test with two models: the ECHO-G model and the HadCM3 model. The results of both tests are compared in the figure. In the ECHO-G model (red lines) the performance of the pseudo-proxies is very poor, as the difference between the full and dotted red lines is huge. In the HadCM3 model (blue lines) however, the performance is much better – those results would not have served as a cause for dramatic headlines.

Figure: Test of proxy climate reconstruction method with two climate models, HadCM3 and ECHO-G. Solid lines show Northern Hemisphere temperature in the models (31-year running means), the dotted lines show simulated proxy reconstructions where the proxies are degraded with 75% noise. The error of the proxy method is the difference between the solid and dotted lines (arrows). For comparison, we show the Mann et al. 40-year-smoothed reconstruction for the Northern Hemisphere temperature (black) with its 95% confidence interval (grey), as shown in the IPCC Third Assessment Report (Fig. 2.21). (Note that results from both models are affected by the calibration error discussed in 2.) We thank Von Storch and coauthors for providing the data of their model experiments.

These rather different results of the two models should have raised a red flag – most scientists would in this situation have paused to investigate the cause of the difference, which could have pointed them to their detrending problem. Von Storch et al. instead show the HadCM3 results only in the online supplement, which very few readers look at. In the paper itself they only show the ECHO-G results where the pseudoproxies perform poorly; about the HadCM3 results we only learn that they are “similar”.

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