Lucia did a recent post on the construction of IPCC Figure 9.5, which I’d also been looking at in light of the Santer model information but I had different issues in mind. IPCC Figure 9.5 says that they extended selected 20th century runs (the “20CEN” models) with A1B models in order to produce the graph shown below up to 2005. The splice is intriguing on a number of counts – not least of which is the first question: how’d they do it?
Original Caption: Figure 9.5a. Comparison between global mean surface temperature anomalies (°C) from observations (black) and AOGCM simulations forced with (a) both anthropogenic and natural forcings …. All data are shown as global mean temperature anomalies relative to the period 1901 to 1950, as observed (black, Hadley Centre/Climatic Research Unit gridded surface temperature data set (HadCRUT3); Brohan et al., 2006) and, in (a) as obtained from 58 simulations produced by 14 models with both anthropogenic and natural forcings. The multimodel ensemble mean is shown as a thick red curve and individual simulations are shown as thin yellow curves. Vertical grey lines indicate the timing of major volcanic events. Those simulations that ended before 2005 were extended to 2005 by using the first few years of the IPCC Special Report on Emission Scenarios (SRES) A1B scenario simulations that continued from the respective 20th-century simulations, where available. … The multi-model ensemble mean is shown as a thick blue curve and individual simulations are shown as thin blue curves. Simulations are selected that do not exhibit excessive drift in their control simulations (no more than 0.2°C per century). Each simulation was sampled so that coverage corresponds to that of the observations. Further details of the models included and the methodology for producing this figure are given in the Supplementary Material, Appendix 9.C. After Stott et al. (2006b).
I hadn’t really thought about it before, but, if I’d been asked, I would have assumed that the A1B simulations were done separately from the 20CEN simulations. If so, it’s not obvious how you’d go about splicing A1B and 20CEN simulations. For example, in many cases, there are multiple realizations of each model – how would you go about linking individual A1B runs to individual 20CEN runs?
There are other interesting aspects of this figure – including the selection of 20CEN runs: not all runs are used. AR4 Chapter 8 SI provides information on which runs were selected. I’ll return to this issue on another occasion. Today I want to walk through the splicing.
Over the past few days, I’ve scraped tropical (20S-20N) averages for all 78 20CEN (25 models) and all 57 A1B runs (24 models) from KNMI (KNMI has some excellent tools, but they are still pretty labor intensive. I’ve done a pretty little scraping program that eliminates 99% of the cut-and-paste drudgery).
Interestingly, virtually all of the A1B runs start in the late 19th century – and have identical start dates as the 20CEN runs. The two exceptions were GISS AOM and FGOALS – for these two models, A1B starts the year after 20CEN ends. This strongly suggested the possibility that individual A1B runs were associated with individual 20CEN runs and that a lexicon linking runs could be constructed. A hint exists in AR4 chapter SI page 9-7, where 28 20CEN runs are shown as being extended with A1B runs.
It appears that this is the case and that accordingly, there is a “natural” extension of the 20CEN runs with A1B runs.
However, there is not a one-to-one map between 20CEN and A1B models. Overall there are 25 20CEN models and 24 A1B models – the one missing A1B model is BCC CM1, which therefore cannot be extended. I wonder whether the absence of an A1B run for BCC CM1 might be a clerical miss – earlier this week, I notified KNMI that two PCM 20CEN models at PCMDI were not on their system. They promptly responded that the models were there, but the linking webpage hadn’t been updated (they promptly fixed things.) Maybe the A1B run for BCC CM1 is around.
For 14 of the remaining 24 models, there are the same number of 20CEN and A1B models and for all 14 (with the numbers ranging from 1 to 5). For each model, I did cross-correlations for the overlap period and in every case, there was one and only one A1B-20CEN map that had a correlation of 0.99 or so. In every case, the “natural” order was preserved in the map. While the values of the cross-correlation “diagonal” were around 0.99 or higher, the “off-diagonal” were significantly lower, with the characteristics varying a lot from model to model. For example, CGCM3.1 had cross-correlations between “different” runs of around 0.9, while they were around 0.6 for CCSM3.0 and a very low 0.05 (with a couple negative) or so for ECHAM5.
Only one model (CCSM3.0) had more A1B (7) than 20CEN runs (6). This meant that only one out of 57 A1B models was left without a “natural” 20CEN link. Again, I wonder whether there might be another CSM3.0 20CEN run somewhere.
Given the existence of this one-to-one map, if the correlation is 0.995 or even 0.9995, it seemed odd that the correlation wasn’t 0.999999 or 1.000000.
This had an interesting explanation, which in turn confirmed the identity of the runs. My comparisons were done using “anomalies”, one of the KNMI options – and the reference period for the 20CEN and A1B datasets is different. As a result the reference means used to create the anomaly differ between the 20CEN and A1B versions. KNMI also permits the retrieval of non-anomaly versions expressed in deg C. I spot checked the CCCma series and these values were identical between versions to all decimal places, confirming that, in this case at least, the 20CEN and A1B runs were identical in the overlap period. The lack of perfect correlation resulted from the fact that the pattern of monthly normals was slightly different between the 20CEN version and A1B version, resulting a slight decorrelation.
The existence of this connection between 20CEN and A1B runs makes one scratch one’s head a little in trying to understand exactly what the IPCC authors meant by saying that the A1B runs were an “extension” of the corresponding 20CEN runs, if, as appears to be the case, they are actually alter egos of the same run. (One odd exception to the IPCC “extensions”: PCM A1B runs are available at KNMI but were not used to “extend” the corresponding 20CEN runs.
There’s an interesting connection to Santer in this, which I’ll visit on another occasion.