date: Tue, 21 Jul 1998 17:40:38 +0100 (BST) from: Jonathan Gregory subject: derivation of headline results to: scenarios@meto.gov.uk Dear Colleagues I believe that it is important to clarify how the "headline" figures for climate change appearing in the TAR and its summaries should be derived from atmospheric concentrations. The choice of approach will affect the contents of more than one chapter. I am not addressing the difficult issues of emissions scenarios and how they are converted to concentrations, which are also being debated in this email list. I am particularly writing to the proposed LAs of the climate-change and sea-level chapters. One possibility would be to restrict the TAR to assessment of results for climate change and sea-level rise which come from coupled GCMs. This would be a clean and attractive approach if the GCMs involved had all been run with the same concentration scenarios. However, it would not give the complete range of uncertainty because most GCMs are unlikely to have been run with the extreme concentration cases. If it is the assessment of the TAR that the extant GCMs do not cover the complete plausible ranges of climate sensitivity and ocean heat uptake, those uncertainties would also not be fully represented. Because of these limitations, it is probable that some means will be needed for calculating results for outlying scenarios and sensitivities. In the SAR, simple climate models were used for this. I think the role of simple models in the TAR should be discussed now. Simple models of aspects of the climate system are very useful for investigating and understanding how things work. Such models should, I think, be discussed in the TAR along with the processes they are used to investigate. Simple models of the whole climate system (such as the two used in the SAR) are also useful in their own right for investigating the effect of different concentration scenarios, for instance. However, I think that a question which has to be clearly addressed in the TAR is whether a simple model is in any way scientifically superior to a coupled GCM for doing this job (setting aside the practical issue of whether you can afford to use a GCM). The answer to this question will determine how simple models are presented. Question 1: (a) If GCMs are the principal tools, producing results which are more reliable in detail than those from any simple model, then simple models are needed only as tools for EXTRAPOLATING or INTERPOLATING between GCM results, to estimate the effect of different scenarios or sensitivities. (b) If the assessment is that some simple models offer independent climate predictions to be treated on the same footing as coupled GCMs, then simple models should be discussed at much greater length in the model assessment and model predictions chapters than they were in the SAR. An assessment of all available simple models and their results should be given. My view is (a), which I think is consistent with the IPCC simple models paper. If (a) is the view taken by the authors of the relevant chapters, then I argue that what we are seeking to do the job is a NUMERICAL TOOL. The test of the tool is that it can reliably reproduce the results of several coupled GCMs, both for temperature change and thermal expansion, for different kinds of scenarios (e.g. 1%, historical forcing, stabilisation). This is a stringent requirement. I do not think that a 2D climate model is necessarily preferable to a 1D climate model for this task. In fact, the more elaborate the simple model - the more physics it has got in it - the less amenable it will probably be to being tuned to reproduce GCM results. It is essential that this calibration can be demonstrated to be good, because in view (a) it is the GCMs which are basically providing the results. The tool must not be allowed to go its own way. It is not necessary to interpret the tuning parameters of a simple model in terms of the physics of the real climate system when it is being used for this purpose. It is not necessarily the case that a simple climate model is the best choice anyway. Question 2: If this view is followed, the calibration of the tool will be needed for both the climate-change and sea-level chapters. Where is the best place to put it? (a) It could be in the climate-change chapter, which is earlier, in which case we will have to take care that the climate-change and sea-level chapters fit well together. (b) It could be in the scenarios chapter, which may be dealing with this kind of issue anyway. (c) It could be in an annex or additional "results" chapter, in which case the climate-change and sea-level chapters would present GCM results only. This would have the drawback that the headline results of the TAR would look rather like a footnote in the main text, even though they would feature prominently in the summary. However, that's what was done in the 1992 report. >From the point of view of clarity, I think (c) would be best. It would have to be presented as a main conclusions chapter of the TAR, resting on the GCM results which had earlier been extensively discussed. Question 3: How many such tools are needed i.e. how many simple climate models or equivalent? I think more than one should be investigated, to see how well they fit the GCMs. If they all fit well, and their extrapolations to outlying scenarios agree closely, only one need be presented in the TAR. If there is a range of results, this gives a measure of the additional uncertainty in the procedure of extrapolation, and should be reported. Thanks for reading all this! Jonathan Gregory jmgregory@meto.gov.uk +44 1344 854542 fax +44 1344 854898 Hadley Centre, Met. Office, London Road, Bracknell, Berks., UK. RG12 2SY