date: Wed, 22 Aug 2001 11:45:49 +0200 from: Rob Swart subject: TGCIA scenario recommendations to: parryml@aol.com, tim.carter@fmi.fi, m.hulme@uea.ac.uk, s.raper@uea.ac.uk, wigley@ucar.edu Dear Sarah, Tom, Tsuneyuki, Martin, Mike and Tim, Back from holidays I found your email exchange. Let me first apologize that I did not inform Sarah about this TGCIA action. I remembered from the IPCC-TGCIA meeting ? apparently wrongly - that Mike and/or Tim would inform Sarah, as they would be in touch with her anyway (I did not even have Sarah's email address at the time). Let me also reiterate the reason for Tsuneyuki's invited proposal. In order to have comparable GCM results available and impact studies based on these results at the time of the IPCC Fourth Assessment Report, and taking into account that GCM teams are unlikely to perform dozens of runs, the IPCC-TGCIA (chaired by Martin) intends to recommend a limited set of both baseline and stabilization scenarios for such runs. In this way, impact modellers in the coming years could base their analysis on different runs from different GCMs for the same socio-economic scenario(s). Evidently, teams are free to run whatever scenario they think interesting, but comparability would be preferable, and many teams have proven responsive to IPCC-TGCIA recommendations in the past as I understand it. The TGCIA has reached agreement on which 4 of the 40 SRES baseline scenarios would be most interesting (see meeting report: 4 scenarios (A1FI, A2, B1 and B2) for 3 time periods 2020s, 2050s and 2080s). The next question was: since a (maybe "the") core policy question is what the benefits (or avoided impacts) would be of stabilizing GHG concentrations at various levels, and since impact analysis should be based directly on GCM results rather than on results from simple climate models/IA models, it would be useful to also recommend a limited set of stabilization cases. To make this a sensible effort, all the cases would have to be distinguishable from one another from a GCM viewpoint. This may allow for combining various scenarios which may be very different socio-economically, but would give very similar climate results for this century, such as the B1 and 550, and the 650 and B2 cases. The stabilization cases would be selected from the following table, of which the cells contain available (post-SRES) scenario runs: |-------------+-------------+-------------+-------------+-------------| | |450 ppm |550 ppm |650 ppm |750 ppm | |-------------+-------------+-------------+-------------+-------------| |A1T | | | | | |-------------+-------------+-------------+-------------+-------------| |A1B | | | | | |-------------+-------------+-------------+-------------+-------------| |A1FI | | | | | |-------------+-------------+-------------+-------------+-------------| |A2 | | | | | |-------------+-------------+-------------+-------------+-------------| |B1 | | | | | |-------------+-------------+-------------+-------------+-------------| |B2 | | | | | |-------------+-------------+-------------+-------------+-------------| It was suggested to select 2-4 cases from the more than 70 scenarios runs in the post-SRES programme co-ordinated by Tsuneyuki. Tom, it may well be that your "post-WRE" work serves the same purpose, but the rationale for selecting post-SRES cases would be: consistency with the SRES narratives and numbers of the IPCC, and the much-acclaimed multi-model characteristics of the (post-)SRES work. To downsize the 70-odd cases to 2-4 cases and not burden Sarah too much, it was suggested to have one model (MAGICC) run a subset of some 10-15 cases which seemed to make sense. Please also note that not all 70-odd cases are useable, either because they do not have all relevant GHG gases, or there have been questions about the consistency/quality of their assumptions, e.g. a correct simulation of the SRES base case by teams participating in post-SRES but not in SRES (right, Tsuneyuki?). More importantly, Tsuneyuki used his intimate knowledge of all cases and their distribution over base cases and stabilization levels to recommend 13 cases. This selection was discussed with me and Naki during a brief meeting in Washington in June and seemed to be a very appropriate one. I noted the remark by Sarah that mean climate change results would be rather be model-independent (for a given climate sensitivity), while Tsuneyuki notes the large differences in the post-SRES work. These differences may not have to do with different approaches with respect to the carbon cycle or radiative forcing calculations, but rather with the freedom modellers had (or rather: took) in selecting the time path (beyond 2100) towards stabilization/time horizon, and the changes in emissions of non-CO2 GHG in the stabilization analyses which focused primarily on CO2 stabilization. This would need to be clarified in detail for the runs to be selected, and I suggest that only those runs are further used for which the authors provide sufficient information on these issues. Concluding, I would like to ask Sarah, if she would be willing to take the material provided by Tsuneyuki and perform the required calculations for the 13 cases (radiative forcing, global mean temperature and sea level rise, right, Mike/Tim?) within the next 1-2 months. The results would be discussed electronically in a small group (the addressees of this message) in October/November and a preliminary proposal based on these discussions would be the input for a discussion on this issue during the next TGCIA meeting in Barbados, in November. Tom's recent work may be useful for this discussion as well, and I wonder if the mentioned (draft) papers could be distributed to this group or even the full TGCIA. Kind regards, Rob Dr. Rob Swart Head, Technical Support Unit Intergovernmental Panel on Climate Change Working Group III: Mitigation P.O. Box 1 3720 BA Bilthoven Netherlands tel. 31-30-2743026 fax. 31-30-2744464 email: rob.swart@rivm.nl