date: Wed Apr 21 15:56:44 1999 from: Mike Hulme subject: Fast-track, CO2 and future worlds to: arnell,white,livermore,kovats,parry,nichollsr,jenkins,d.viner Dear Fast-trackers, There have been some requests re. CO2 and associated scenario data for the S550 and S750 runs. For now, let me suggest the following. This may require more debate on May 12th. 1. S550 CO2 concentrations are as listed in my Fast-track note of 21 July 1998, namely: 1961-90 = 334ppmv 1990 = 354ppmv 2020s = 410ppmv 2050s = 458 2080s = 498 2110s = 530 2140s = 546 2170s (and thereafter) = 550ppmv [Note: these are 1-2 ppmv different from what I have just recalculated now (19/4/99), but if anyone's model is that sensitive to 1-2 ppmv differences then we really are kidding ourselves about what we are up to]. IMPORTANT: This scenario says nothing about non-CO2 gases, i.e., stabilisation at 550ppmv is CO2 stabilisation at 550ppmv *not* anthropogenic forcing stabilisation at 550ppmv CO2-equivalent. If people want the latter then we are in to a completely new ball game because: a) IPCC do not specify non-CO2 species for stabilisation scenarios (IPCC Tech. Note simply explores effects of different assumption), and b) it brings us back to the role of aerosol forcing. In the original Enting/Wigley stabilisation scenarios they explored a number of different assumptions for these non-CO2 species, but made it clear that none of them were a priori consistent (see Comment 3 below). In effect, you could think of my suggestion as assuming that non-CO2 GHG and aerosol forcings cancel out and we are left with CO2-only forcing. Of course, this may not be what DETR had in mind when thinking about stabilisation since it does not meet the Article 2 FCCC definition of stabilisation which is 'greenhouse gas stabilisation' and not CO2-only stabilisation (but note: the FCCC Article 2 says nothing about aerosols so are we supposed to ignore them?!). ***************** So if people have a problem with this then they better shout now and loud, because the alternative will take some thought and more time than I have right now. ***************** 2. For S750 CO2 concentrations we (i.e., Hadley and I) have determined the following, but are calculated with the same assumption as for S550 (i.e., it is CO2 that is stabilised at 750ppmv and we are assuming that non-CO2 GHG and aerosol forcings cancel): 1961-90 = 334ppmv 1990 = 354ppmv 2020s = 424ppmv 2050s = 501ppmv 2080s = 577ppmv 2110s = 643ppmv 2140s = 692ppmv 2170s = 724ppmv 2200s = 742ppmv 2230s = 749ppmv 3. Socio-economic assumptions. This is a really difficult issue, to which there are at least three solutions (see below). It is a difficult issue because IPCC (and may be no-one else yet) have created a set of consistent socio-economic indicators to accompany stabilisation scenarios. And, furthermore, it all depends on how stabilisation is reached. e.g. is S550 reached through climate policy in a world that looks like IS92a or SRES A1, or is it reached through climate policy in a world that otherwise would have looked liked IS92d or SRES B1? And what sort of climate policies and what effect do they have on economic production? If you believe the economic skeptics then any climate policy will have a detrimental effect on world production, thus lowering GDP whether from an IS92a or IS92d-type world! The three solutions are: Solution 1: fudge the issue. Just accept that we are Fast-trackers and can therefore get away with anything. The best thing may be to take IS92d-type assumptions for S550 and stick with IS92a assumptions for S750. From the socio-economics perspective this is incorrect and we miss an interesting issue in thinking about the effects that climate policy has on future world growth. But it will give us something to work with. Solution 2: try and tap in to emerging work in this area by IIASA (Arnulf Grubler). He has run the IIASA economic model (the one used by SRES) to generate consistent socio-economic indicators for a S550 outcome in a SRES-A1 type world. He is also (in principle) willing to re-run the model for us to generate an S750 outcome in a SRES-A2 (I think) world. Note: S750 is a daft outcome in B1, B2 and A1 world's since 750ppmv is never reached anyway. I will need to investigate just what we can get and when but Arnulf has shown willing. Solution 3: wait to see what IPCC decide. There is a WGIII meeting the first week of June in Copenhagen when mitigation/stabilisation scenarios is the focal point. There is some pressure for WGIII to come up with something on this (SRES will not do it), something that thinks through the issue of just what stabilisation scenarios mean for gas species (non-CO2, see point 1 above) and socio-economic indicators (e.g. effect of different climate policies on economics). Needless to say this is sensitive material and it is not clear how WGIII will handle it. So this is the situation as seen by me right now. I guess Solution 1 would not pass decent Nature reviewers and Solution 3 may never materialise. If people want Solution 2 then I can ask Grubler for what he can give us. Mike