cc: m.hulme@uea.ac.uk, parryml@aol.com, Rob Swart <Rob.Swart@rivm.nl>,  steve smith <ssmith@pnl.gov>, s.raper@uea.ac.uk,  Tsuneyuki MORITA <t-morita@nies.go.jp>, tim.carter@fmi.fi
date: Thu, 30 Aug 2001 09:43:07 +0200
from: Rob Swart <Rob.Swart@rivm.nl>
subject: Re: TGCIA scenario recommendations
to: wigley@ucar.edu


Dear Tom,

   Let me take the liberty to ask you some questions and give some
   suggestions with respect to your draft paper for Climatic Change. I
   think the paper is very interesting and I still envy you because of your
   writing skills. A lot of concise information in only 12 pages. Some
   issues here, some of which may help the consideration of TGCIA
   recommendations:

     On page 2 you suggest WG2 to have come up with a threshold (2-3
   degrees average global T) above which dangerous interference becomes
   more likely. Not only does WG2 not use such political terms, I also
   fundamentally disagree. The "reasons for concern" indicate that for
   certain vulnerable systems adverse impacts are already expected at lower
   levels of temperature change, and in fact already have been observed.
   There is a general finding that for temperatures above 2-3 degrees
   adverse impacts are ubiquitous while below this level there may be some
   sectors in some countries (e.g. agriculture in northern areas) which may
   even benefit. So, choosing the 2-3 degrees is fine for the purpose of
   youre paper, but it is not a threshold determining what constitutes
   "dangerous".
     Page 5, figure 3: there seems to be a significant discrepancy of more
   than 50 ppm between your lowest 2100 CO2 concentration (below 500) and
   the value in WG1-TAR (about 550). An explanation may be useful.
     Page 7, the acronymns of the 4 cases in Figure 7 have to be derived
   with some effort from the text. It would help define them either in the
   text or in the figure legend/caption.
     Page 7: other stabilization levels. The points discussed for the
   delay of emissions controls for the 550 case are well taken and the same
   as in the WRE paper. The EMF and "post-SRES" findings that economic
   costs may increase exponentially when stabilisation targets are
   tightened below 550 ppm suggest that it would be interesting to add a
   discussion of what the effects of delay would be for a 450 case.
     Page 9, Figure 10, para 1. In some scenarios the contribution by
   non-CO2 forcing is negligible: did you analyse to what extend this is
   caused by SO2 emissions cancelling out the effect of the other non-CO2
   compounds?
     Page 9, 2nd para: you may want to add that the separation between CO2
   and non-CO2 gases is a fictitious assumption. In reality, in mitigation
   scenarios a large portion of the non-CO2 emissions are abated along with
   CO2 through fuel shifts and decreasing energy intensity.
     Page 9, 3rd para. Your rather arbitrary selection of the average
   value for non-CO2 forcing is probably appropriate for the purpose of
   your paper. However, I wonder to what extend we may want to recommend
   something similar for the TGCIA. It seems better to use the modelled
   non-CO2 emissions with the CO2 emissions from the same model runs, but
   here there is a problem because not all post-SRES models generate
   non-CO2 GHGs.
      Page 9, bottom. Reading your paper made me go back to the 2000
   Hansen et al. paper. Apart from the uncertainty about black carbon
   forcing, do you have any strong feelings about Hansen's "alternative
   scenario"?  I have so far discarded  this scenario as a scenario that
   was too primitive and implausible from the socio-economic view point
   (limiting CO2 forcing to 1 W/m2 and non-CO2 to zero), but it may make
   sense from the climate viewpoint and thus taken into account considering
   recommendations by the TGCIA.
     Page 10: I do have problems with this section in the sense that it
   supports Hansen in the sense of saying that much of the benefits of
   abating CO2 would be cancelled out by sulfur aerosols anyway, and the
   implication seems to be: so why bother. It does suggest implicitly that
   the CO2 only case is a realistic one, while it is only an unrealistic
   construct to make your point. In my view, the emphasis could be on the
   implications for detection  rather than for policy.
     Page 11, 2nd para of conclusions: does this conclusion also hold for
   450 ppm?
     Page 11, 3rd para of conclusions: again, the gap between 550 and 400
   is mainly artificial since in reality much of the non-CO2 gases would be
   abated together with CO2. See e.g. UNDP's World Energy Assessment for
   the percentages of these gases directly tied to fossil fuel
   combustion.Also: the statement that SRES did not consider policies for
   non-CO2 substances is incorrect: policies for NOx, VOC, CO were taken
   into account, albeit in less detail than SO2 (see SRES report, and the
   figures for these gases).

   I hope these comments are helpful to you. Some answers may help TCGIA.

   Regards,

   Rob
   


