Sunday, June 10, 2012


cc:,, Rob Swart <>, steve smith <>,, Tsuneyuki MORITA <>,
date: Thu, 30 Aug 2001 09:43:07 +0200
from: Rob Swart <>
subject: Re: TGCIA scenario recommendations

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

� 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
� 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
� 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.



No comments:

Post a Comment