date: Wed, 22 Aug 2001 11:45:49 +0200
from: Rob Swart <Rob.Swart@rivm.nl>
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

