date: Mon, 19 Apr 1999 21:08:18 +0100
from: "Mike Hulme" <m.hulmeatXYZxyz.ac.uk>
subject: Fast-track scenarios
There have been some requests re. CO2 and associated scenario data for the
S550 and S750 runs. For now, let me suggest the following:
1. S550 CO2 concentrations are as listed in my Fast-track note of 21 July
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, 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). 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.
2. S750 CO2 concentrations will be circulated as soon as Hadley and I can
agree them (i.e., tomorrow I hope). These again will be CO2-only
concentrations and will say nothing about other gas species.
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.