Are the IPCC emissions scenarios equally plausible?
55. The IPCC takes the view that its emissions scenarios "reflect
a wide range of future possibilities that characterize our current
understanding of the uncertainties of the drivers of future emissions
patterns". They say that "The SRES was designed to provide
insights on uncertainty from a range of plausible scenarios, and
not to assign likelihood to any of the alternative futures described
by the set of 40 scenarios".
This is indeed the standard procedure in scenario building, as
it is practised in the world of business. But, while this may
have been the purpose of the scenario exercise, the reasonableness
of constraining the exercise in this way must be brought into
question. Whatever the intent of the IPCC, the public perception
of the scenario exercise is often that each scenario is equally
plausible: by not assigning levels of significancequantitative
or qualitativeto the scenarios, the impression given is
that each has the same probability of occurrence. One of the salient
features of the Henderson-Castles critique was that the high-emission
scenarios rest on assumptions that are not credible. We were therefore
concerned to hear from Dr Nakicenovic that IPCC had no intention
of undertaking any significant reappraisal of the SRES for the
IPCC Fourth Assessment exercise (AR4) for 2007.
It seems to us that there is an urgent need for a wholesale reappraisal
of the emissions scenario exercise.
56. One of our witnesses, Professor Richard Tol of Hamburg
University, has assessed the relative likelihood of each of the
IPCC scenarios. One
important feature of Professor Tol's work is that he has sought
to validate the scenarios using long term historical data, something
that the SRES did not do, and an issue raised in the original
Henderson-Castles critique. On the underlying assumptions, Professor
Tol finds: (a) the population projections are credible, although
the A1 and B1 scenarios unaccountably have the same populations;
(b) the per capita income growth for developed economies is credible;
(c) the per capita income growth for the developing countries
diverges from long-term (though not more recent) historical precedent,
and for Africa there is a clear break with the past; (d) the assumption
of convergence of per capita incomes is not consistent with much
of the longer term past record; (e) projections of energy intensity
are only partially confirmed by history. Professor Tol concludes
on scenario assumptions that:
"The [previous observations] suggest that the SRES modellers
know a lot about the supply side of the energy system, but less
about the demand for energy. Their knowledge of economic development
57. As to which scenarios are more likely, Professor Tol argues
that the A2 scenario "is by far the most realistic"
"The SRES scenarios do not accord with past trends. On the
one hand, this makes for interesting scenarios. On the other hand,
it is odd that all SRES scenarios break with past trends at the
same time, and that this trend break is sometimes at the point
where data end and scenarios start".
58. The A2 scenario is, however, one of the scenarios with
high cumulative CO2 emissionssee Box 4. The
high emissions result from the population projection of 15 billion
people in 2100, a projection not borne out by any of the population
forecasts made elsewhere.
59. We find Professor Tol's analysis telling. He suggests
that many of the likely errors in the scenarios cancel out, and
he suggests that the scenarios do result in emissions that are
within the range of "not implausible" futures. But the
shortcomings in the scenarios identified by Professor Tol do further
underline our call for their thorough reassessment.
60. In short, serious questions have been raised about
the IPCC emissions scenarios, andas we have already
noteda reappraisal of the scenarios exercise is
Are the economic growth assumptions credible?
61. Table 1 indicates that world GDP is expected to grow at
2.2 to 3.0% p.a. in the IPCC scenarios. In his evidence to us,
Professor Angus Maddison, a leading expert on the historical record
of the world economy, produced estimates of expected growth in
the world economy up to 2030 which are consistent with a 3% growth
rate. and with the historical record from 1900 to 1990.
However, we were interested to hear from Paul Johnson of HM Treasury
that he found the high economic growth scenarios "relatively
unlikely" and that "the 3% a year growth for 100 years
is certainly extremely unprecedented".
Table 1 shows data for historical growth rates taken from the
work of Professor Maddison.
Past economic growth rates for world and