A given concentration ceiling can be achieved through a variety
of emission pathways. This is illustrated in
Figure 8.12.
The top panel shows alternative concentration profiles for stabilization at
350-750ppmv. The bottom panel shows the corresponding emission trajectories.
In each case, two different routes to stabilization are shown: the IPCC Working
Group I profiles (from IPCC, 1995) and Wigley, Richels and Edmonds (WRE) profiles
(from Wigley
et al., 1996). The choice of emission pathways can be
thought of as a carbon budget allocation problem. To a first approximation,
a concentration target defines an allowable amount of carbon to be emitted
into the atmosphere between now and some date in the future. The issue is
how best to allocate this budget over time. A number of modellers have attempted
to address this issue. Unfortunately, to model stabilization costs is a daunting
task. It is difficult enough to forecast the evolution of the energy and economic
system to 2010. Projections over a century or more are necessary, but must
be treated with considerable caution. They provide useful information, but
their value lies not in the specific numbers but in the insights.
This section examines how mitigation costs might vary both with the stabilization
level and with the pathway to stabilization. Also discussed are key assumptions
that influence mitigation cost projections. Important, this discussion begins
with the assumption that the stabilization ceiling is known with certainty and
neglects the costs of different damages associated with different pathways (discussed
in Chapter 10). Here, the challenge is to identify
the least-cost mitigation pathway to stay within the prescribed ceiling. In
Chapter 10, the issue of decision-making under uncertainty
is discussed regarding the ultimate target and impacts of different pathways.
Decision making under uncertainty requires indeed examining symmetrically the
costs of accelerating the abatement in case of negative surprises about damages
of climate change and adopting a prudent near-term hedging strategy. That is,
one that balances the risks of acting too slowly to reduce emissions with the
risks of acting too aggressively.
8.4.2 Studies of the Costs of Alternative Pathways for Stabilizing
Concentrations at a Given Level
Some insight into the characteristics of the least-cost
mitigation pathway can be obtained from two EMF studies (EMF-14, 1997; EMF-16,
1999) and from Chapter 2 in the SRES mitigation scenarios
(IPCC, 2000). In the first EMF study, modellers compared mitigation costs associated
with stabilizing concentrations at 550ppmv using the WGI and WRE profiles (see
Figure 8.12), Note that the WGI pathway entails lower
emissions in the early years, with less rapid reductions later on. The WRE pathway
allows for a more gradual near-term transition away from carbon-venting fuels.
Figure 8.13 shows that in these models the more gradual
near-term transition of the two examined results in lower mitigation costs.
The above experiment compares mitigation costs for two emission pathways for
stabilizing concentrations at 550ppmv. It does not identify the least-cost mitigation
pathway, however. This was done in the subsequent EMF (1997) study. The results
are presented in Figure 8.14. In these studies the least-cost
mitigation pathway tends to follow the models reference case in the early years
with sharper reductions later on.
The selection of a 550ppmv target was purely arbitrary and not meant to imply
an optimal concentrations target. Given the present lack of consensus on what
constitutes “dangerous” interference with the climate system, three
models in the EMF-16 study examined how mitigation costs are projected to vary
under alternative targets. The results are summarized in Figure
8.15. As would be expected, mitigation costs increase with more stringent
stabilization targets.
In Chapter 2, nine modelling groups reported scenario
scenario results using different baseline scenarios. An analysis focused on
the results of stabilizing the SRES A1B scenario at 550 and 450ppmv provides
additional insight into the relationship between mitigation and baseline emissions.
For the 550ppmv case, there are eight relevant trajectories (see Figure
8.16) giving the carbon reductions necessary to achieve a stabilization
level of 550ppmv, where the models which impose a long-term cost minimization
(LTCM) are represented as solid lines, and the models which use an external
trajectory as the basis for their mitigation strategy are presented as dashed
lines. The first impression of Figure 8.16 is
that even given common assumptions about GDP, population, and final energy use,
and a common stabilization goal, there is still a lot of difference in the model
results. A preliminary examination suggests that, in contrast to the non-optimization
model results, a common characteristic among the LTCM models is that the near-term
emissions pathways departs only gradually from the baseline.
Figure 8.17 clarifies the results by converting
the absolute reduction to a percent reduction basis and averages them for the
two classes of models. LTCM models show clearly a more gradual departure from
the emissions baseline. Figure 8.17 also gives comparable results for the four
cases with a 450ppmv target. The LTCM show a very similar decoupling until 2030,
when this decoupling increases rapidly, and exceeds the other models by 2050,
earlier in the 450ppmv case than in the 550ppmv case.
Figure 8.14: Rate of departure from the baseline corresponding
to least-cost mitigation pathway for a 550ppmv stabilization target.
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Figure 8.15: Relationship between present discounted costs for stabilizing
the concentrations of CO2 in the atmosphere at alternative
levels.
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