The appropriate timing of mitigation pathways depends upon many factors including the economic characteristics of different pathways, the uncertainties about the ultimate objective, and the risks and damages implied by different rates and levels of atmospheric change. This section focuses upon the mitigation costs of different pathways towards a predetermined concentration ceiling. No policy conclusion should be derived from it before reading Chapter 10, which discusses mitigation timing in the wider context of uncertainties, risks and impacts.

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

Figure 8.13: Costs of stabilizing concentrations at 550ppmv; discounted to 1990 at 5%.

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.

Figure 8.15: Relationship between present discounted costs for stabilizing the concentrations of CO₂ in the atmosphere at alternative levels.