Scenarios are unfolding sequences of interrelated events stemming from prior and ongoing decisions. With regard to climate change and carbon emissions, the outputs of climate and carbon cycle models under different scenarios of population growth, economic development, and fossil and other fuel use have proved to be useful in exploring possible policy options for mitigation of climate change due to greenhouse gas emissions. Owing to the contingent nature of historical evolution, economic and technological development may in fact be unknowable in advance. Nevertheless, the scenario approach is useful because it allows asking "what if?" questions. A variety of scenarios have been published by several research groups. There is, of course, an infinite variety of scenarios that will lead, say, to keeping the future level of CO2 below a certain level. One of the earliest sets is that presented in the first of the scientific reports from the Intergovernmental Panel on Climate Change (IPCC). Subsequent scenarios have been suggested by Wigley et al. (1996) and by Nakicenovic et al. (1998).

2.1.1 The Intergovernmental Panel on Climate Change (IPCC) scenarios

The authors of the IPCC report point out that scenarios are not predictions of the future, but merely illustrate the effects of a range of economic, demographic, and policy assumptions. In Climate Change 1992 the IPCC examined six scenarios, including the one called IS92a, which has become known as the Business as Usual scenario (BAU). This scenario assumes a world population growing to 11.3 billions by 2100, economic growth of 2.9% per year until 2025 and 2.3% thereafter, and a set of assumptions about the availability and cost of various fuels. Other scenarios are either more or less optimistic. The reader should refer to the IPCC report for details (Houghton et al., 1992). The IPCC scenarios were constructed under the following constraints: (1) prescribed initial concentrations of CO2 and the rate of change of concentration set at 1990 values, (2) a range of prescribed stabilization levels and attainment dates, and (3) a requirement that the implied emissions should not change too abruptly. An inverse calculation using a carbon cycle model was then used to determine emission rates required to attain stabilization via the set pathways. These are shown in Figure 1.8 in Chapter 1. We return to the BAU scenario later.

2.1.2 The Wigley, Richels, Edmonds (WRE) scenarios

Wigley et al. (1996) added an initial constraint. They required that the initial emissions trajectories initially track the BAU scenario but that the higher the concentration target for stabilizing CO2, the longer the adherence to BAU. Concentration targets can be met when departures from BAU are delayed by 10 to 30 years. Of course, the atmospheric loading of CO2 increases more rapidly initially than in the IPCC scenarios, but stabilization is ultimately achieved at a prescribed level in either the IPCC scenarios or the WRE scenarios. Cumulative emissions are notably higher when emissions follow the BAU scenario longer. Reducing emissions later allows greater total CO2 production, and therefore total use of fossil fuels. Important economic considerations are implied by this conclusion. Several studies (Nordhaus, 1979; Manne and Richels, 1995; Richels and Edmonds, 1994) have shown that emission reduction pathways that are modest in the early stages and sharper later on are less expensive than those requiring early sharp reductions. This is because (1) future environmental impacts have low present values in economic analyses, (2) capital stock for energy is long-lived and one pays a penalty for retiring it prematurely, and (3) future energy technologies will presumably be more efficient and less costly than present ones. The carbon emissions pathways from WRE are shown in Figure 1.8 in Chapter 1, along with the IPCC pathways for comparison.

Wigley et al. (1996) are careful to point out that their results do not suggest a "wait and see" policy. "No regrets" options should be adopted immediately where appropriate. Since capital stocks do have a long lifetime and since new supply options typically take many years to enter the marketplace a sustained effort in research, development and deployment should begin now.

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