Mitigation scenarios are usually defined as a description and a quantified projection of how GHG emissions can be reduced with respect to some baseline scenario. They contain new emission profiles as well as costs associated with the emission reduction. Stabilization scenarios are mitigation scenarios that aim at a pre-specified GHG reduction target. Usually the target is the concentration of CO₂ or the CO₂-equivalent concentration of a “basket” of gases by 2100 or at some later date when atmospheric stabilization is actually reached.

There are two common difficulties associated with the formulation and quantification of mitigation scenarios. First, in certain cases there is not a clear-cut distinction between intervention and non-intervention scenarios, that is, scenarios with or without explicit climate policy. This is discussed in detail in Box 2.1. The second important problem regarding mitigation scenarios has to do with the difference between top-down and bottom-up models. Whereas the latter focus on engineering trends and technology costs, the former view resource development from a macroeconomic price-mediated perspective. Although, as discussed in the SAR (IPCC, 1995), the differences between these approaches are continiously narrowing as each incorporates elements of the other, there is still quite a difference in their formulation of emission reduction strategies. This suggests the importance of including multiple methodological approaches in scenario analysis.

The climate issue is embedded in the larger question of how combined social, economic, and environmental subsystems interact and shape one another over many decades. There are multiple links. Economic development depends on maintenance of ecosystem resilience; poverty can be both a result and a cause of environmental degradation; material-intensive lifestyles conflict with environmental and equity values; and extreme socio-economic inequality within societies and between nations undermines the social cohesion required for effective policy responses.

It is clear that climate policy, and the impacts of climate change, will have significant implications for sustainable development at both the global and sub-global levels. In addition, policy and behavioural responses to sustainable development issues may affect both our ability to develop and successfully implement climate policies, and our ability to respond effectively to climate change. In this way, climate policy response will affect the ability of countries to achieve sustainable development goals, while the pursuit of those goals will in turn affect the opportunities for, and success of, climate policy responses.

In this report and its Working Group II companion report, climate change impacts, mitigation, and adaptation strategies are discussed in the broader context of DES (see Munasinghe, 1999).

The issues raised by a consideration of DES are of particular relevance to the scenarios discussed in this chapter. Because they are necessarily based upon assumptions about the socio-economic conditions that give rise to emissions profiles, mitigation and stabilization scenarios implicitly or explicitly contain information about DES. In principle, each stabilization or mitigation scenario describes a particular future world, with particular economic, social, and environmental characteristics. Given the strong interactions between development, environment, and equity as aspects of a unified socio-ecological system and the interplay between climate policies and DES policies, emissions scenarios are viewed in this report as an aspect of broad sustainable development scenarios.

The allocation of emissions in a scenario is coupled closely to an important policy question in climate negotiations: the fair distribution of future emission rights among nations, or “burden sharing”. For example, an egalitarian formulation of the rights of developing countries to future “climate space” is often expressed in terms of equal per capita emissions allocations. Alternative assumptions on burden sharing have important implications for equity, sustainable development, and the economics of emissions abatement. However, it is noteworthy that this critical conditioning variable is usually not explicitly treated in mitigation scenarios in the literature (see section 2.3). Indeed, documentation of scenarios generally does not address the implications of the scenarios for equity and burden sharing. In rare cases, mitigation scenarios have been developed which explicitly impose the simultaneous co-constraints of climate and equity goals (e.g., Raskin et al., 1998).

In this and other ways scenario analysis could become an important way of linking DES issues to climate policy considerations. However, as discussed in more detail in section 2.4, many quantitative mitigation and stabilization scenarios have not been designed with this purpose in mind. As a result, it is not always easy to draw out the DES implications of particular stabilization and mitigation scenarios.

Although this chapter focuses on mitigation and stabilization scenarios, it is important to note that DES issues are also implicit in the base case or reference scenarios that underlie mitigation and stabilization scenarios. Since the difference between reference case scenarios and stabilization and mitigation scenarios is simply the addition of deliberate climate policy, it can be the case that the DES differences among different reference case scenarios are greater than between any one such scenario and its stabilization or mitigation version. This is of particular relevance in the discussion below in section 2.5.2 of scenarios based on the baselines produced in the IPCC’s SRES (Nakicenovic et al., 2000).

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