Co-ordinating Lead Authors: TSUNEYUKI MORITA (JAPAN),

This chapter reviews three scenario literatures: general mitiga-tion scenarios produced since the Second Assessment Report (SAR), narrative-based scenarios found in the general futures literature, and mitigation scenarios based on the new reference scenarios developed in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES). Scenarios A long-term view of a multiplicity of future possibilities is required to consider the ultimate risks of climate change, assess critical interactions with other aspects of human and environmental systems and guide policy responses. Scenarios offer a structured means of organizing information and gleaning insight into the possibilities. Each mitigation scenario describes a future world with particular economic, social, and environmental characteristics, and therefore implicitly or explicitly contains information about development, equity, and sustainability (DES). Since the difference between reference case scenarios and their corresponding mitigation scenarios is simply the addition of deliberate climate policy, it can be the case that the differences in emissions among reference case scenarios are greater than between any one such scenario and its mitigation version. This chapter considers the results of 519 quantitative emission scenarios from 188 sources, mainly produced after 1990. The review focuses on 126 mitigation scenarios that cover global emissions and have a time horizon encompassing the coming century. These mitigation scenarios include concentration stabilization scenarios, emission stabilization scenarios, tolerable win-dows/safe emission corridor scenarios, and other mitigation scenarios. They all include energy-related carbon dioxide (CO 2) emissions; several also include CO 2 emissions from land-use changes and industrial processes and other important greenhouse gases (GHGs). Mitigation options used in the reviewed mitigation scenarios take into account energy systems, industrial processes, and land use, and depend on the underlying model structure. Most of the scenarios introduce simple carbon taxes or constraints on emissions or concentration levels to reflect measures that are taken to implement such options. Regional targets are introduced in the models with regional disaggregation. Emission trading is introduced in more recent work. Some models employ supply-side technology introduction, while others emphasize efficient demand-side technology options. Allocation of emission reduction among regions is a contentious issue. Only some studies, particularly recent ones, make explicit assumptions about such allocations in their scenarios. Some studies offer global emission trading as a mechanism to reduce mitigation costs. Technological improvement is a critical element in all the general mitigation scenarios. Detailed analysis of the characteristics of 31 scenarios for stabilization at 550ppmv (and their respective baseline …