The 1994 IPCC evaluation of the usefulness of the IPCC 1992 (IS92) scenarios found that for the purposes of driving atmospheric and climate models, the CO₂ emissions trajectories in these provided a reasonable reflection of variations found in the open literature. Specifically, their global CO₂ emissions spanned most of the range of other scenarios identified in the literature. Figure 1-2 shows the global, energy-related and industrial CO₂ emissions of the IS92 scenarios ranging from very high emissions of 35.8 GtC to very low emissions of 4.6 GtC by 2100 (corresponding to a sixfold increase and a decline by a third compared to 1990 levels, respectively). The shaded area in Figure 1-2 indicates the coverage of the IS92 scenarios while the "spaghetti-like" curves indicate other energy-related emissions scenarios found by the IPCC review to be representative of the scenarios available in the open literature at that time (Alcamo et al., 1995). In the open literature, emissions trajectories for other gases were extremely thin in many instances, but the IS92 cases were not dissimilar to these.

Another important recommendation of the 1994 IPCC review was that, given the degree of uncertainty about future climate change, analysts should use the full range of IS92 emissions as input to climate models rather than a single scenario. This is in stark contrast to the actual use of one particular scenario in the set, the IS92a scenario, as the reference case in numerous studies. In fact, the IS92a scenario is often referred to in climate change modeling and impact studies as the "business-as- usual" scenario and used as the only reference emissions trajectory. The review concluded that the mere fact of the IS92a being an intermediate, or central, CO₂ emissions scenario at the global level does not equate it with being the most likely scenario. Indeed, the conclusion was that there was no objective basis on which to assign likelihood to any of the scenarios. Furthermore, the IS92a scenario was shown to be "central" for only a few of its salient characteristics such as global population growth, global economic development and global CO₂ emissions. In other ways, IS92a was found not to be central with respect to the published literature, particularly in some of its regional input assumptions. The same is the case with the new set of SRES scenarios, as is shown below. No single scenario can be central with respect to all the characteristics relevant for different uses of emissions scenarios and there is no objective way to assign likelihood to any of the scenarios.

The 1994 IPCC evaluation of the IS92 scenarios found that the scenarios were innovative at the time of their publication and path-breaking in their coverage of the full range of GHG and SO₂ emissions, on both a global and a regional basis. The review also identified a number of weaknesses. These included the limited range of CO₂ intensities of energy (CO₂ emissions per unit energy) reflected in the six scenarios and the absence of any scenario with significant closure in the income gap between developed and developing countries, even after a full century (Parikh, 1992). Across all six scenarios the per capita income in the developing countries grows only to a share of 17% to 26% of that of the industrial countries, compared to 6% today.

Furthermore, since the development of the IS92 scenarios much has changed in our understanding of both the possible future GHG emissions and the climate impacts that might result. The most straightforward change arises because the IS92 scenarios had to estimate data for the base year 1990 (actual data were unavailable at the time); 1990 is no longer a forecast year and actual data can be used. But other changes are at least as important. It is now recognized that in some regions SO₂ emissions may be as important to climate change as all non-CO₂ greenhouse related gases combined - at least in the near-term. As a result the rapid growth of SO₂ emissions in the IS92 scenarios has been questioned. The 1994 evaluation of the IS92 scenario series (Alcamo et al., 1995) concluded, for instance, that projected emissions in the Organization for Economic Cooperation and Development (OECD) countries did not reflect recent legislative changes such as the Amendments of the Clean Air Act in the US or the Second European Sulfur Protocol. Thus, factors other than climate change, namely regional and local air quality, may well prompt limits on future SO₂ emissions independent of global warming concerns. Restructuring in Eastern and Central Europe and the Newly Independent States of the former Soviet Union has powerfully affected economic activity, with reductions in CO₂ emissions unforeseen in the IS92 scenarios. The advent of integrated assessment (IA) models has also made it possible now to construct internally consistent emissions scenarios that jointly consider the interactions among energy use, the economy, and land-use change. Some IA models account for interactions in both directions between driving forces of GHG and SO₂ emissions and possible impacts of climate change. Progress has also been made in achieving greater consistency among scenario characteristics such as the rates of technological change in different sectors.

Owing to these advances, the 1994 IPCC review of the IS92 scenarios concluded that new scenarios, if developed, should include these improvements:

• estimation of emissions baselines and future non-CO₂ emissions, particularly from land use;
• incorporation of the latest information on economic restructuring throughout the world;
• expand the range of economic-development pathways, including a narrowing of the income gap between developing and industrialized countries;
• examine different trends in and rates of technological change;
• evaluate possible consequences of trade and market liberalization and privatization; and
• reflect current emission commitments in connection with the United Nations Framework Convention on Climate Change (UNFCCC, 1997).

However, the Terms of Reference for SRES explicitly preclude additional initiatives, measures and policies specifically designed to reduce climate change (see Appendix I).

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