Introduction: Summary of the Second Assessment Report and progress since this report.

This chapter reviews three scenario literatures: general mitigation 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.

General Greenhouse Gas Emissions Mitigation Scenarios

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 windows/safe emission corridor scenarios, and other mitigation scenarios. They all include energy-related carbon dioxide (CO₂) emissions; several also include CO₂ 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 scenarios) yielded several insights¹.

There was a wide range in baselines, reflecting a diversity of assumptions, mainly with respect to economic growth and low-carbon energy supply. High economic growth scenarios tend to assume high levels of progress in the efficiency of end-use technologies; carbon intensity reductions were found to be largely independent of economic growth assumptions. The range of future trends shows greater divergence in scenarios that focus on developing countries than in scenarios that look at developed nations. There is little consensus with respect to future directions in developing regions.

The reviewed 550ppmv stabilization scenarios vary with respect to reduction time paths and the distribution of emission reductions among regions. Some scenarios show that emission trading lowers overall mitigation cost by shifting mitigation to non-OECD countries, where abatement costs are assumed to be lower. The range of assumed mitigation policies is very wide. In general, scenarios in which there is an assumed adoption of high-efficiency measures in the baseline show less scope for further introduction of efficiency measures in the mitigation scenarios. In part this is due to the structure of the models, which do not assume major technological breakthroughs. Conversely, baseline scenarios with high carbon intensity reductions show larger carbon intensity reductions in their corresponding mitigation scenarios. Global macroeconomic costs of mitigation in the reviewed scenarios range from 0% to 3.5% of gross domestic product (GDP), while a few simple models estimate more increase in the second half of the 21^st century. No clear relationship was discovered between the GDP loss and the GDP growth assumptions in the baselines.

Only a small set of studies has reported on scenarios for mitigating non-CO₂ gases. This literature suggests that small reductions of GHG emissions can be accomplished at lower cost by including non-CO₂ gases; that both CO₂ and non-CO₂ emissions would have to be controlled in order to reduce emissions sufficiently to meet assumed mitigation targets; and that methane (CH₄) mitigation can be carried out more rapidly, with a more immediate impact on the atmosphere, than CO₂ mitigation.

In most cases it is clear that mitigation scenarios and mitigation policies are strongly related to their baseline scenarios, but no systematic analysis in this class of literature has been published on the relationship between mitigation and baseline scenarios.

Global Futures Scenarios

Global futures scenarios do not specifically or uniquely consider GHG emissions. Instead, they are more general “stories” of possible future worlds. They can complement the more quantitative emission scenario assessment because they consider dimensions that elude quantification, such as governance and social structures and institutions, but which are nonetheless important to the success of mitigation policies. Addressing these issues reflects the different perspectives presented in Chapter 1 on cost-effectiveness, equity, and sustainability.

A survey of this literature has yielded a number of insights. First, a wide range of future conditions has been identified by futurists, ranging from variants of sustainable development to collapse of social, economic, and environmental systems. Since the underlying socio-economic drivers of emissions may vary widely in the future, it is important that climate policies should be designed so that they are resilient against widely different future conditions.

Second, the global futures scenarios that show falling GHG emissions tend to show improved governance, increased equity and political participation, reduced conflict, and improved environmental quality. They also tend to show increased energy efficiency, shifts to non-fossil energy sources, and/or shifts to a post-industrial economy. Furthermore, population tends to stabilize at relatively low levels, in many cases as a result of increased prosperity, expanded provision of family planning, and improved rights and opportunities for women. A key implication is that sustainable development policies can make a significant contribution to emission reduction.

Third, different combinations of driving forces are consistent with low emission scenarios. The implication of this would seem to be that it is important to consider the linkage between climate policy and other policies and conditions associated with the choice of future paths in a general sense.

Special Report on Emission Scenarios

Six new GHG emission reference scenario groups (not including specific climate policy initiatives), organised into 4 scenario “families”, were developed by the IPCC and published as the Special Report on Emission Scenarios (SRES). Scenario families A1 and A2 emphasize economic development but differ with respect to the degree of economic and social convergence; B1 and B2 emphasize sustainable development but also differ in terms of degree of convergence. In all, six models were used to generate 40 scenarios that comprise the six scenario groups. In each group of scenarios, which should be considered equally sound, one illustrative case was chosen to illustrate the whole set of scenarios. These six scenarios include marker scenarios for each of the scenario families as well as two scenarios, A1FI and A1T, which illustrate alternative energy technology developments in the A1 world.

The SRES scenarios lead to the following findings:

• Alternative combinations of driving-force variables can lead to similar levels and structure of energy use, land-use patterns and emissions.
  
• Important possibilities for further bifurcations in future development trends exist within each scenario family.
  
• Emissions profiles are dynamic across the range of SRES scenarios. They portray trend reversals and indicate possible emissions cross-over among different scenarios.
  
• Describing potential future developments involves inherent ambiguities and uncertainties. One and only one possible development path (as alluded to for instance in concepts such as “business-as-usual scenario”) simply does not exist. The multi-model approach increases the value of any scenario set, since uncertainties in the choice of model input assumptions can be more explicitly separated from the specific model behaviour and related modelling uncertainties.

Review of Post-SRES Mitigation Scenarios

Recognizing the importance of multiple baselines in evaluating mitigation strategies, recent studies analyze and compare mitigation scenarios using as their baselines the new SRES scenarios. This allows for the assessment in this report of 76 “Post-SRES Mitigation Scenarios” produced by nine modelling teams.

These mitigation scenarios were quantified on the basis of storylines for each of the six SRES scenarios which describe the relationship between the kind of future world and its capacity for mitigation.

Quantifications differ with respect to the baseline scenario including assumed storyline, the stabilization target, and the model that was used. The post-SRES scenarios cover a very wide range of emission trajectories but the range is clearly below the SRES range. All scenarios show an increase in CO₂ reduction over time. Energy reduction shows a much wider range than CO₂ reduction, because in many scenarios a decoupling between energy use and carbon emissions takes place as a result of a shift in primary energy sources.

In general, the lower the stabilization target and the higher the level of baseline emissions, the larger the CO₂ divergence from the baseline that is needed, and the earlier that it must occur. The A1FI, A1B, and A2 worlds require a wider range and more strongly implemented technology and/or policy measures than A1T, B1, and B2. The 450 ppmv stabilization case requires very rapid emission reduction over the next 20 to 30 years.

A key policy question is what kind of emission reductions in the medium term (after the Kyoto protocol commitment period) would be needed. Analysis of the post-SRES scenarios (most of which assume developing country emissions to be below baselines by 2020) suggests that stabilization at 450ppmv will require emissions reductions in Annex I countries after 2012 that go significantly beyond their Kyoto Protocol commitments. It also suggests that it would not be necessary to go much beyond the Kyoto commitments for Annex I countries by 2020 to achieve stabilization at 550ppmv or higher. However, it should be recognized that several scenarios indicate the need for significant Annex I emission reductions by 2020 and that none of the scenarios introduces other constraints such as a limit to the rate of temperature change.

An important policy question already mentioned concerns the participation of developing countries in emission mitigation. A preliminary finding of the post-SRES scenario analysis is that, if it is assumed that the CO₂ emission reduction needed for stabilization would occur in Annex I countries only, Annex I per capita CO₂ emissions would fall below non-Annex I per capita emissions during the 21^st century in nearly all of the stabilization scenarios, and before 2050 in two-thirds of the scenarios. This suggests that the stabilization target and the baseline emission level are both important determinants of the timing when developing countries’ emissions might need to diverge from their baseline.

Climate policy would reduce per capita final energy consumption in the economy-emphasized worlds (A1FI, A1B, and A2), but not in the environment-emphasized worlds (B1 and B2). The reduction in energy use caused by climate policies would be larger in Annex I than in non-Annex I. However, the impact of climate policies on equity in per capita final energy use would be much smaller than that of the future development path.

No single measure will be sufficient for the timely development, adoption, and diffusion of mitigation options to stabilize atmospheric GHGs. Instead, a portfolio based on technological change, economic incentives, and institutional frameworks could be adopted. Combined use of a broad array of known technological options has a long-term potential which, in combination with associated socio-economic and institutional changes, is sufficient to achieve stabilization of atmospheric CO₂ concentrations in the range of 450–550ppmv or below.

Assumed mitigation options differ among scenarios and are strongly dependent on the model structure. However, common features of mitigation scenarios include large and continuous energy efficiency improvements and afforestation as well as low-carbon energy, especially biomass, over the next one hundred years and natural gas in the first half of the 21^st century. Energy conservation and reforestation are reasonable first steps, but innovative supply-side technologies will eventually be required. Possible robust options include using natural gas and combined-cycle technology to bridge the transition to more advanced fossil fuel and zero-carbon technologies, such as hydrogen fuel cells. Solar energy along with either nuclear energy or carbon removal and storage would become increasingly important for a higher emission world or lower stabilization target.

Integration between global climate policies and domestic air pollution abatement policies could effectively reduce GHG emissions in developing regions for the next two or three decades; however, control of sulphur emissions could amplify possible climate change, and partial trade-offs are likely to persist for environmental policies in the medium term.

Policies governing agriculture and land use and energy systems need to be linked for climate change mitigation. Supply of biomass energy as well as biological CO₂ sequestration would broaden the available options for carbon emission reductions, although the post-SRES scenarios show that they cannot provide the bulk of the emission reductions required. That has to come from other options.

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