Using Scenarios to Develop a Plausible Range of Outcomes

The IPCC commissioned a Special Report on Emission Scenarios (SRES)86 both to broaden assessments to include a range of outcomes and to focus analysis on a coherent set of scenario outcomes to facilitate comparison. The scenarios concentrate on assumptions about economic growth, technological developments, and population growth, arguably the three most critical variables affecting the uncertainty over future climate change and policy options. To the extent possible, the Third Assessment Report (TAR)87 has referred to the SRES to inform and guide the assessment. Box 2.2 describes the baseline SRES scenarios; Fig. 2.6 demonstrates how the SRES scenarios have been used to evaluate projected temperature changes.88 However, IPCC did not assign subjective

BOX 2.2. The Emission Scenarios of the Special Report on Emission Scenarios (SRES)

A1: The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence between regions, capacity building, and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B) (where balance is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end use technologies).

BOX 2.2. Continued

A2: The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in continuously increasing population. Economic development is primarily regionally oriented, and per capita economic growth and technological change are more fragmented and slower than in other storylines. B1: The B1 storyline and scenario family describes a convergent world with the same global population (which peaks in midcentury and declines thereafter) as in the A1 storyline but with rapid change in economic structures toward a service and information economy, with reductions in material intensity and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity but without additional climate initiatives. B2: The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with continuously increasing global population (at a rate lower than in A2), intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. Although the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

An illustrative scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1, and B2 represented in Fig. 2.6. The SRES authors consider the scenarios equally sound, which offers no guidance on which scenarios are more or less likely. A subjective probability assessment of the likelihood of the scenarios would offer policymakers a useful characterization of which scenarios may entail dangerous outcomes.

The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included that explicitly assume implementation of the United Nations Framework Convention on Climate Change or the emission targets of the Kyoto Protocol or any next generation agreements.

FIGURE 2.6. The global climate of the twenty-first century will depend on natural changes and the response of the climate system to human activities. Climate models project the response of many climate variables—such as increases in global surface temperature and sea level—to various scenarios of greenhouse gas and other human-related emissions. (a) CO2 emissions of the six illustrative SRES scenarios, summarized Box 2.2, along with IS92a for comparison purposes with the SAR. (b) Projected CO2 concentrations. (c) Anthropogenic SO2 emissions. Emissions of other gases and other aerosols were included in the temperature change model but are not shown in the figure. (d), (e)

The projected temperature and sea level responses, respectively. The "several models all SRES envelope" in (d) and (e) shows the temperature and sea level rise, respectively, for the simple model when tuned to a number of complex models with a range of climate sensitivities. The "all SRES envelopes" refer to the full range of 35 SRES scenarios. The "model average all SRES envelope" shows the average from these models for the range of scenarios. Note that the warming and sea level rise from these emissions would continue well beyond 2100. Also note that this range does not allow for uncertainty relating to ice dynamic changes in the West Antarctic ice sheet, nor does it account for uncertainties in projecting nonsulfate aerosols and greenhouse gas concentrations. (From IPCC, 2001a, Working Group I Summary for Policymakers, available online at

probabilities to the SRES scenarios or to various climate model uncertainties, making it difficult for policymakers to compare risks or evaluate tradeoffs.89

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