Scenarios Based on Outputs from Climate Models
The Emissions Scenarios of the Special Report on Emissions Scenarios (SRES)
The IPCC published a Special Report on Emissions Scenarios (SRES) in 2000, describing the new set of emissions scenarios used in the Third Assessment Report. The SRES scenarios have been constructed to explore future developments in the global environment with special reference to the production of greenhouse gases and aerosol precursor emissions. All are 'non-mitigation' scenarios with respect to climate change.
They use the following terminology:
- Storyline: a narrative description of a scenario (or a family of scenarios), highlighting the main scenario characteristics and dynamics, and relationships among key driving forces.
- Scenario: projections of a potential future, based on a clear logic and a quantified storyline.
- Scenario family: one or more scenarios that have the same demographic, politico-societal, economic and technological storyline.
The approach was to develop a set of four "scenario families". The storyline of each scenario family describes one possible demographic, politico-economic, societal and technological future. Within each family, one or more scenarios explore the global energy industry and other developments and their implications for greenhouse gas emissions and other pollutants. The scenarios have been built to explore two main questions for the twenty first century, neither of which we know the answer to:
- Can adequate governance - institutions and agreements - be put in place to manage global problems?
- Will society's values focus more on enhancing material wealth or be more
broadly balanced, incorporating environmental health and social well-being?
The way these questions are answered leads to four families of scenarios. Within these scenario families, plausible energy industry and other developments that will contribute to greenhouse gas emissions are examined. Although the storylines do not contain explicit climate change policy measures, some scenarios have examples of indirect mitigation measures. The scenario quantifications of the main indicators related to growth of population and economy, the characteristics of the energy system and the associated greenhouse gas emissions all fall within the range of prior studies.
Figure 3. A Qualitative Description of the SRES Scenarios
A1. The A1 storyline and scenario family describe a future world of very rapid economic growth, global population that peaks in the mid-21st century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family has three groups that describe alternative directions of technological change in the energy system:
- fossil intensive (A1FI),
- non-fossil energy sources (A1T), or
- a balance across all sources (A1B) (where balanced 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).
A2. The A2 storyline and scenario family describe 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 other storylines.
B1. The B1 storyline and scenario family describe a convergent world with the same global population, that peaks in mid-century 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 describe 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 A2, and with intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is oriented toward environmental protection and social equity, it focuses on local and regional levels.
SRES chose an illustrative scenario for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All are considered equally sound.
Some subsequent studies, recognizing that A1F1 would assume unlimited fossil fuel supplies, and that A1T would seem unlikely within the next 50 years, use the A2 and B2 storyline scenarios as the outer limits of the envelope.
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 emissions targets of the Kyoto Protocol.
The figures below show the future projections with respect to global temperatures, precipitation and sea level rise. The first table and box help to explain how each scenario was developed. For more interpretation and discussion, the reader is referred to the IPCC report. Table 2 indicates the estimates of confidence of the projections.
Table 1
Note: Some of the 40 scenarios were not judged ready, so only 35 were used in the final modeling process.