Emission scenarios

A principal reason for the development of climate models is to learn about the detail of the likely climate change this century and beyond. Because model simulations into the future depend on assumptions regarding future anthropogenic emissions of greenhouse gases, which in turn depend on assumptions about many factors involving human behaviour, it has been thought inappropriate and possibly misleading to call the simulations of future climate so far ahead 'predictions'. They are therefore generally called 'projections' to emphasise that what is being done is to explore likely future climates which arise from a range of assumptions regarding human activities.

A starting point for any projections of climate change into the future is a set of descriptions of likely future global emissions of greenhouse gases. These will depend on a variety of assumptions regarding human behaviour and activities, including population, economic growth, energy use and the sources of energy generation. As was mentioned in Chapter 3, such descriptions of future emissions are called scenarios. A wide range of scenarios was developed by the IPCC in a Special Report on Emission Scenarios (SRES)1 in preparation for its 2001 Report (see box below). It is these scenarios that have been used in developing the projections of future climate presented in this chapter. In addition, because it has been widely used in modelling studies, results are also presented using a scenario (IS 92a) taken from a set developed by the IPCC in 1992 and widely described as representative of 'business-as-usual'.2 Details of these scenarios are presented in Figure 6.1.

The storylines on which the SRES scenarios are based incorporate a wide range of different assumptions regarding population, economic growth, technological innovation and attitudes to social and environmental sustainability. None of them, however, takes account of deliberate action to combat climate change and reduce greenhouse gases. Scenarios including such action will be presented in Chapters 10 and 11 where the possibilities for stabilisation of carbon dioxide concentration in the atmosphere is considered.

The SRES scenarios include estimates of greenhouse gas emissions resulting from all sources including land-use change. Estimates in the different scenarios begin from the current values for land-use change including deforestation (see Table 3.1). Assumptions in different scenarios vary, from continued deforestation, although reducing as less forest remains available for clearance, to substantial afforestation leading to an increased carbon sink. The next stage in the development of projections of climate change is to turn the emission profiles of

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Figure 6.1 Anthropogenic emissions of carbon dioxide, methane, nitrous oxide and sulphur dioxide for the six illustrative SRES scenarios, A1B, AIT, A1FI, A2, B1 and B2. For comparison the IS 92a scenario is also shown.

greenhouse gases into greenhouse gas concentrations (Figure 6.2) and then into radiative forcing (Table 6.1). The methods by which these are done are described in Chapter 3, where the main sources of uncertainty are also mentioned. For the carbon dioxide concentration scenarios these uncertainties, especially those concerning the magnitude of the climate feedback from the terrestrial biosphere (see box on page 48-9), amount to a range of about -10% to +30% in 2100 for each profile.3

For most scenarios, emissions and concentrations of the main greenhouse gases increase during the twenty-first century. However, despite the increases projected in fossil fuel burning - very large increases in some cases - emissions of sulphur dioxide (Figure 6.1) and hence the concentrations of sulphate

The emission scenarios of the Special Report on Emission Scenarios (SRES)

The SRES scenarios are based on a set of four different storylines within each of which a family of scenarios has been developed - leading to a total of 35 scenarios.4

A1 storyline

The A1 storyline and scenario family describes a future world of very rapid economic growth, a 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 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 develops into three groups which describe alternative directions of technological change in the energy system. The three groups are distinguished by their technological emphasis: fossil fuel intensive (A1FI), non-fossil fuel 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.

A2 storyline

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 a continuously increasing population. Economic development is primarily regionally oriented and per capita economic growth and technological change more fragmented and slower than other storylines.

B1 storyline

The B1 storyline and scenario family describes 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 towards 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-related initiatives.

B2 storyline

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 a 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. While the storyline is also oriented towards environmental protection and social equity, it focuses on local and regional levels.

From the total set of 35 scenarios, an illustrative scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All should be considered equally sound. It is mostly for this set of six illustrative scenarios that data are presented in this chapter.

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.

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Figure 6.2 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide resulting from the six illustrative SRES scenarios and from the IS 92a scenario. Uncertainties for each profile, especially those due to possible carbon feedbacks, have been estimated as from about -10% to +30% in 2100.

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Figure 6.2 Atmospheric concentrations of carbon dioxide, methane and nitrous oxide resulting from the six illustrative SRES scenarios and from the IS 92a scenario. Uncertainties for each profile, especially those due to possible carbon feedbacks, have been estimated as from about -10% to +30% in 2100.

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particles are expected to fall substantially because of the spread of policies to abate the damaging consequences of air pollution and 'acid rain' deposition to both humans and ecosystems.5 The influence of sulphate particles in tending to reduce the warming due to increased greenhouse gases is therefore now projected to be much less than for projections made in the mid 1990s (see the IS 92a scenario for sulphur dioxide in Figure 6.1).6 In fact it is likely that sulphate particles will be reduced to well below their 1990 levels during the twenty-first century.7 The other anthropogenic sources of particles in the atmosphere included in Figure 3.11 will also contribute small amounts of positive or negative radiative forcing during the twenty-first century.8 Table 6.1 includes a 2005 estimate of future total radiative forcing from all aerosol sources.

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