Working Group I: The Scientific Basis

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In 1996, the IPCC began the development of a new set of emissions scenarios, effectively to update and replace the well-known IS92 scenarios. The approved new set of scenarios is described in the IPCC Special Report on Emission Scenarios (SRES) (Nakic´enovic´ et al., 2000; see more complete discussion of SRES scenarios and forcing in Chapters 3, 4, 5 and 6). Four different narrative storylines were developed to describe consistently the relationships between emission driving forces and their evolution and to add context for the scenario quantification (see Box 9.1). The resulting set of forty scenarios (thirty-five of which contain data on the full range of gases required for climate modelling) cover a wide range of the main demographic, economic and technological driving forces of future greenhouse gas and sulphur emissions. Each scenario represents a specific quantification of one of the four storylines. All the scenarios based on the same storyline constitute a scenario “family”. (See Box 9.1, which briefly describes the main characteristics of the four SRES storylines and scenario families.) The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included that explicitly assume implementation of the UNFCCC or the emissions targets of the Kyoto Protocol. However, greenhouse gas emissions are directly affected by non-climate change policies designed for a wide range of other purposes. Furthermore, government policies can, to varying degrees, influence the greenhouse gas emission drivers and this influence is broadly reflected in the storylines and resulting scenarios.

Box 9.1: The Emissions Scenarios of the Special Report on Emissions Scenarios (SRES)

A1. The A1 storyline and scenario family describe 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 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 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 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 in 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, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented towards environmental protection and social equity, it focuses on local and regional levels.

Because SRES was not approved until 15 March 2000, it was too late for the modelling community to incorporate the scenarios into their models and have the results available in time for this Third Assessment Report. Therefore, in accordance with a decision of the IPCC Bureau in 1998 to release draft scenarios to climate modellers (for their input to the Third Assessment Report) one marker scenario was chosen from each of four of the scenario groups based on the storylines (A1B, A2, B1 and B2) (Box 9.1). The choice of the markers was based on which initial quantification best reflected the storyline, and features of specific models. Marker scenarios are no more or less likely than any other scenarios but these scenarios have received the closest scrutiny. Scenarios were also selected later to illustrate the other two scenario groups (A1FI and A1T), hence there is an illustrative scenario for each of the six scenario groups. These latter two illustrative scenarios were not selected in time for AOGCM models to utilise them in this report. In fact, time and computer resource limitations dictated that most modelling groups could run only A2 and B2, and results from those integrations are evaluated in this chapter. However, results for all six illustrative scenarios are shown here using a simple climate model discussed below. The IS92a scenario is also used in a number of the results presented in this chapter in order to provide direct comparison with the results in the SAR.

The final four marker scenarios contained in SRES differ in minor ways from the draft scenarios used for the AOGCM experiments described in this report. In order to ascertain the likely effect of differences in the draft and final SRES scenarios each of the four draft and final marker scenarios were studied using a simple climate model tuned to the AOGCMs used in this report. For three of the four marker scenarios (A1B, A2 and B2) temperature change from the draft and final scenarios are very similar. The primary difference is a change to the standardised values for 1990 to 2000, which is common to all these scenarios. This results in a higher forcing early in the period. There are further small differences in net forcing, but these decrease until, by 2100, differences in temperature change in the two versions of these scenarios are in the range 1 to 2%. For the B1 scenario, however, temperature change is significantly lower in the final version, leading to a difference in the temperature change in 2100 of almost 20%, as a result of generally lower emissions across the full range of greenhouse gases. For descriptions of the simulations, see Section 9.3.1.

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