The SRES scenarios quantified emissions of CO2, CH4, N2O, NOx, carbon monoxide, VOCs, SOx, chlorofluorocarbons (CFCs), hydrofluorochlorocarbons, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. A detailed discussion of the trajectories and the most relevant driving forces is presented in Chapter 5. In all cases herein, future emissions of these gases are affected by environmental policies not related to climate-change concerns, as in the SRES Terms of Reference (see Appendix I). Efforts to protect the ozone layer will result in a significant decline in CFCs and some of their substitutes. Urban air quality concerns will have a significant influence on future emissions of indirect GHGs and of sulfate aerosols. As discussed in Chapter 3, for sulfur emissions the SRES scenarios reflect the evaluation of the IS92 scenario series (Alcamo et al., 1995), the recent sulfur scenario literature (for a review see Gr�bler, 1998b), and sulfur control policies in the OECD countries and the beginning of similar developments in many developing countries (see Chapter 3). Thus, a range of environmental measures is assumed in all four SRES scenario families, although the magnitude and timing differ as a function of the four different scenario storylines.
In the A1 scenario family, environmental policies are implemented as incomes rise and societies choose to protect environmental amenities. This first of all would concern water and air quality (health concerns), but also traffic congestion and noise, as well as land-use policies (preservation of recreational spaces). Economic instruments are assumed to be the preferred policy instrument in an A1 world. In some of the more "technology intensive" A1 scenario groups, cleaner energy systems and lower GHG emissions are achieved as a by-product of high economic growth coupled with rapid technological change.
Environmental concerns in their own right are perhaps the least important in the A2 world and they are mostly local in nature. For instance, no environmentally related barriers to nuclear energy development or environmental costs of fossil energy use are assumed. However, A2 scenarios include indirect control options for several GHGs that adversely effect local air quality. They also require vigorous environmental controls on pollutants that affect the availability of water, quality of soils, and productivity of agricultural crops to ensure food security is not jeopardized in a 15 billion person world. Hence, in terms of a number of traditional pollutants, A2 is far from an environmental "worst case" scenario, even if it generally has the highest GHG emissions.
The B1 scenario family assumes the most comprehensive environmental measures (see Section 4.3). Sulfur and nitrogen oxide emissions decline with vigorous efficiency improvements and "dematerialization" of the economy. Economic structural change (toward services) and cleaner energy systems (electricity, gas, district heat) to improve urban air quality also result in lower emissions levels compared to other scenarios. Finally, additional "add-on" technologies, such as flue-gas desulfurization and de-nitrification equipment, catalytic converters, etc., bridge the transition gap before inherently clean energy systems (e.g. IGCC based on coal or biomass, hydrogen-powered fuel cell vehicles) come into use on a global scale. Environmental consciousness would also lead less-developed regions to take such measures earlier and at lower income levels than occurred in the history of presently industrialized regions (as indeed already actually occurs; see Chapter 5).
In B2 scenarios, environmental concerns are prominent, but in contrast to the B1 world it is assumed that measures are implemented effectively only at local and regional scales. For instance, it is assumed that sulfur emissions would be progressively controlled and other local and regional problems concerning, for example, air quality, human health, and food supply would also be the focus of policies.
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