Aerosol concentrations and forcing will change in the future, both as a result of changing emissions and as a result of changing climate. The uncertainties associated with our knowledge of the present day distribution of aerosols noted in previous sections will carry over into uncertainties in analysis of future scenarios. Nonetheless, models are the best available tool for making an assessment of what changes might follow. To estimate these future changes, we specified a set of emissions for the IPCC model intercomparison workshop (Section 5.4.1) based on the draft scenarios developed for the IPCC Special Report on Emissions Scenarios (SRES) (Nakic´enovic´ et al., 2000). The results from the workshop form the basis of the future aerosol forcing reported in Chapter 6 and contribute to the climate change scenarios reported in Chapter 9.

Separate estimates for the amount of biomass-burning activity were not available for the SRES scenarios (though growth in biomass burning was included in the SRES analysis). Also, estimates of emissions of organic carbon and black carbon aerosols from fossil fuels and industrial activity were not available. Therefore, these were constructed using the ratio of source strengths for CO in 2030 and 2100 to that in 2000, respectively. This ratio was then used to scale the emissions for organic carbon and black carbon from fossil fuel and biomass burning. Because the scenarios do not provide a breakdown of emissions for CO by source category, this scaling implicitly assumes that as a given region develops, the ratio of emissions of CO by biomass burning and by fossil fuel burning remains roughly constant. We note that our projected carbon particle emissions may be too large if countries choose to target particle emissions for reduction to a greater extent than they target emissions of CO.

In addition to the emissions of carbon particles, we constructed emissions for NH₃ in order to examine possible changes in the emissions of NH₃ and HNO₃ to aerosol abundance and forcing. Only the A2 scenario in 2100 was considered. For this simulation, the growth in anthropogenic NH₃ emissions was assumed to follow the growth in anthropogenic N₂O emissions. Anthropogenic emissions grew from 46.9 TgN/yr in 2000 to 111.5 TgN/yr in 2100. The anthropogenic NO_x emissions were 39.5 TgN/yr in 2000 and grew to 109.7 TgN/yr in 2100.

The emissions for 2030 and 2100 from the draft A2 and B1 scenarios that were considered in the IPCC workshop are shown in Table 5.7 (see the Appendix to this volume for the final SRES emissions). As noted there, SO₂ emissions in 2030 are about a factor of 1.6 higher than those in 2000 in the draft A2 scenario, but decrease thereafter to global average levels that are less than the present-day estimates in 2100. Carbon aerosol emissions grow by a factor of 1.3 in 2030 in the A2 scenario and continue to grow to 2100 by an additional factor of 1.8. In the B1 scenario, both SO₂ and carbon aerosol emissions are controlled by 2100, falling by 60 and 3%, respectively, compared to 2000. For comparison, the table also shows emissions for the IS92a scenario, with carbon aerosol emissions constructed as above for the SRES scenarios. In the IS92a scenario, growth continues throughout the time period for both SO₂ and carbon emissions: SO₂ and CO emissions are a factor of 1.8 and 1.6 larger in 2100, respectively, than the same emissions in 2000.

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