Figure 4.14: Atmospheric composition and properties predicted using the six SRES Marker-Illustrative scenarios for anthropogenic emissions: A1B (green dashed line), A1T (yellow dash-dotted), A1FI (orange dash-dot-dotted), A2 (red solid), B1 (cyan dashed), B2 (solid dark blue). Abundances prior to year 2000 are taken from observations, and the IS92a scenario computed with current methodology is shown for reference (thin black line). Results are shown for CH4 (ppb), N2O (ppb), tropospheric O3 (DU), HFC-134a (ppt), CF4 (ppt), SF6 (ppt), the lifetime of CH4 (yr), and the global annual mean abundance of tropospheric OH (scaled to year 2000 value). All SRES A1-type scenarios have the same emissions for HFCs, PFCs, and SF6 (appearing a A1B), but the HFC-134a abundances vary because the tropospheric OH values differ affecting its lifetime. The IS92a scenario did not include emissions of PFCs and SF6. For details, see chapter text and tables in Appendix II.

Mean tropospheric abundances of greenhouse gases and other chemical changes in the atmosphere are calculated by this chapter for years 2000 to 2100 from the SRES scenarios for anthropogenic emissions of CH₄, N₂O, HFCs, PFCs, SF₆, NO_x, CO, and VOC (corresponding emissions of CO₂ and aerosol precursors are not used). The emissions from the six SRES marker/illustrative scenarios (A1B, A1T, A1FI, A2, B1, B2) are tabulated in Appendix II, as are the resulting greenhouse gas abundances, including CO₂ and aerosol burdens. Chlorine- and bromine-containing greenhouse gases are not calculated here, and we adopt the single baseline scenario from the WMO assessment (Montreal Protocol Scenario A1 of Madronich and Velders, 1999), which is reproduced in Appendix II. Also given in Appendix II are the parallel data for the SRES preliminary marker scenarios (A1p, A2p, B1p, B2p) and, in many cases, the SAR scenario IS92a as a comparison with the previous assessment.

Greenhouse gas abundances are calculated using a methodology similar to the SAR: (1) The troposphere is treated as a single box with a fill-factor for each gas that relates the burden to the tropospheric mean abundance (e.g., Tg/ppb). (2) The atmospheric lifetime for each gas is recalculated each year based on conditions at the beginning of the year and the formulae in Table 4.11. (Changes in tropospheric OH are used to scale the lifetimes of CH₄ and HFCs, and the abundance of N₂O is used to calculate its new lifetime.) (3) The abundance of a gas is integrated exactly over the year assuming that emissions remain constant for 12 months. (4) Abundances are annual means, reported at the beginning of each year (e.g., year 2100 = 1 January 2100).

In the SAR, the only OH feedback considered was that of CH₄ on its own lifetime. For this report, we calculate the change in tropospheric OH due to CH₄ abundance as well as the immediate emissions of NO_x, CO and VOC. Likewise, the increase in tropospheric O₃ projected in the SAR considered only increases in CH₄; whereas now it includes the emissions of NO_x, CO and VOC. Thus the difference between IS92a in the SAR and in this report is similar to that noted by Kheshgi et al. (1999). Also, the feedback of N₂O on its lifetime is included here for the first time and shows up as reduction of 14 ppb by year 2100 in this report�s IS92a scenario as compared to the SAR.

The 21st century abundances of CH₄, N₂O, tropospheric O₃, HFC-134a, CF₄, and SF₆ for the SRES scenarios are shown in Figure 4.14. Historical data are plotted before year 2000; and the SRES projections, thereafter to year 2100. CH₄ continues to rise in B2, A1FI, and A2 (like IS92a), with abundances reaching 2,970 to 3,730 ppb, in order. For A1B and A1T, CH₄ peaks in mid-century at about 2,500 ppb and then falls. For B1, CH₄ levels off and eventually falls to 1980-levels by year 2100. N₂O continues to rise in all scenarios, reflecting in part its long lifetime, and abundances by the end of the century range from 350 to 460 ppb. Most scenarios lead to increases in tropospheric O₃, with scenarios A1FI and A2 projecting the maximum tropospheric O₃ burdens of 55 DU by year 2100. This increase of about 60% from today is more than twice the change from pre-industrial to present. Scenario B1 is alone in projecting an overall decline in tropospheric O₃ over most of the century: the drop to 30 DU is about halfway back to pre-industrial values. HFC-134a, the HFC with the largest projected abundance, is expected to reach about 900 ppt by year 2100 for all scenarios except B1. Likewise by 2100, the abundance of CF₄ rises to 340 to 400 ppt in all scenarios except B1. The projected increase in SF₆ is much smaller in absolute abundance, reaching about 60 ppt in scenarios A1 and A2. For the major non-CO₂ greenhouse gases, the SRES A2 and A1FI increases are similar to, but slightly larger than, those of IS92a. The SRES mix of lesser greenhouse gases (HFCs, PFCs, SF₆) and their abundances are increased substantially relative to IS92a. The summed radiative forcings from these gases plus CO₂ and aerosols are given in Chapter 6.

The chemistry of the troposphere is changing notably in these scenarios, and this is illustrated in Figure 4.14 with the lifetime (LT) of CH₄ and the change in mean tropospheric OH relative to year 2000. In all scenarios except B1, OH decreases 10% or more by the end of the century, pushing the lifetime of CH₄ up from 8.4 years, to 9.2 to 10.0 years. While increasing emissions of NO_x in most of these scenarios increases O₃ and would tend to increase OH (see notes to Table 4.11), the increase in CH₄ abundance and the greater CO emissions appear to dominate, driving OH down. In such an atmosphere, emissions of CH₄ and HFCs persist longer with greater greenhouse impact. In contrast the B1 atmosphere is more readily able to oxidise these compounds and reduce their impact.

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