|Working Group I: The Scientific Basis|
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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, NOx, 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 NOx, 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 NOx, 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 reportís 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 NOx 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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