Climate Change 2001:
Working Group I: The Scientific Basis
Other reports in this collection

4.3 Projections of Future Emissions

The IPCC SRES (Nakic´enovic´ et al., 2000) developed 40 future scenarios that are characterised by distinctly different levels of population, economic, and technological development. Six of these scenarios were identified as illustrative scenarios and these were used for the analyses presented in this chapter. The SRES scenarios define only the changes in anthropogenic emissions and not the concurrent changes in natural emissions due either to direct human activities such as land-use change or to the indirect impacts of climate change. The annual anthropogenic emissions for all greenhouse gases, NOx, CO, VOC and SO2 (sulphur dioxide) are given in the SRES for the preliminary marker scenarios (Nakic´enovic´ et al., 2000, Appendix VI) and the final marker/illustrative scenarios (Nakic´enovic´ et al., 2000, Appendix VII). Much of these data is also tabulated in Appendix II to this report. There are insufficient data in the published SRES (Nakic´enovic´ et al., 2000) to break down the individual contributions to HFCs, PFCs, and SF6, but these emissions were supplied by Lead Authors of the SRES (available at and are also reproduced in this Appendix. The geographic distribution of emissions of the short-lived compounds - NOx, CO, VOC, and SO2 is an important factor in their greenhouse forcing, and the preliminary gridded emissions were likewise supplied by the SRES Lead Authors (Tom Kram and Steven Smith, December 1998) and used in the OxComp model studies. A synopsis of the regional shift in CO and NOx emissions projected by 2100 is given in Tables 4.6 and 4.8.

This chapter evaluates the SRES emissions from year 2000 to year 2100 in terms of their impact on the abundances of non-CO2 greenhouse gases. A new feature of this report, i.e., use of NOx, CO and VOC emissions to project changes in tropospheric O3 and OH, represents a significant advance over the level-of-science in the SAR. The original four preliminary marker scenarios (December 1998) are included here because they have been used in preliminary model studies for the TAR and are designated A1p, A2p, B1p, B2p. In January 1999, these emissions were converted into greenhouse gas abundances using the level-of-science and methodology in the SAR, and the radiative forcings from these greenhouse gas abundances were used in this report for some climate model simulations.

The recently approved six marker/illustrative scenarios (March 2000) are also evaluated and are designated A1B-AIM, A1T-MESSAGE, A1FI-MiniCAM, A2-ASF, B1-IMAGE, B2-MESSAGE (hereafter abbreviated as A1B, A1T, A1FI, A2, B1, B2). For comparison with the previous assessment, we also evaluate the IPCC emissions scenario IS92a used in the SAR; for the full range of IS92 scenarios, see the SAR. An agreed-upon property of all SRES scenarios is that there is no intervention to reduce greenhouse gases; but, in contrast, regional controls on SO2 emissions across the illustrative SRES scenarios lead to emissions in the last two decades of the century that are well below those of 1990 levels. There appear to be few controls on NOx, CO and VOC emissions across all scenarios; however, the large increases in surface O3 abundances implied by these results may be inconsistent with the SRES storylines that underpin the emissions scenarios. As understanding of the relationship between emissions and tropospheric O3 abundances improves, particularly on regional scales, more consistent emissions scenarios can be developed. The SRES scenarios project substantial emissions of HFC-134a as in IS92a, but only half as much HFC-125, and no emissions of HFC-152a. The SRES emissions scenarios do include a much larger suite of HFCs plus SF6 and PFCs, which are not included in IS92a. The emissions of greenhouse gases under the Montreal Protocol and its Amendments (CFCs, HCFCs, halons) have been evaluated in WMO (Madronich and Velders, 1999). This report adopts the single WMO baseline Montreal Protocol Scenario A1 (no relation to SRES A1) for emissions and concentrations of these gases, while the SRES adopted a similar WMO Scenario A3 (maximum production); however, the differences between scenarios in terms of climate forcing is inconsequential. The resulting abundances of greenhouse gases are given in Appendix II and discussed in Section 4.4.5.

Other reports in this collection