There are some obvious gaps in these projections where processes influencing the greenhouse gas abundances have been omitted. One involves coupling of tropospheric chemistry with the stratosphere. For one, we did not include the recovery of stratospheric ozone expected over the next century. The slow recovery of stratospheric ozone depletion from the halogens will lead to an increase in the flux of ozone into the troposphere and also to reduced solar UV in the troposphere, effectively reversing over the next century what has occurred over the past two decades. A more important impact on the Y2100 stratosphere, however, is the response to increases in CH₄ and N₂O projected by most scenarios (see Hofmann and Pyle, 1999), which in terms of coupled stratosphere-troposphere chemistry models could be evaluated in only one of the OxComp models (ULAQ, Université degli studi dell’ Aguila) and is not included here.

Another major gap in these projections is the lack of global models coupling the atmospheric changes with biogeochemical models. There have been studies that tackled individual parts of the problem, e.g., deposition of reactive N (Holland et al., 1997), crop damage from O₃ (Chameides et al., 1994). Integrated assessment studies have coupled N₂O and CH₄ emission models with lower dimension or parametrized climate and chemistry models (e.g., Alcamo, 1994; Holmes and Ellis, 1999; Prinn et al., 1999). However, the inherent local nature of this coupling, along with the possible feedbacks through, for example NO and VOC emissions, point to the need for coupled 3-D global chemistry and ecosystem models in these assessments.

Finally, there is an obvious need to couple the physical changes in the climate system (water vapour, temperature, winds, convection) with the global chemical models. This has been partially accomplished for some cases that are highlighted here (Section 4.5.2), but like other gaps presents a major challenge for the next assessment.

In order to assess the overall impact of changing industry or agriculture, it would be necessary to combine all emissions from a specific sector or sub-sector as has been done with the IPCC assessment of aviation (Penner et al., 1999). Further, the impact on natural emissions and land-use change (e.g., albedo, aerosols) would also need to be included. Such a sector analysis would cut across Chapters 3, 4, 5 and 6 of this report (e.g., as in Prather and Sausen, 1999). Such an analysis cannot be done for the SRES emissions scenarios, which lack a breakdown by sector and also lack numbers for the changes in the land area of agriculture or urbanisation.

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