SRES scenarios do not consider the changes in natural emissions and sinks of reactive gases that are induced by alterations in land use and agriculture or land-cover characteristics. (Land-use change statistics, however, are reported, and these could, in principle, be used to estimate such changes.) In some sense these altered emissions must be considered as anthropogenic changes. Examples of such changes may be increased NO_x, N₂O and NH₃ emissions from natural waters and ecosystems near agricultural areas with intensified use of N-fertiliser. A change of land cover, such as deforestation, may lead to reduced isoprene emissions but to increases in soil emissions of NO_x. At present we can only point out the lack of projecting these parallel changes in once natural emissions as an uncertainty in this assessment.

Calculating the abundances of chemically reactive greenhouse gases in response to projected emissions requires models that can predict how the lifetimes of these gases are changed by an evolving atmospheric chemistry. This assessment focuses on predicting changes in the oxidative state of the troposphere, specifically O₃ (a greenhouse gas) and OH (the sink for many greenhouse gases). Many research groups have studied and predicted changes in global tropospheric chemistry, and we seek to establish a consensus in these predictions, using a standardised set of scenarios in a workshop organised for this report. The projection of stratospheric O₃ recovery in the 21st century also a factor in radiative forcing and the oxidative state of the atmosphere is reviewed extensively in WMO (Hofmann and Pyle, 1999), and no new evaluation is made here. The only stratospheric change included implicitly is the N₂O feedback on its lifetime. Overall, these projections of atmospheric composition for the 21st century include the most extensive set of trace gas emissions for IPCC assessments to date: greenhouse gases (N₂O, CH₄, HFCs, PFCs, SF₆) plus pollutants (NO_x, CO, VOC).

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