4.3.3 Projections of Natural Emissions in 2100
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 NOx, 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 NOx.
At present we can only point out the lack of projecting these parallel changes
in once natural emissions as an uncertainty in this assessment.
4.4 Projections of Atmospheric Composition for the 21st Century
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 (NOx, CO, VOC).