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In addition to land use and land-use change, several other factors-many of which are anthropogenic in origin-affect large-scale sources and sinks of atmospheric CO2. These factors contribute to the observed variability and upward trend in CO2 concentration, thus complicate the determination of how much of the observed changes in carbon stocks in vegetation and soils during a commitment period should be attributed to the direct activities initiated in accordance with Articles 3.3 or 3.4 of the Kyoto Protocol. For example, changes in climate and climate variability influence sources and sinks of CO2 from vegetation and soils and vice versa, and these may have natural or anthropogenic causes. Projections of future atmospheric CO2 concentrations will, therefore, be quite uncertain, at least beyond the next few decades (Tans and Wallace, 1999).
Other factors that may have contributed to the rate of carbon sequestration into vegetation and soils include the increase in atmospheric CO2 concentration during 1850-1998 from about 285 to 366 ppmv (see Section 1.3.2.3) and increasing atmospheric concentrations of NOX (NO and NO2) and NO3 that enhance the atmospheric deposition of nitrogen-a limiting plant nutrient in many ecosystems (Schimel et la.,1995) (see Section 1.3.2.4). On the other hand, emissions of NOX and SO2 also lead to atmospheric deposition of nitrogen and sulfur compounds that cause acidification of soils and waters, which may negatively affect plant growth and reduce carbon uptake. In addition, elevated concentrations of surface ozone that also reduce plant growth (Semenov et la.,1998, 1999) result from NOX emissions in the presence of volatile organic compounds (Houghton et la.,1996). All of these factors affect the net removal of CO2 from the atmosphere by terrestrial ecosystems, as given in Table 1-2, in addition to the direct effects of land use and land-use change.
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