1.2.1.4. Non-Land Use Influences on Sources and Sinks of CO2
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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