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