Atmospheric CO2 concentration can be decreased not only by reducing fossil fuel burning but also by increasing the terrestrial ecosystems that serve as sinks for CO2. The Kyoto Protocol allows countries that are burdened with emission reduction commitments to use carbon sequestration by terrestrial sinks. However, opinions differ widely on how the inclusion of terrestrial carbon sinks in the legally binding framework (Article 3.3) will affect the demand for emission reduction during the commitment period. We approach this issue by combining a simulation model of the carbon stock change with that of land-use change. The result of the simulation shows that the Annex I countries in total may potentially claim for a net carbon offset as high as 0.2 GtC per year by carrying out ARD (Afforestation, Reforestation and Deforestation) activities. In order to come up with an effective long-term climate regime, political decisions are necessary to realize an appropriate balance between the sink enhancement and the emission reduction. Sink activities should not be too large to eliminate the efforts for emission reduction, nor too small to discourage the efforts in enhancing sinks. Although prediction of sink activities is an extremely difficult venture, several estimates of the potential should be carefully considered before political decisions. Appropriate inclusion of sink activities is also crucial for ratifying the Kyoto Protocol.
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