Soil carbon sequestration is perhaps more hotly debated in the political arena than in the scientific arena. Terrestrial sinks have received close political scrutiny since their inclusion in the Kyoto Protocol at the fourth-Conference of Parties (COP4) to the United Nations Framework Convention on Climate Change (UNFCCC). Under Articles 3.3 and 3.4 of the Kyoto Protocol, biospheric sinks and sources of carbon can be included by parties in meeting targets for the reduction of GHG emissions, by comparing emissions in the commitment period (CP; first CP = 2008-2012) with baseline (1990) emissions. The exact nature of the sinks and sources that would be allowed as well as the modalities and methods that could be used were addressed in subsequent meetings of the COP culminating in agreement at COP7 in 2001, leading to the publication of the Marrakesh Accords.
Under the Marrakesh Accords, Kyoto Article 3.3, activities limited to afforestation, reforestation and deforestation are included, but are subject to a complex series of rules to attempt to separate any impacts that are not directly human-induced (see UNFCCC web site at www.unfccc.de). Under the Marrakesh Accords, Kyoto Article 3.4, activities under forest management, cropland management, grazing land management and revegetation can be included. The details of the rules governing how sinks can be used are complex and contain some safeguards to avoid double-counting of sinks, to avoid carbon credit claims and the land subsequently reverting to low carbon stock levels (e.g. by deforestation), and to ensure that accounting is carried out in a transparent and verifiable way. Soil carbon changes occurring during afforestation, reforestation or deforestation are included under Article 3.3 of the Kyoto Protocol, whilst soil carbon sequestration in croplands, grazing lands, managed forests and land subject to revegetation is included under Kyoto Article 3.4.
An interesting outcome of the Marrakesh Accords is that cropland management, grazing land management and revegetation will be reported on a net-net basis, i.e. net emissions during the CP will be compared to net emissions during the baseline year. Under this form of accounting, a carbon credit will arise if a party reduces a source, even though a sink (i.e. a net removal of carbon from the atmosphere) has not been created. In this respect, decreasing a source is equivalent to creating a sink.
Scientific issues relating to the use of biospheric sinks in the Kyoto Protocol were addressed in the IPCC (2000a). Although many of these issues no longer apply (since the political negotiations have moved on), this text is still a useful source of information on the potential of biospheric sinks, including soils, for climate mitigation.
Soil carbon sequestration will remain in the political spotlight at least until the end of the first CP in 2012, but with further CPs set for negotiation in the near future, sinks are likely to remain politically important for the foreseeable future (see Reilly et al., Chapter 8, this volume).
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