National inventories of terrestrial carbon sources and sinks: The UK experience

The U.K. has extensive databases on soils, land cover and historic land use change which have made it possible to construct a comprehensive inventory of the principal terrestrial sources and sinks of carbon for approximately the year 1990, using methods that are consistent with, and at least as accurate as, the revised 1996 guidelines recommended by IPCC where available - and including categories which are not currently considered under the UN Framework Convention on Climate Change. This country inventory highlights issues concerning methodology, uncertainty, double counting, the importance of soils and the relative magnitude of sources and sinks which are reported to the UNFCCC relative to other sources and sinks. The carbon sinks (negative values in MtC a(-1)) for categories reported to the UNFCCC, based on the IPCC categories, were estimated to be: forest trees and litter (-2.1), U.K. forest products (-0.5, ignoring imports and exports), non-forest biomass (-0.3), forest soils (-0.1) and soils on set-aside land (-0.4). The carbon sources (positive values) reported under the UNFCCC were estimated to be: losses of soil organic carbon resulting from cultivation of semi-natural land (6.2) and from urbanization (1.6), drainage of peatlands (0.3) and fenlands (0.5), and peat extraction (0.2). A range of other sources and sinks not covered by the IPCC guidelines were also quantified, namely, the accumulation of carbon in undrained peatlands (-0.7, ignoring methane emission), sediment accretion in coastal marshes (-0.1), the possible U.K. share of the CO2 and N fertilization carbon sink (-2.0) and riverine organic and particulate carbon export to the sea (1.4, which may be assumed to be a source if most of this carbon is released as CO2 in the sea). All sinks totalled -6.2 and sources 10.2, giving a net flux to the atmosphere in 1990 of 4.0 MtC a(-)1. Uncertainties associated with categories, mostly based on best guesses, ranged from +/-15% for forest biomass and litter to +/-60% for CO2 and N fertilization.