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 accu
rate as, the revised 1996 guidelines recommended by IPCC where available -
and including categories which are not currently considered under the UN Fr
amework Convention on Climate Change. This country inventory highlights iss
ues concerning methodology, uncertainty, double counting, the importance of
soils and the relative magnitude of sources and sinks which are reported t
o 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 tree
s and litter (-2.1), U.K. forest products (-0.5, ignoring imports and expor
ts), 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 cultiva
tion of semi-natural land (6.2) and from urbanization (1.6), drainage of pe
atlands (0.3) and fenlands (0.5), and peat extraction (0.2). A range of oth
er sources and sinks not covered by the IPCC guidelines were also quantifie
d, namely, the accumulation of carbon in undrained peatlands (-0.7, ignorin
g methane emission), sediment accretion in coastal marshes (-0.1), the poss
ible U.K. share of the CO2 and N fertilization carbon sink (-2.0) and river
ine organic and particulate carbon export to the sea (1.4, which may be ass
umed 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 atmosphe
re in 1990 of 4.0 MtC a(-)1. Uncertainties associated with categories, most
ly based on best guesses, ranged from +/-15% for forest biomass and litter
to +/-60% for CO2 and N fertilization.