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Abstract:
Terrestrial and oceanic carbon cycle processes remove
55% of global carbon emissions, with the remaining
45 %, known as the “airborne fraction”, accumulating in
the atmosphere. The long-term dynamics of the component
fluxes contributing to the airborne fraction are challenging
to interpret, but important for informing fossil-fuel emission
targets and for monitoring the trends of biospheric carbon
fluxes. Climate and land-cover forcing data for terrestrial
ecosystem models are a largely unexplored source of uncertainty
in terms of their contribution to understanding airborne
fraction dynamics. Here we present results using a single
dynamic global vegetation model forced by an ensemble experiment
of climate (CRU, ERA-Interim, NCEP-DOE II),
and diagnostic land-cover datasets (GLC2000, GlobCover,
MODIS). For the averaging period 1996–2005, forcing uncertainties
resulted in a large range of simulated global carbon
fluxes, up to 13% for net primary production (52.4 to
60.2 Pg C a−1) and 19% for soil respiration (44.2 to 54.8 Pg
C a−1). The sensitivity of contemporary global terrestrial
carbon fluxes to climate strongly depends on forcing data
(1.2–5.9 Pg C K−1 or 0.5 to 2.7 ppmv CO2 K−1), but weakening
carbon sinks in sub-tropical regions and strengthening
carbon sinks in northern latitudes are found to be robust. The
climate and land-cover combination that best correlate to the
inferred carbon sink, and with the lowest residuals, is from
observational data (CRU) rather than reanalysis climate data
and with land-cover categories that have more stringent criteria
for forest cover (MODIS). Since 1998, an increasing
positive trend in residual error from bottom-up accounting of
global sinks and sources (from 0.03 (1989–2005) to 0.23 Pg
C a−1 (1998–2005)) suggests that either modeled drought sensitivity of carbon fluxes is too high, or that carbon emissions from net land-cover change is too large.
