Abstract
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.
Highlights
Fossil fuel emission targets to limit global warming below a certain threshold are determined by the sensitivity of the climate system to greenhouse gas concentrations (Meinshausen et al, 2009)
Differences in incoming solar radiation can be partly explained by the use of atmospheric transmissivity constants (Linacre, 1968) used to convert Climatic Research Unit (CRU) cloud cover to shortwave radiation, previous studies have confirmed a systematic positive bias to be common in both reanalysis datasets (Hicke, 2005; Zhang et al, 2007)
CRU was noticeably warmer over tropical South America (1–1.5 ◦C) and NCEP was approximately 1 ◦C cooler over North Africa and Tropical Asia
Summary
Fossil fuel emission targets to limit global warming below a certain threshold are determined by the sensitivity of the climate system to greenhouse gas concentrations (Meinshausen et al, 2009). About 55 % of fossil fuel carbon emissions are removed from the atmosphere by a suite of terrestrial and oceanic processes with uptake rates governed by factors such as climate variability and changes in land cover (Le Quere et al, 2009). We systematically explore the choice of forcing data and how the resulting differences in model output (i) affects the interpretation of recent trends in the airborne fraction and (ii) contributes to residual error using standard source-sink accounting procedures (as in Le Quere et al, 2009). We estimate the range of terrestrial carbon cycle-climate sensitivities arising from three climate and four land-cover forcing combinations, assess the robustness of carbon flux trends. Our discussion focuses on the main features of residual error that arise from bottom-up carbon-budget assessments to better understand process-based interpretations of the airborne fraction
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