Abstract
Resolving regional carbon budgets is critical for informing land-based mitigation policy. For nine regions covering nearly the whole globe, we collected inventory estimates of carbon-stock changes complemented by satellite estimates of biomass changes where inventory data are missing. The net land–atmospheric carbon exchange (NEE) was calculated by taking the sum of the carbon-stock change and lateral carbon fluxes from crop and wood trade, and riverine-carbon export to the ocean. Summing up NEE from all regions, we obtained a global ‘bottom-up’ NEE for net land anthropogenic CO2 uptake of –2.2 ± 0.6 PgC yr−1 consistent with the independent top-down NEE from the global atmospheric carbon budget during 2000–2009. This estimate is so far the most comprehensive global bottom-up carbon budget accounting, which set up an important milestone for global carbon-cycle studies. By decomposing NEE into component fluxes, we found that global soil heterotrophic respiration amounts to a source of CO2 of 39 PgC yr−1 with an interquartile of 33–46 PgC yr−1—a much smaller portion of net primary productivity than previously reported.
Highlights
Net ecosystem exchange (NEE) is defined as the land–atmosphere flux of carbon excluding fossil-fuel emissions [1,2]
This flux is followed by carbon emissions from fires (1.6 PgC yr−1), the consumption of harvested crop products (1.5 PgC yr−1), land-use-change emissions (1.0–1.2 PgC yr−1), emissions from grazing (1.0 PgC yr−1), biogenic reduced-carbon emissions such as methane and volatile biogenic compounds (0.8 PgC yr−1) and the decay and burning of wood products (0.7 PgC yr−1). These fluxes represent a globally large source of carbon to the atmosphere of 8.3 ± 0.9 PgC yr−1. From these data combined with net primary productivity (NPP), we infer a global median value of soil heterotrophic respiration (SHR) equal to 39 PgC yr−1 with an interquartile range (IQR) of 33–46 PgC yr−1 and a non-Gaussian uncertainty distribution obtained from a Monte-Carlo analysis across different available data sets and their internal uncertainty (Fig. 2)
We conclude that soil carbon is temporally and spatially decoupled from NPP by lateral carbon fluxes from biomass harvest, grazing and carbon export to rivers, as well as by emissions of reduced biogenic carbon compounds
Summary
We obtained a bottom-up anthropogenic NEE of –2.2 ± 0.6 PgC yr−1, totally independently of the value derived from the global CO2 budget [5] of –2.4 ± 0.7 PgC yr−1 but remarkably consistent with it. Once fixed by NPP, carbon turns over in ecosystem pools and is returned back to the atmosphere mainly by SHR, and by land-use-change emissions; fires; livestock grazing and the harvest wood and crop products subsequently oxidized by humans and animals; outgassing of carbon by lakes, rivers, and estuaries; and biogenic emissions of reduced-carbon compounds including methane and biogenic volatile organic compounds (Supplementary Fig. 3) All these gross fluxes and their uncertainties were estimated for each region using observational data sets, considering wherever possible different independent estimates for consistency checking (‘Methods’ section and Supplementary Table 1). These fluxes represent a globally large source of carbon to the atmosphere of 8.3 ± 0.9 PgC yr−1 From these data combined with NPP (and our bottom-up NEE estimates), we infer a global median value of SHR equal to 39 PgC yr−1 with an IQR of 33–46 PgC yr−1 and a non-Gaussian uncertainty distribution obtained from a Monte-Carlo analysis across different available data sets and their internal uncertainty (Fig. 2). Other scenarios such as the RCP2.6, with its vast areas of bio-energy crops and harvested residues, should significantly affect βsoil and γ soil
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