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Revision of global carbon fluxes based on a reassessment of oceanic and riverine carbon transport


Measurements of atmospheric CO2 concentration provide a tight constraint on the sum of the land and ocean sinks. This constraint has been combined with estimates of ocean carbon flux and riverine transport of carbon from land to oceans to isolate the land sink. Uncertainties in the ocean and river fluxes therefore translate into uncertainties in the land sink. Here, we introduce a heat-based constraint on the latitudinal distribution of ocean and river carbon fluxes, and reassess the partition between ocean, river and land in the tropics, and in the southern and northern extra-tropics. We show that the ocean overturning circulation and biological pump tightly link the ocean transports of heat and carbon between hemispheres. Using this coupling between heat and carbon, we derive ocean and river carbon fluxes compatible with observational constraints on heat transport. This heat-based constraint requires a 20–100% stronger ocean and river carbon transport from the Northern Hemisphere to the Southern Hemisphere than existing estimates, and supports an upward revision of the global riverine carbon flux from 0.45 to 0.78 PgC yr−1. These systematic biases in existing ocean/river carbon fluxes redistribute up to 40% of the carbon sink between northern, tropical and southern land ecosystems. As a consequence, the magnitude of both the southern land source and the northern land sink may have to be substantially reduced.

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L.R. was granted support by the Climate Program Office of the National Oceanic and Atmospheric Administration grant NA13OAR4310219. S.K. was supported by US National Science Foundation (NSF) grant OCE 10-60804. K.B.R. was supported by NASA award NNX14AL85G. N.C.A.R. is sponsored by the NSF. The authors also thank the groups developing MOM, MITgcm, NEMO and CMIP5 models for providing their model results.

Author information

L.R. directed the analysis of the several data sets used here and shared responsibility for writing the manuscript. R.F.K. and B.B.S. shared responsibility for writing the manuscript. C.R. computed the land sinks. L.B., M.C.L. and K.B.R. provided model results. All authors contributed to the final version of the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to L. Resplandy.

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Fig. 1: Overturning circulation and biological pump link carbon and heat transport in ocean basins.
Fig. 2: Schematic of ocean transport asymmetries.
Fig. 3: Link between heat and carbon asymmetries.
Fig. 4: Observation-based estimates of carbon transport asymmetry AC.
Fig. 5: Impact of ocean/river carbon asymmetry on land sinks.
Fig. 6: Revised ocean/river flux is consistent with atmospheric CO2 data.