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The land-to-ocean loops of the global carbon cycle

Abstract

Carbon storage by the ocean and by the land is usually quantified separately, and does not fully take into account the land-to-ocean transport of carbon through inland waters, estuaries, tidal wetlands and continental shelf waters—the ‘land-to-ocean aquatic continuum’ (LOAC). Here we assess LOAC carbon cycling before the industrial period and perturbed by direct human interventions, including climate change. In our view of the global carbon cycle, the traditional ‘long-range loop’, which carries carbon from terrestrial ecosystems to the open ocean through rivers, is reinforced by two ‘short-range loops’ that carry carbon from terrestrial ecosystems to inland waters and from tidal wetlands to the open ocean. Using a mass-balance approach, we find that the pre-industrial uptake of atmospheric carbon dioxide by terrestrial ecosystems transferred to the ocean and outgassed back to the atmosphere amounts to 0.65 ± 0.30 petagrams of carbon per year (±2 sigma). Humans have accelerated the cycling of carbon between terrestrial ecosystems, inland waters and the atmosphere, and decreased the uptake of atmospheric carbon dioxide from tidal wetlands and submerged vegetation. Ignoring these changing LOAC carbon fluxes results in an overestimation of carbon storage in terrestrial ecosystems by 0.6 ± 0.4 petagrams of carbon per year, and an underestimation of sedimentary and oceanic carbon storage. We identify knowledge gaps that are key to reduce uncertainties in future assessments of LOAC fluxes.

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Fig. 1: Approaches to quantify the pre-industrial carbon budget.
Fig. 2: The global carbon budget with LOAC fluxes.
Fig. 3: Bottom-up estimates of the pre-industrial open-ocean carbon budget in three latitudinal bands.

Data availability

Source data for Figs. 13, Supplementary Figs. 1, 2 are provided with the paper. All numbers and their associated uncertainties shown in Fig. 2 are synthesized in Supplementary Table 1 and described in detail in Supplementary Sections 1, 2.

Code availability

No code was used to generate the figures in this study.

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Acknowledgements

P.R. and P.C. acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska‐Curie grant agreement number 643052 (C-CASCADES). P.R. also received financial support from BELSPO through the project ReCAP, which is part of the Belgian research programme FedTwin and from the European Union’s Horizon 2020 research and innovation programme under grant agreements number 776810 (VERIFY) and number 101003536 (ESM2025–Earth System Models for the Future). P.C. has been co-funded by the French Agence Nationale de la Recherche (ANR) Convergence Lab Changement climatique et usage des terres (CLAND), the European Space Agency Climate Change Initiative ESA-CCI RECCAP2 project 1190 (ESRIN/ 4000123002/18/I-NB) and Observation-based system for monitoring and verification of greenhouse gases (VERIFY, grant agreement number 776810). L.R. gratefully acknowledges support from the Alfred P. Sloan Foundation Research Fellowship, the Princeton Catalysis Initiative at Princeton University and the NASA OCO‐2 Science Team Grant 80NSSC18K0893. R.G.N. acknowledges support from NASA Carbon Cycle Science and Interdisciplinary Science Programs and NSF Chemical Oceanography Program. This study benefitted from discussions with W.-J. Cai, A. Coppola, P. Friedlingstein and N. Gruber.

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P.R. and P.C. initiated the design of the study that led to this paper. P.R. and L.R. directed the analysis and coordinated the conception and writing of the paper. L.R. designed all the figures. P.R. and R.G.N. co-led the synthesis of LOAC fluxes. P.C., R.G.N., L.R. and P.R. contributed to the budget analysis and the writing of the paper. P.R. and L.R. have contributed equally to this study.

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Correspondence to Pierre Regnier.

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Regnier, P., Resplandy, L., Najjar, R.G. et al. The land-to-ocean loops of the global carbon cycle. Nature 603, 401–410 (2022). https://doi.org/10.1038/s41586-021-04339-9

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