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Uncertain response of ocean biological carbon export in a changing world

Abstract

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long-term storage of photosynthetically fixed carbon. This ‘biological carbon pump’ is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by ~200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including important fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, Earth system models disagree on the projected response of the global export flux to climate change, with estimates ranging from −41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritizes the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterization, and hence improved model parameterization, of export flux and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean.

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Fig. 1: Uncertain response of export flux to climate change.
Fig. 2: Potential response of export processes to climate change.
Fig. 3: Feedbacks between changing export flux mechanisms and climate.

Data availability

All CMIP6 model output used in our analysis is freely available from https://esgf-data.dkrz.de/projects/cmip6-dkrz/.Source data are provided with this paper.

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Acknowledgements

This work was supported by a European Research Council Consolidator grant (GOCART, agreement number 724416) to S.A.H. S.A.H. and S.L.C.G. received funding from the Natural Environment Research Council through the COMICS project (Controls over Ocean Mesopelagic Interior Carbon Storage; NE/M020835/1). C.L. acknowledges support from the Swiss National Science Foundation under grant 174124. H.I.P. acknowledges support from the US National Science Foundation (award #1946072). E.L.C. was supported by an Imperial College Research Fellowship, funded by Imperial College London.

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S.A.H. conceived the manuscript, and all authors contributed extensively to the work presented in this paper.

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Correspondence to Stephanie A. Henson.

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Nature Geoscience thanks Ken Buesseler, Judith Hauck and Jessica Luo for their contribution to the peer review of this work. Primary Handling Editors: Xujia Jiang, Kyle Frischkorn and James Super in collaboration with the Nature Geoscience team.

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Supplementary Tables 1–4 and Fig. 1.

Source data

Source Data Fig. 1

Globally averaged annual mean export flux (GtC yr–1) and percentage change in export flux from 1850 to 2100 from 19 coupled climate models in the CMIP6 archive run under the SSP5–8.5 scenario. Percentage change is calculated with respect to the mean of years 1850–1900 for each model.

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Henson, S.A., Laufkötter, C., Leung, S. et al. Uncertain response of ocean biological carbon export in a changing world. Nat. Geosci. 15, 248–254 (2022). https://doi.org/10.1038/s41561-022-00927-0

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