The ocean has absorbed 25 ± 2% of the total anthropogenic CO2 emissions from the early 1960s to the late 2010s, with rates more than tripling over this period and with a mean uptake of –2.7 ± 0.3 Pg C year–1 for the period 1990 through 2019. This growth of the ocean sink matches expectations based on the increase in atmospheric CO2, but research has shown that the sink is more variable than long assumed. In this Review, we discuss trends and variations in the ocean carbon sink. The sink stagnated during the 1990s with rates hovering around –2 Pg C year–1, but strengthened again after approximately 2000, taking up around –3 Pg C year–1 for 2010–2019. The most conspicuous changes in uptake occurred in the high latitudes, especially the Southern Ocean. These variations are caused by changes in weather and climate, but a volcanic eruption-induced reduction in the atmospheric CO2 growth rate and the associated global cooling contributed as well. Understanding the variability of the ocean carbon sink is crucial for policy making and projecting its future evolution, especially in the context of the UN Framework Convention on Climate Change stocktaking activities and the deployment of CO2 removal methods. This goal will require a global-level effort to sustain and expand the current observational networks and to better integrate these observations with models.
The long-term trend in the ocean carbon sink since the early 1960s was primarily driven by the increasing uptake of anthropogenic CO2. Although the ocean is expected to have lost a few petagrams of natural CO2 to the atmosphere in response to ocean warming, this loss cannot be quantified conclusively with observations.
The oceanic uptake of anthropogenic CO2 scaled proportionally with the increase in atmospheric CO2 between the early 1960s and late 2010s, as expected given the quasi-exponential growth of atmospheric CO2 during this period.
The average ocean uptake rate of –2.7 ± 0.3 Pg C year–1 for the period 1990 through 2019 is commensurate with a sensitivity β of 1.4 ± 0.1 Pg C per ppm atmospheric CO2, suggesting a trend in the uptake of –0.4 ± 0.1 Pg C year–1 per decade.
The annual mean ocean carbon sink varies by about ±20% around this trend, primarily caused by changes in the sources and sinks of natural CO2, with a lesser role for variations in atmospheric CO2 growth rates impacting the uptake of anthropogenic CO2.
The net oceanic uptake rate of CO2 will likely decrease in the future owing to several converging trends: reduced emissions of CO2 leading to reduced atmospheric CO2 growth rates in response to climate policy; reduced storage capacity owing to continuing ocean acidification; and enhanced outgassing of natural CO2 owing to ocean warming and changes in ocean circulation and biology.
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N.G., J.D.M., L.G. and P.L. acknowledge support from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 821003 (project 4C). N.G. also acknowledges support from the EU Horizon project no. 821001 (SO-CHIC). The work of D.C.E.B. was supported by the EU Horizon project no. 820989 (COMFORT). The work reflects only the authors’ views; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. G.A.M. acknowledges funding from the National Science Foundation (NSF) through LEAP STC (2019625) and OCE (1948624), the National Aeronautics and Space Administration (NASA) (80NSSC22K0150) and the National Oceanic and Atmospheric Administration (NOAA) (NA20OAR4310340). J.H. received funding from the Helmholtz Young Investigator Group Marine Carbon and Ecosystem Feedbacks in the Earth System (MarESys) (grant number VH-NG-1301). T.D. acknowledges support from NSF award OCE-1948955.
The authors declare no competing interests.
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- Air–sea gas exchange
A diffusion-driven process governing the transfer of gases across the air–sea interface, driven by the concentration gradient of the gas across the interface and controlled by the level of turbulence at the interface.
- Buffer factor
How well seawater is able to buffer an increase in surface ocean CO2 (ocean partial pressure of CO2), which is crucial for determining the amount of anthropogenic CO2 the surface ocean can hold; also called the Revelle factor.
- Dissolved inorganic carbon
(DIC). The sum of all dissolved inorganic carbon species in the seawater, including dissolved CO2 (CO2aq), carbonic acid (H2CO3), bicarbonate (HCO3–) and carbonate (CO32−).
- El Niño Southern Oscillation
(ENSO). A quasi-periodic oscillation of the coupled ocean–atmosphere system with the majority of the action being focused on the eastern tropical Pacific; it is globally the dominant mode of climate variability.
- External forcing
Processes leading to changes in the ocean carbon sink driven by processes external to the climate system, such as volcanic eruptions.
- Forward models
A class of models that start from initial conditions and solve the governing balance equations by time-integrating them forward using a set of provided boundary conditions.
- Inverse models
A class of models that fuse observations and models in order to improve our quantitative understanding of a set of processes.
- Internal forcing
Processes leading to changes in the ocean carbon sink, primarily associated with (internally generated) weather and climate variations.
- Ocean partial pressure of CO2
(Ocean pCO2). The partial pressure of CO2 measured in the air in equilibrium with the water parcel under consideration at 1 atm total pressure and at the in situ temperature of the water parcel; often also referred to as pCO2oc.
- Ocean acidification
Change in the ocean’s seawater chemistry (pH, [CO32−], CaCO3 saturation state and so on) as a consequence of the oceanic uptake of anthropogenic CO2.
- Ocean biogeochemical models
A class of ocean models where the most important biogeochemical processes are explicitly represented, namely air–sea gas exchange, chemical speciation and biological processes.
- Southern Annular Mode
(SAM). A mode of variations in the polar atmosphere of the southern hemisphere, characterized by fluctuations in the strength of the circumpolar vortex.
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Gruber, N., Bakker, D.C.E., DeVries, T. et al. Trends and variability in the ocean carbon sink. Nat Rev Earth Environ 4, 119–134 (2023). https://doi.org/10.1038/s43017-022-00381-x
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