The oceanic storage of anthropogenic CO2 (Cant) that humans have emitted into the atmosphere has been pivotal for counteracting climate change. Yet multi-decadal trends in the ocean interior storage of Cant have not been assessed at global scale. Here, we determine storage changes of Cant by applying the eMLR(C*) regression method to ocean interior observations collected between 1989 and 2020. We find that the global ocean storage of Cant grew by 29 ± 3 Pg C dec-1 and 27 ± 3 Pg C dec-1 (±1σ) from 1994 to 2004 and 2004 to 2014, respectively. Although the two growth rates are not significantly different, they imply a reduction of the oceanic uptake fraction of the anthropogenic emissions from 36 ± 4 % to 27 ± 3 % from the first to the second decade. We attribute this reduction to a decrease of the ocean buffer capacity and changes in ocean circulation. In the Atlantic Ocean, the maximum storage rate shifted from the Northern to the Southern Hemisphere, plausibly caused by a weaker formation rate of North Atlantic Deep Waters and an intensified ventilation of mode and intermediate waters in the Southern Hemisphere. Between 1994 and 2004, the oceanic Cant accumulation exceeded the net air-sea flux by 8 ± 4 Pg C dec-1, suggesting a loss of natural carbon from the ocean during this decade. Our results reveal a substantial sensitivity of the ocean carbon sink to climate variability and change.

A budget approach is used to disentangle drivers of the seasonal mixed layer carbon cycle at Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the North Pacific Subtropical Gyre (NPSG). The budget utilizes data from the WHOTS (Woods Hole - Hawaii Ocean Time-series Site) mooring, and the ship-based Hawai‘i Ocean Time-series (HOT) in the North Pacific Subtropical Gyre (NPSG), a region of significant oceanic carbon uptake. Parsing the carbon variations into process components allows an assessment of both the proportional contributions of mixed layer carbon drivers, and the seasonal interplay of drawdown and supply from different processes. Annual net community production reported here is at the lower end of previously published data, while net community calcification estimates are 4- to 7-fold higher than available sediment trap data, the only other estimate of calcium carbonate export at this location. Although the observed seasonal cycle in dissolved inorganic carbon (DIC) in the NPSG has a relatively small amplitude, larger fluxes offset each other over an average year, with major supply from physical transport, especially lateral eddy transport throughout the year and entrainment in the winter, and biological carbon uptake in the spring. Gas exchange plays a smaller role, supplying carbon to the surface ocean between Dec-May, and outgassing in Jul-Oct. Evaporation-precipitation (E–P) is variable with precipitation prevailing in the first- and evaporation in the second half of the year. The observed total alkalinity signal is largely governed by E–P, with a somewhat stronger net calcification signal in the wintertime.

The decline in global emissions of carbon dioxide due to the COVID-19 pandemic provides a unique opportunity to investigate the sensitivity of the global carbon cycle and climate system to emissions reductions. Recent efforts to study the response to these emissions declines has not addressed their impact on the ocean, yet ocean carbon absorption is particularly susceptible to changing atmospheric carbon concentrations. Here, we use ensembles of simulations conducted with an Earth system model to explore the potential detection of COVID-related emissions reductions in the partial pressure difference in carbon dioxide between the surface ocean and overlying atmosphere (ΔpCO2), a quantity that is regularly measured. We find a unique fingerprint in global-scale ΔpCO2 that is attributable to COVID and potentially detectable in observations, but only with much larger emissions reductions than those that have been observed to date.

Significant advancements have been made over the past four decades in understanding and quantifying the stocks and flows of carbon between the reservoirs. However, knowledge of the complex oceanic processes influencing the carbon cycle has been largely compartmentalized into physico-chemical and biological studies. The connections between coastal and open-ocean carbon processes have also been understudied. To fully appreciate the ocean carbon cycle, and its anticipated changes in the future, a holistic and integrated approach to ocean carbon cycle research is needed. In particular, a greater quantitative understanding of how biological processes interact with the physical and chemical drivers in the open ocean and in coastal waters is needed. Moreover, the carbon cycle needs to be understood in the current socio-economic context and large anticipated changes in the next decades. To address these issues, the Integrated Ocean Carbon Research (IOC-R), a formal IOC working group, was formed in 2018. The working group is a response to the UN Decade of Ocean Science for Sustainable Development 2021-2030, “the Decade”. The IOC-R will contribute to the science elements of an overarching Implementation Plan for the Decade. The Implementation Plan is a high-level framework to guide actions by which ocean science can more effectively deliver its contribution to achieving the societal outcomes outlined for the Decade. The IOC-R focusses the ocean carbon cycle component of the Implementation Plan by addressing key issues in ocean carbon research through a combined strategy of research and observational goals. The research will be framed by four key questions that were formulated at the inaugural Expert Workshop on Integrated Ocean Carbon Research at the IOC-UNESCO Headquarters in Paris, France on Oct. 28-30, 2019: 1) Will the ocean uptake of anthropogenic carbon dioxide (CO2) continue as primarily an abiotic process? 2) What is the role of biology in the ocean carbon cycle? 3) What are the exchanges of carbon between the land-ocean continuum and how are they evolving over time? 4) How are humans altering the ocean carbon cycle, and what are the feedbacks?