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.

The formation of sea ice in the Arctic Ocean, as well as other physical processes such as injection of air and rapid cooling, plays a crucial role in determining the physical and chemical properties of its waters, which in turn drive the circulation in the Arctic [1]. Such processes can be constrained by conservative tracers which are biologically and chemically nonreactive, such as the noble gases. The full suite of stable noble gases (He, Ne, Ar, Kr and Xe) have been measured for the first time in the Arctic Ocean—along with CFC-12, SF6, and other transient tracers—during the Ventilation and Anthropogenic Carbon in the Arctic Ocean (VACAO) project of the wider Synoptic Arctic Survey 2021 (SAS21) [2]. The noble gas profiles indicate a water column strongly influenced by rapid cooling and excess air injection, with a surface signature characteristic of solute rejection by sea ice formation.We have compared multiple Arctic Ocean gas exchange models (based on similar models used in the Antarctic by Loose et al. [3] and in the Labrador Sea by Hamme et al. [4]) to constrain the fractions of Arctic water composed of Pacific, Atlantic and sea ice melt-derived origin waters, as well as the amount of sea ice being formed and air being injected into the water via bubbles. These parameters are estimated using a χ­2-minimisation procedure, where the misfit between fitted parameters and data is minimised. Preliminary results indicate a non-negligible sea ice term in the equations describing gas saturation anomalies in the Arctic.Another key goal of VACAO is to use transient tracers to study the ventilation timescales of the Arctic Ocean, with application towards the study of the efficacy of its CO2 solubility pump/storage. CFC-12/SF6 is one tracer pair with which this is attempted; water dating with this pair requires knowledge of the concentration history of the tracers in the surface water, for which there are no direct measurements. Thus, the physical gas exchange parameters modeled from the noble gas data can be used in conjunction with observed atmospheric histories to more accurately describe the surface water histories of CFC-12 and SF6. This in turn can be used to better constrain the Transit Time Distribution parameters used when dating Arctic waters [5]. Comparisons with other VACAO age tracer data (39Ar and 129I/236U) may act as validation tools to this “correction” to CFC-12/SF6 dating.References for Abstract[1] Rudels, B., and Carmack, E. 2022. Arctic ocean water mass structure and circulation. Oceanography, 35(3–4), 52–65 pp.[2] Snoeijs-Leijonmalm, P. and the SAS-Oden 2021 Scientific Party (2022). Expedition Report SWEDARCTIC Synoptic Arctic Survey 2021 with icebreaker Oden. Swedish Polar Research Secretariat. 300 pp.[3] Loose, B., Stammerjohn, S., Sedwick, P., & Ackley, S. (2023). Sea ice formation, glacial melt and the solubility pump boundary conditions in the Ross Sea. Journal of Geophysical Research: Oceans, 128, e2022JC019322.[4] Hamme, R. C., Emerson, S. R., Severinghaus, J. P., Long, M. C., & Yashayaev, I. (2017). Using noble gas measurements to derive air-sea process information and predict physical gas saturations. Geophysical Research Letters, 44, 9901–9909 pp.[5] Jeansson, E., Tanhua, T., Olsen, A., Smethie, W. M., Rajasakaren, B., Ólafsdóttir, S. R., & Ólafsson, J. (2023). Decadal changes in ventilation and anthropogenic carbon in the Nordic Seas. Journal of Geophysical Research: Oceans, 128, e2022JC019318.

Fossil fuel combustion, land use change and other human activities have increased the atmospheric carbon dioxide (CO2) abundance by about 50% since the beginning of the industrial age. The atmospheric CO2 growth rates would have been much larger if natural sinks in the land biosphere and ocean had not removed over half of this anthropogenic CO2. As these CO2 emissions grew, uptake by the ocean increased in response to increases in atmospheric CO2 partial pressure (pCO2). On land, gross primary production (GPP) also increased, but the dynamics of other key aspects of the land carbon cycle varied regionally. Over the past three decades, CO2 uptake by intact tropical humid forests declined, but these changes are offset by increased uptake across mid- and high-latitudes. While there have been substantial improvements in our ability to study the carbon cycle, measurement and modeling gaps still limit our understanding of the processes driving its evolution. Continued ship-based observations combined with expanded deployments of autonomous platforms are needed to quantify ocean-atmosphere fluxes and interior ocean carbon storage on policy-relevant spatial and temporal scales. There is also an urgent need for more comprehensive measurements of stocks, fluxes and atmospheric CO2 in humid tropical forests and across the Arctic and boreal regions, which are experiencing rapid change. Here, we review our understanding of the atmosphere, ocean, and land carbon cycles and their interactions, identify emerging measurement and modeling capabilities and gaps and the need for a sustainable, operational framework to ensure a scientific basis for carbon management.