Increased oceanic uptake of CO­2 due to rising anthropogenic emissions has caused lowered pH levels (ocean acidification) that are hypothesized to inhibit biotic calcification and reduce the export of total alkalinity (AT) as carbonate minerals from the surface ocean. This “CO2-biotic calcification feedback” is a negative feedback on atmospheric CO2, as elevated levels of surface AT increase the ocean’s capacity to uptake CO2. We detect signatures of this feedback in the global ocean for the first time using repeat hydrographic measurements and seawater property prediction algorithms. Over the course of the past 30 years, we find an increase in global surface AT of 0.072 ± 0.023 µmol kg-1 yr-1, which would have caused approximately 20 Tmol of additional AT to accumulate in the surface ocean. This finding suggests that anthropogenic CO2 emissions are measurably perturbing the cycling of carbon on a planetary scale by disrupting biological patterns. More observations of AT would be required to understand the effects of this feedback on a regional basis and to fully characterize its potential to reduce the efficiency of marine carbon dioxide removal technology.

The ocean plays a major role in the moderation of anthropogenically-induced climate change by absorbing roughly a quarter of anthropogenic CO2 (Cant). This absorption of Cant by the ocean leads to ocean acidification, threatening marine’s life. The North Atlantic Ocean encompasses the highest ocean storage capacity of Cant per unit area. The subpolar North Atlantic gyre is subject to a large seasonal to decadal variability that might impact Cant storage. To investigate Cant evolution over the 2011-2021 period and its relationship with ocean dynamics in this region, we use the Argo-O2 array combined with neural networks and a back-calculation method (φCTO method). We compute monthly time-series of Cant in the Labrador and Irminger Seas. We show that Cant concentrations in the first 2000 dbar of the Labrador and Irminger Seas are strongly affected by winter deep convection, especially between winter 2015 and winter 2018. The Cant inventories in the top 2000 dbar of the Labrador and Irminger Seas increase through time, at rates of 1.63±0.32% yr-1 and 1.49±0.30% yr-1, respectively. Our monthly Argo-based Cant estimates complement high-quality ship-based measurements acquired at a biennial or lower frequency. Additionally, this study shows that Cant concentrations and Cant inventories in deep convection areas may depend on the method employed to calculate Cant. As a consequence, we take over the model ensemble idea and propose to use several methods to compute Cant, which would give its methodological uncertainty.

The ocean is a major carbon sink and takes up 25-30% of the anthropogenically emitted CO2. A state-of-the-art method to quantify this sink are global ocean biogeochemistry models (GOBMs) but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in-depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter-model differences and ensemble-mean biases our study identifies the (i) model set-up, such as the length of the spin-up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (ii) the ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (iii) the oceanic buffer capacity. Our analysis suggests that the late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low but the current set-up prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short-term to (1) start simulations in 1765, when atmospheric CO2 started to increase, (2) conduct a sufficiently long spin-up such that the GOBMs reach steady-state, and (3) provide key metrics for circulation, biogeochemistry, and the land-ocean interface. In the long-term, we recommend improving the representation of these metrics in the GOBMs.

A document by Bieito Fernández Castro. Click on the document to view its contents.

A document by Fiz F. Pérez. Click on the document to view its contents.

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.