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International audience; The Arctic Ocean, more than any other ocean, is influenced by riverine input of carbon and nutrients. That riverine delivery is likely to change with climate change as runoff increases, permafrost thaws, and tree lines advance. But it is unknown to what extent these changes in riverine delivery will affect Arctic Ocean primary production, air-to-sea CO 2 fluxes, and acidification. To test their sensitivity to changing riverine delivery, we made sensitivity tests using an ocean circulation model coupled to an ocean biogeochemical model. In separate idealized simulations, riverine inputs of dissolved inorganic carbon (C T), dissolved organic carbon (DOC), and nutrients were increased by 1%/year until doubling. Doubling riverine nutrient delivery increased primary production by 11% on average across the Arctic basin and by up to 34-35% locally. Doubling riverine DOC delivery resulted in 90% of that added carbon being lost to the atmosphere, partly because it was imposed that once delivered to the ocean, the riverine DOC is instantaneously remineralized to C T. That additional outgassing, when considered alone, reduced the net ingassing of natural CO 2 into the Arctic Ocean by 25% while converting the Siberian shelf seas and the Beaufort Sea from net sinks to net sources of carbon to the atmosphere. The remaining 10% of DOC remained in the Arctic Ocean, but having been converted to C T , it enhanced acidification. Conversely, doubling riverine C T increased the Arctic Ocean's average surface pH by 0.02 because riverine total alkalinity delivery increased at the same rate as riverine C T delivery.

International audience; Estimates of the ocean's large-scale transport of anthropogenic CO2 are based on one-time hydrographic sections, but the temporal variability of this transport has not been investigated. The aim of this study is to evaluate how the seasonal and mesoscale variability affect data-based estimates of anthropogenic CO2 transport. To diagnose this variability, we made a global anthropogenic CO2 simulation using an eddy-permitting version of the coupled ocean sea-ice model ORCA-LIM. As for heat transport, the seasonally varying transport of anthropogenic CO2 is largest within 20° of the equator and shows secondary maxima in the subtropics. Ekman transport generally drives most of the seasonal variability, but the contribution of the vertical shear becomes important near the equator and in the Southern Ocean. Mesoscale variabilty contributes to the annual-mean transport of both heat and anthropogenic CO2 with strong poleward transport in the Southern Ocean and equatorward transport in the tropics. This "rectified" eddy transport is largely baroclinic in the tropics and barotropic in the Southern Ocean due to a larger contribution from standing eddies. Our analysis revealed that most previous hydrographic estimates of meridional transport of anthropogenic CO2 are severely biased because they neglect temporal fluctuations due to non-Ekman velocity variations. In each of the three major ocean basins, this bias is largest near the equator and in the high southern latitudes. In the subtropical North Atlantic, where most of the hydrographic-based estimates have been focused, this uncertainty represents up to 20% and 30% of total meridional transport of heat and CO2. Generally though, outside the tropics and Southern Ocean, there are only small variations in meridional transport due to seasonal variations in tracer fields and time variations in eddy transport. For the North Atlantic, eddy variability accounts for up to 10% and 15% of the total transport of heat and CO2. This component is not accounted for in coarse-resolution hydrographic surveys.

This study aims to evaluate the potential for impacts of ocean acidification on North Atlantic deep-sea ecosystems in response to IPCC AR5 Representative Concentration Pathways (RCPs). Deep-sea biota is likely highly vulnerable to changes in seawater chemistry and sensitive to moderate excursions in pH. Here we show, from seven fully coupled Earth system models, that for three out of four RCPs over 17% of the seafloor area below 500m depth in the North Atlantic sector will experience pH reductions exceeding -0.2 units by 2100. Increased stratification in response to climate change partially alleviates the impact of ocean acidification on deep benthic environments. We report on major pH reductions over the deep North Atlantic seafloor (depth >500 m) and at important deep-sea features, such as seamounts and canyons. By 2100, and under the high CO2 scenario RCP8.5, pH reductions exceeding -0.2 (-0.3) units are projected in close to 23% (similar to 15 %) of North Atlantic deep-sea canyons and similar to 8% (3 %) of seamounts - including seamounts proposed as sites of marine protected areas. The spatial pattern of impacts reflects the depth of the pH perturbation and does not scale linearly with atmospheric CO2 concentration. Impacts may cause negative changes of the same magnitude or exceeding the current target of 10% of preservation of marine biomes set by the convention on biological diversity, implying that ocean acidification may offset benefits from conservation/management strategies relying on the regulation of resource exploitation.

The first symposium on “The Ocean in a High-CO2 World” in 2004 proved to be a landmark event in our understanding of the seriousness of ocean acidification, as reported in Oceanography (Cicerone et al., 2004). The scientific community reunited in 2008 for a second symposium on “The Ocean in a High-CO2 World.” During the four years between the two symposia, more scientific papers were published on the topic of ocean acidification than during the preceding 55 years. Ocean acidification is now widely cited in the press and is familiar to many nonscientists. Participants at the 2008 symposium identified new research priorities and stressed the importance of improving international coordination to facilitate agreements on protocols, methods, and data reporting in order to optimize limited resources by greater sharing of materials, facilities, expertise, and data. Despite major uncertainties, the research community must find ways to scale up understanding of individual organisms’ responses to provide meaningful predictions of ocean acidification’s effects on food webs, fisheries, marine ecosystems, coastal erosion, and tourism. Easy-to-understand information, such as simple indicators of change and of thresholds beyond which marine ecosystems will not recover, is also needed for management and policymaking.

International audience; Global-scale tracer simulations are typically made at coarse resolution without explicitly modelling eddies. Here we ask what role do eddies play in ocean uptake, storage, and meridional transport of transient tracers. We made global anthropogenic transient-tracer simulations in coarse-resolution (2°cosf×2°, ORCA2) and eddy-permitting (½°cosf×½°, ORCA05) versions of the ocean general circulation model OPA9. Our focus is on surface-to-intermediate waters of the southern extratropics where air-sea tracer fluxes, tracer storage, and meridional tracer transport are largest. Eddies have little effect on global and regional bomb ?14C uptake and storage. Yet for anthropogenic CO2 and CFC-11, refining the horizontal resolution reduced southern extratropical uptake by 25% and 28%, respectively. There is a similar decrease in corresponding inventories, which yields better agreement with observations. With higher resolution, eddies strengthen upper ocean vertical stratification and reduce excessive ventilation of intermediate waters by 20% between 60° S and 40° S. By weakening the residual circulation, i.e., the sum of Eulerian mean flow and the opposed eddy-induced flow, eddies reduce the supply of tracer-impoverished deep waters to the surface near the Antarctic divergence, thus reducing the air-sea tracer flux. Thus in the eddy permitting model, surface waters in that region have more time to equilibrate with the atmosphere before they are transported northward and subducted. As a result, the eddy permitting model's inventories of CFC-11 and anthropogenic CO2 are lower in that region because mixed-layer concentrations of both tracers equilibrate with the atmosphere on relatively short time scales (15 days and 6 months, respectively); conversely, bomb ?14C's air-sea equilibration time of 6 years is so slow that, even in the eddy permitting model, there is little time for surface concentrations to equilibrate with the atmosphere, i.e., before surface waters are subducted.