Today, the ocean absorbs ~25% of the human-induced carbon emissions. Earth System Models (ESMs) indicate that the absorption increases by 0.79±0.07PgC per ppm of atmospheric CO2 increase (carbon-concentration feedback), but diminishes by -17.3±5.5PgC per degree of warming (carbon-climate feedback). Due to limited computational capacity, ESMs parameterize flows at scales smaller than their horizontal grid resolution, typically ~1°. We conduct simulations of global warming using increasingly finer horizontal resolutions (from 1° to 1/27°), with an ocean-biogeochemical model, in an idealized mid-latitude double-gyre circulation. Our findings demonstrate that these ocean carbon cycle feedbacks are highly influenced by resolution. This sensitivity primarily stems from how the overturning circulation’s mean state depends on resolution, as well as how it responds to global warming. Although being a fraction of the intricate response to climate change, it emphasizes the significance of an accurate representation of small-scale ocean processes to better constrain the future ocean carbon uptake.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.

The Cenomanian-Turonian period recorded one of the largest disruptions to the oxygen and carbon cycles, the Oceanic Anoxic Event 2 (OAE2, 94 Ma). This event is global, yet paleo-reconstructions document heterogeneous ocean oxygenation states and sedimentary carbon contents, both temporally and spatially, suggesting that several mechanisms are at play. To better understand the long-term controls on oceanic oxygen and the initial oxygenation conditions prevailing at the beginning of OAE2, we perform numerical simulations of the Cenomanian using the IPSCL-CM5A2 Earth System Model, which includes a marine biogeochemistry component. We examine the control of the biogeochemical states of the global and Central Atlantic oceans by the depth of the Central American Seaway (CAS). The simulations show that a vigorous ocean circulation existed during the Cenomanian and that dysoxia/anoxia was caused by paleogeography rather than by ocean stagnation. The existence of restricted basins, disconnected from the deep global circulation and supplied with oxygen-depleted waters from Oxygen Minimum Zones of the surrounding basins, played a key role in the development of dysoxic/anoxic regions. A comparison with redox-proxy data suggests that a deep connection existed between the Pacific and Central Atlantic prior to OAE2. A shallowing of the CAS may have contributed to the establishment of enhanced anoxia in the Central Atlantic during OAE2. The paleogeographic configuration and that of gateways and submarine topographic barriers appear as major long-term controllers of the oceanic circulation and oxygen distribution, leading to low-oxygen concentrations in extended parts of the ocean as prerequisite conditions for OAEs to occur.