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As part of the second phase of the Regional Carbon Cycle Assessment and Processes project (RECCAP2), we present an assessment of the carbon cycle of the Atlantic Ocean, including the Mediterranean Sea, between 1985 and 2018 using global ocean biogeochemical models (GOBMs) and estimates based on surface ocean carbon dioxide (CO2) partial pressure (pCO2 products) and ocean interior dissolved inorganic carbon observations. Estimates of the basin-wide long-term mean net annual CO2 uptake based on GOBMs and pCO2 products are in reasonable agreement (-0.47 ± 0.15 PgC yr-1 and -0.36 ± 0.06 PgC yr-1, respectively), with the higher uptake in the GOBM-based estimates likely being a consequence of a deficit in the representation of natural outgassing of land derived carbon. In the GOBMs, the CO2 uptake increases with time at rates close to what one would expect from the atmospheric CO2 increase, but pCO2 products estimate a rate twice as fast. The largest disagreement in the CO2 flux between GOBMs and pCO2 products is found north of 50°N, coinciding with the largest disagreement in the seasonal cycle and interannual variability. The mean accumulation rate of anthropogenic CO2 (Cant) over 1994-2007 in the Atlantic Ocean is 0.52 ± 0.11 PgC yr-1 according to the GOBMs, 28% ± 20% lower than that derived from observations. Around 70% of this Cant is taken up from the atmosphere, while the remainder is imported from the Southern Ocean through lateral transport. [ABSTRACT FROM AUTHOR]

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). In this second phase of the Regional Carbon Cycle Assessment and Processes (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 year−1, 1998–2018, coastal ocean extending to 300 km offshore or 1,000 m isobath with 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 coastal ocean CO2 sink has increased in the past decades but the available time‐resolving observation‐based products and models show large discrepancies in the magnitude of this increase. The global coastal ocean is a major source of N2O (+0.70 PgCO2‐e year−1 in observational product and +0.54 PgCO2‐e year−1 in model median) and CH4 (+0.21 PgCO2‐e year−1 in observational product), which offsets a substantial proportion of the coastal CO2 uptake in the net radiative balance (30%–60% in CO2‐equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate. Plain Language Summary: The coastal ocean regulates greenhouse gases. It acts as a sink of carbon dioxide (CO2) but also releases nitrous oxide (N2O) and methane (CH4) into the atmosphere. This synthesis contributes to the second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2) and provides a comprehensive view of the coastal air‐sea fluxes of these three greenhouse gases at the global scale. We use a multi‐faceted approach combining gap‐filled observation‐based products and ocean biogeochemical models. We show that the global coastal ocean is a net sink of CO2 in both observational products and models, but the coastal uptake of CO2 is ∼60% larger in models than in observation‐based products due to model‐product differences in seasonality. The coastal CO2 sink is strengthening but the magnitude of this strengthening is poorly constrained. We also find that the coastal emissions of N2O and CH4 counteract a substantial part of the effect of coastal CO2 uptake in the atmospheric radiative balance (by 30%–60% in CO2‐equivalents), highlighting the need to consider these three gases together to understand the influence of the coastal ocean on climate. Key Points: We synthesize air‐sea fluxes of CO2, nitrous oxide and methane in the global coastal ocean using observation‐based products and ocean modelsThe coastal ocean CO2 sink is 60% larger in ocean models than in observation‐based products due to systematic differences in seasonalityCoastal nitrous oxide and methane emissions offset 30%–60% of the CO2 coastal uptake in the net radiative balance [ABSTRACT FROM AUTHOR]

The Southern Ocean is a major sink of atmospheric CO2, but the nature and magnitude of its variability remains uncertain and debated. Estimates based on observations suggest substantial variability that is not reproduced by process-based ocean models, with increasingly divergent estimates over the past decade. We examine potential constraints on the nature and magnitude of climate-driven variability of the Southern Ocean CO2 sink from observation-based air–sea O2 fluxes. On interannual time scales, the variability in the air–sea fluxes of CO2 and O2 estimated from observations is consistent across the two species and positively correlated with the variability simulated by ocean models. Our analysis suggests that variations in ocean ventilation related to the Southern Annular Mode are responsible for this interannual variability. On decadal time scales, the existence of significant variability in the air–sea CO2 flux estimated from observations also tends to be supported by observation-based estimates of O2 flux variability. However, the large decadal variability in air–sea CO2 flux is absent from ocean models. Our analysis suggests that issues in representing the balance between the thermal and non-thermal components of the CO2 sink and/or insufficient variability in mode water formation might contribute to the lack of decadal variability in the current generation of ocean models. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were carried out on the systematic assessment of the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. Mean-state assessments routinely compared model hindcasts to available modern biogeochemical observations. However, these assessments considered neither the extent of equilibrium in modeled biogeochemical reservoirs nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESM) contribute to model-to-model differences in the simulated fields. We take advantage of a 500 year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and is consistent when confronted against a larger ensemble of CMIP5 models. This shows that drift has implications on their performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercices in order to provide realistic ESM results on marine biogeochemistry and carbon cycle feedbacks. [ABSTRACT FROM AUTHOR]

This model study addresses the change in pelagic calcium carbonate production (CaCO3, as calcite in the model) and dissolution in response to rising atmospheric CO2. The parameterization of CaCO3 production includes a dependency on the saturation state of seawater with respect to calcite. It was derived from laboratory and mesocosm studies on particulate organic and inorganic carbon production in Emiliania huxleyi as a function of pCO2. The model predicts values of CaCO3 production and dissolution in line with recent estimates. The effect of rising pCO2 on CaCO3production and dissolution was quantified by means of model simulations forced with atmospheric CO2 increasing at a rate of 1% per year from 286 ppm to 1144 ppm over a 140 year time-period. The simulation predicts a decrease of CaCO3 production by 27%. The combined change in production and dissolution of CaCO3 yields an excess uptake of CO2 from the atmosphere by the ocean of 5.9 GtC over the period of 140 years. [ABSTRACT FROM AUTHOR]

This study focuses on an improved representation of the biological soft tissue pump in the global three-dimensional biogeochemical ocean model PISCES. We compare three parameterizations of particle dynamics: (1) the model standard version including two particle size classes, aggregation-disaggregation and prescribed sinking speed; (2) an aggregation-disaggregation model with a particle size spectrum and prognostic sinking speed; (3) a mineral ballast parameterization with no size classes, but prognostic sinking speed. In addition, the model includes a description of surface sediments and organic carbon early diagenesis. The integrated representation of material fluxes from the productive surface ocean down to the sediment-water interface allows taking advantage of surface ocean observations, sediment trap data and exchange fluxes at the sediment-water interface. The capability of the model to reproduce yearly averaged particulate organic carbon fluxes and benthic oxygen demand does at first order not dependent on the resolution of the particle size spectrum. Model results obtained with the standard version and with the one including a particle size spectrum and prognostic sinking speed are not significantly different. Both model versions overestimate particulate organic carbon between 1000 and 2000 m, while deep fluxes are of the correct order of magnitude. Predicted benthic oxygen fluxes correspond with respect to their large scale distribution and magnitude to data based estimates. Modeled particulate organic C fluxes across the mesopelagos are most sensitive to the intensity of 20 zooplankton flux feeding. An increase of the intensity of flux feeding in the standard version results in lower mid- and deep-water particulate organic carbon fluxes, shifting model results to an underestimation of particulate organic carbon fluxes in the deep. The corresponding benthic oxygen fluxes are too low. The model version including the mineral ballast parameterization yields an improved fit between modeled and observed particulate organic carbon fluxes below 2000m and down to the sediment-water interface. Our results suggest that aggregate formation alone might not be sufficient to drive an intense biological pump. The later is most likely driven by the combined effect of aggregate formation and mineral ballasting. [ABSTRACT FROM AUTHOR]