While the preindustrial ocean was assumed to be in equilibrium with the atmosphere, the modern ocean is a carbon sink, resulting from natural variability and anthropogenic perturbations, such as fossil fuel emissions and changes in riverine exports over the past two centuries. Here we use a suite of sensitivity experiments based on the ECCO-Darwin global-ocean biogeochemistry model to evaluate the response of air-sea CO2 flux and carbon cycling to present-day lateral fluxes of carbon, nitrogen, and silica. We generate a daily export product by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral fluxes from 5171 watersheds worldwide. From 2000 to 2019, carbon exports increase CO2 outgassing by 0.22 Pg C yr-1 via the solubility pump, while nitrogen exports increase the ocean sink by 0.17 Pg C yr-1 due to phytoplankton fertilization. On regional scales, exports to the Tropical Atlantic and Arctic Ocean are dominated by organic carbon, which originates from terrestrial vegetation and peats and increases CO2 outgassing (+10 and +20%, respectively). In contrast, Southeast Asia is dominated by nitrogen from anthropogenic sources, such as agriculture and pollution, leading to increased CO2 uptake (+7%). Our results demonstrate that the magnitude and composition of riverine exports, which are determined in part from upstream watersheds and anthropogenic perturbations, substantially impact present-day regional-to-global-ocean carbon cycling. Ultimately, this work stresses that lateral fluxes must be included in ocean biogeochemistry and Earth System Models to better constrain the transport of carbon, nutrients, and metals across the land-ocean-aquatic-continuum.

As a marine Carbon Dioxide Removal (mCDR) approach, Ocean Alkalinity Enhancement (OAE) is emerging as a viable method for removing anthropogenic CO2 emissions from the atmosphere to mitigate climate change. To achieve substantial carbon reduction using this method, OAE would need to be widespread and scaled-up across the global ocean. However, the efficiency of OAE varies substantially across a range of space-time scales and as such field deployments must be carefully planned to maximize efficiency and minimize logistical costs and risks. Here we develop a mCDR efficiency framework based on the data-assimilative ECCO-Darwin ocean biogeochemistry model, which examines two key factors over seasonal to multi-decadal timescales: 1) mCDR potential, which quantifies the CO2 solubility of the upper ocean; and 2) dynamical mCDR efficiency, representing the full-depth impact of ocean advection, mixing, and air-sea CO2 exchange. To isolate and quantify the factors that determine dynamical efficiency, we develop a reduced complexity 1-D model, rapid-mCDR, as a computationally-efficient tool for evaluation of mCDR efficiency. Combining the rapid-mCDR model with ECCO-Darwin allows for rapid characterization of OAE efficiency at any location globally. This research contributes to our understanding and optimization of OAE deployments (i.e., deploying experiments in the real-world ocean) as an effective mCDR strategy and elucidates the regional differences and mechanistic processes that impact mCDR efficiency. The modeling tools developed in this study can be readily employed by research teams and industry to plan and complement future field deployments and provide essential Monitoring, Reporting, and Verification (MRV).

The diverse range of mechanisms driving the Arctic amplification are not completely understood and, moreover, the role of the greenhouse gas methane in Arctic warming remains unclear. Strong sources of methane at the ocean seabed in the Barents Sea and other polar regions are well documented. Nevertheless, those data suggest that negligible amounts of methane fluxed from the seabed enter the atmosphere, with roughly 90% of the methane consumed by bacteria. The observations are taken during summer, which is favorable for collecting data but also characterized by a strongly-stratified water column. In winter the stratification weakens and after a breakdown of the pycnocline, convection, storms, and turbulent diffusion can mix the full-depth water column in high latitudes.TheMixed Layer Depth (MLD) in the ice-free Central/Southern Barents Sea is deepening and the ocean-atmosphere methane exchange increases.. An additional barrier for the air-sea flux is seasonally and interannually variable sea-ice cover in partially ice-covered seas. We present Thermal IR space-based spectrometer data between 2002 and 2019 that shows increased methane concentration anomalies over the Barents and Kara seas in winter months. The seasonal methane cycle amplitude north of the Kara Sea has more than doubled since the beginning of the century; this may be interpreted as an effect of sea-ice decline and/or an evidence for growth of seabed emissions. A progressing degradation of Arctic sea-ice cover may lead to increased methane flux and, through a positive feedback loop, to further warming.