A document by Lina Garcia-Suarez. Click on the document to view its contents.

Ocean alkalinity enhancement (OAE) can potentially remove gigatons of CO2 from the atmosphere for durable storage in the ocean. Before implementing OAE at climate-relevant scales, questions about its safety and verifiability must be addressed. Operational deployment poses a dilemma between pursuing large detectability, essential for effective monitoring, reporting, and verification (MRV), and ensuring environmental safety and satisfying regulatory requirements. In this study, we present a computationally efficient approach, based on a high-resolution, coupled circulation-dissolution model of Halifax Harbour, to simulating the addition, transportation, dissolution, and sinking of various theoretical alkaline feedstocks for different dosages, seasons, and addition sites. Detectability and exposure risk of OAE are quantified and an approach for optimizing OAE deployment is demonstrated. Mean residence times (MRT) are calculated for different subregions and seasons. Results show that for a given amount of feedstock, summer is more favourable from the perspective of detectability but also creates higher exposure risks than other seasons because of a longer MRT. The exposure risk can be mitigated while maintaining large detectability by choosing optimal feedstocks with different characteristics for different seasons. The exposure risk can also be reduced by spreading alkalinity over multiple addition sites. The optimum allocation, where the largest detectability is sought without violating regulatory requirements, is specific to each season, dosage, and choice of feedstock. OAE deployments should be tailored taking into account local hydrography, season, dosage, and feedstock characteristics. Our approach provides a practical avenue for optimizing deployments.

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.