Nagwekar, TanviTanviNagwekar2025-08-182025-08-182025-04-24https://media.suub.uni-bremen.de/handle/elib/22495https://doi.org/10.26092/elib/4364Anthropogenic CO2 emissions have driven global warming since the Industrial Revolution, necessitating a phasing out of fossil fuels to limit warming to well below 2degC, preferably 1.5degC, as outlined in the Paris Agreement. However, emission reductions alone are unlikely to be sufficient to meet these targets, highlighting the need for large-scale carbon dioxide removal (CDR). While current CDR efforts primarily focus on land-based methods, challenges, such as land competition and biodiversity loss have led to growing interest in ocean-based approaches as a complementary solution. Ocean Alkalinity Enhancement (OAE) is one such ocean-based CDR method with high theoretical carbon sequestration potential that enhances CO2 uptake by shifting carbonate equilibria in the surface ocean. The ocean has absorbed ~25% of anthropogenic CO2 since the Industrial Revolution and thus plays a crucial role in the global carbon cycle. Subduction regions in the Southern Ocean and North Atlantic are particularly important for anthropogenic carbon uptake as they transport carbon into the deep ocean, sequestering it for centuries to millennia. Building on this natural process, I hypothesize that deploying OAE in subduction regions will enhance deep ocean sequestration and OAE efficiency. To test this, OAE is simulated in subduction regions and globally using a low-resolution ocean circulation and biogeochemistry model (Publication I), a fully coupled emission-driven Earth System Model (ESM; Publication II), and a high-resolution ocean circulation and biogeochemistry model (Publication III). This thesis identifies three key factors influencing simulated OAE efficiency: the amount of added alkalinity, climate feedbacks, and model resolution. Among these, alkalinity addition is the primary driver, which exhibits a strong linear relationship with oceanic CO2 uptake and atmospheric CO2 reduction after 60-70 years of OAE deployment, a relationship that is consistent across emission scenarios and aligns with previous literature. However, in contrast to this scaling observed after decades of alkalinity addition, the largest differences and highest uncertainties in OAE efficiency occur in the initial decade of deployment (2030s). During this period, OAE efficiency in subduction regions (0.60 in Publication I, 0.70 in Publication III) surpasses global OAE in ocean-only models (0.56 in Publication I, 0.57 in Publication III). In contrast, the ESM-based regional OAE simulation (Publication II) yields a lower efficiency of 0.38, with a ±55% uncertainty attributed to climate-feedback driven variability. As alkalinity perturbations disperse over time, regional differences diminish, reducing uncertainty and leading to a convergence of efficiency estimates between global and regional applications by the 2090s, with values of 0.85 (global and regional, Publication I), 0.72 (global and regional, Publication II), and 0.85-0.90 (global-regional; Publication III). The persistently lower efficiency in Publication II underscores the role of climate feedbacks as the second most influential factor in OAE efficiency. Model resolution has only a third-order impact. Notably, Publication III reports the highest regional OAE efficiency in the 2030s, potentially due to a combination of different dynamics in the high-resolution model and differences in the background state of carbonate chemistry across model setups, which further contributes to early-phase uncertainty. Further, both high- and low-resolution ocean-only models demonstrate efficient deep-ocean carbon sequestration, with high-resolution global and regional simulations storing a relatively higher amount of carbon below 1 km throughout the simulation period than their low-resolution counterparts. Moreover, these models indicate that subduction regions transport nearly twice as much carbon to depths below 1 km as global OAE in their respective setups, although their smaller surface area leads to a lower total dissolved inorganic carbon (DIC) increase. In contrast, the regional OAE using the ESM model (Publication II) does not replicate such efficient deep transport, largely due to internal climate variability and feedbacks. The ocean-only simulations also reveal that subduction regions can be viable for OAE, but exhibit strong seasonal variability in excess carbon uptake, surface alkalinity, and DIC accumulation, driven by the seasonality of the mixed layer depth (MLD). A shallower summer MLD retains excess surface alkalinity, allowing CO2-deficient waters to equilibrate with the atmosphere and enhance CO2 uptake and DIC accumulation, while a deeper winter MLD increases mixing, leading to alkalinity loss and reduced CO2 uptake. Therefore, to optimize OAE efficiency in regional deployments, future strategies must account for these seasonal dynamics. Based on the findings of this thesis, model selection for OAE research should align with the specific goals: low-resolution models capture large-scale ocean processes, high-resolution models can be useful to resolve small-scale dynamics that are potentially important for deep ocean carbon sequestration, and ESMs incorporate Earth system feedbacks vital for assessing how CDR deployment can affect the climate trajectory with respect to climate targets. Most importantly, as all model setups in this study exhibit the highest uncertainty during the initial phase of OAE deployment, future modeling efforts should focus on understanding uncertainties in estimates of OAE efficiency in the early phases of regional applications, which are crucial for developing a robust Monitoring, Reporting, and Verification (MRV) framework.enhttps://creativecommons.org/licenses/by/4.0/Ocean Alkalinity EnhancementSubduction RegionsCarbon Dioxide RemovalCarbon SequestrationCarbon and Climate FeedbacksEarth System ModelingOcean-only ModelingDissertation10.26092/elib/4364urn:nbn:de:gbv:46-elib224957