The North Atlantic subpolar gyre is a key region for the North Atlantic phytoplankton bloom (NAB), the foundation of the regional foodweb. The NAB is dependent on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, and the NAB may “collapse” as a result, representing a potential “tipping point” in the Earth system. We investigate change in winter mixing and the impacts on the SPG and the broader northern North Atlantic using 1. a spread of future projections from a low-resolution Earth system model (UKESM) and 2. a single, high-warming projection of a high-resolution ocean-only configuration of the same model (NEMO-MEDUSA). For both models we find significant declines in the strength of the NAB during the 21st century. In UKESM, this occurred across all projections, but with low spatiotemporal coherence. In NEMO-MEDUSA, changes in upper mixed layer depth, surface nutrients and chlorophyll concentrations were noticeably abrupt and more highly spatiotemporally-correlated. We also find a large (>30 day) phenological shift in the peak of the bloom aligned with the timing of this change, which may affect foodweb dynamics. Overall, defining “collapse” as a halving of surface chlorophyll, we find that the NAB collapses by the end of the century regardless of future projection. However, the timing, abruptness and coherence of this collapse differs in high and low resolution models, suggesting the need for higher resolution for prediction of abrupt and irreversible changes, especially those involving ecosystem dynamics.

The Paris Climate Accords plan for “net-zero” carbon dioxide (CO2) by 2050. However, reducing emissions from some sectors is challenging, and “net-zero” permits carbon dioxide removal (CDR) activities. One CDR scheme is ocean alkalinity enhancement (OAE), which proposes to dissolve basic minerals into seawater to increase its total alkalinity (TA) and buffering capacity for CO2. While modelling studies have often investigated OAE by adding TA to the ocean’s surface at basin or global scale, some proposals focus on readily-accessible coastal shelves, with TA added through the dissolution of olivine sands. Critically, by settling and dissolving sands on shallow seafloors, this retains the added TA in near-surface waters in direct contact with atmospheric CO2. To investigate this, we add dissolved TA to the global shelves (<100m) of an Earth system model (UKESM1) running a high emissions scenario. As UKESM1 is fully-coupled, wider effects of OAE-mediated increase in ocean CO2 uptake –e.g. atmospheric xCO2, air temperature and marine pH– are fully-quantified. Applying OAE from 2020-2100 decreases atmospheric xCO2 ~10 ppm, and increases air-to-sea CO2 uptake ~8%. Due to advection of added TA, ~50% of this uptake occurs remotely from OAE operations. In-line with other studies, CO2 uptake per unit of TA added occurs at a rate of ~0.8~mol~C~(mol~TA)$^{-1}$, though this is elevated in enclosed regions. Meanwhile, changes in air temperature and marine pH are indistinguishable from natural variability. While practical uncertainties and model representation caveats remain, this analysis estimates the effectiveness of this specific OAE scheme to assist with net-zero planning.