Mengyang Zhou

Induction of a CO ₂ partial pressure ( p CO ₂ ) deficit in the surface-ocean through ocean alkalinity enhancement (OAE) or direct ocean removal (DOR) methods has been recognized as a promising approach to meet the projected need for negative CO ₂ emissions. The difficulty of directly measuring the counterfactual CO ₂ flux due to rapid spreading of the DIC-deficient plume has put ocean circulation models in the center of the Measurement, Reporting and Verification (MRV) challenge. Confidence in the results of such models is essential for the emerging industry to access carbon credit markets and grow at the required pace, to reach substantial negative emissions by 2050, as envisioned by the Intergovernmental Panel on Climate Change (IPCC). The kinetics and equilibration time of such a DIC deficit have been shown to vary substantially depending on the location and season of the initial induction point. A major component of this variance is the vertical transport and mixing of the DIC-deficient plume; however, air-sea CO ₂ gas exchange and carbonate chemistry are also important. Currently, it is poorly understood how much the results of OAE pulse simulations depend on the models chosen. To help close this knowledge gap, we investigate two global circulation models, the CESM2/MARBL model (1°) and the data-assimilative ECCO-Darwin model (1/3°). We perform pulse injection simulations at twelve locations with both models, matched precisely in terms of injection patch geometry, release year and season. We analyze the differences in CO ₂ uptake curves, vertical mixing, gas exchange and carbonate chemistry. We show that in some locations, such as subtropical regions, substantial differences exist between these two models – well beyond the expected intrinsic variation of each model. Furthermore, we demonstrate that the majority of the differences are attributable to the representation of vertical transport, especially mixed layer depth, followed by the effect of wind parameterizations; a small amount of difference is attributable to carbonate chemistry parameterization. In some locations, there exists good agreement between the models. In most injection locations, the largest differences between models are found in the first 7 years post alkalinity injection; in many this is followed by slow convergence towards the expected theoretical maximums.

Induction of a surface-ocean DIC (dissolved organic carbon) deficit through alkalinity-based or direct CO₂ removal methods has been recognized as a promising approach to meet the projected need for negative emissions. The difficulty of directly measuring the counter-factual CO₂ flux due to rapid spreading of the DIC-deficient plume has put ocean circulation models in the center of the Measurement, Reporting and Verification (MRV) challenge. Confidence in the results of such models is essential for the emerging industry to access carbon credit markets and grow at the required pace, to reach substantial negative emissions by 2050, as envisioned by the Intergovernmental Panel on Climate Change (IPCC). The kinetics and equilibration time of such a DIC deficit have been shown to vary substantially depending on the location and season of the initial induction point. A major component of this variance is the vertical transport and mixing of the DIC-deficient plume; however, air-sea CO₂ gas exchange and carbonate chemistry are also important. Currently, it is poorly understood how much the results of DIC-deficit pulse simulations depend on the models chosen. To help close this knowledge gap, we investigate two global circulation models, the CESM2/MARBL model (1°) and the data-assimilative ECCO-Darwin model (1/3°). We perform pulse injection simulations at twelve locations with both models, matched precisely in terms of injection patch geometry, release year and season. We analyze the differences in CO₂ uptake curves, vertical mixing, gas exchange and carbonate chemistry. We show that in some locations, such as subtropical regions, substantial differences exist between these two models — well beyond the expected intrinsic variation of each model. Furthermore, we demonstrate that the majority of the differences are attributable to the representation of vertical transport, followed by the effect of wind parameterizations. A small amount of difference is attributable to carbonate chemistry parameterization. In some locations, there exists good agreement between the models. In most injection locations, the largest differences between models are found in the first 7 years post alkalinity injection, followed by slow convergence towards the expected theoretical maximums.

Limiting global warming to 2 °C by the end of the century requires dramatically reducing CO₂ emissions, and also implementing carbon dioxide removal (CDR) technologies. Ocean-based CDR through ocean alkalinity enhancement (OAE) offers a particularly scalable and promising pathway. However, quantifying carbon removal achieved by OAE deployments is challenging because it requires determining air-to-sea CO₂ transfer over large spatiotemporal scales – and there is the possibility that ocean circulation will remove alkalinity from the surface ocean before complete equilibration. This challenge makes it difficult to establish robust accounting frameworks suitable for an effective carbon market. Here, we propose using impulse response functions (IRFs) to address such challenges. We perform model simulations of a short-duration alkalinity release (the “impulse”), compute the resultant air-sea CO₂ flux as a function of time, and generate a characteristic carbon uptake curve for the given location (the IRF). Applying the IRF method requires a linear and time-invariant system. We attempt to meet these conditions by using small alkalinity forcing values and creating an IRF ensemble accounting for seasonal variability. The IRF ensemble is used to predict carbon uptake for an arbitrary-duration alkalinity release. We test whether the IRF approach provides a reasonable approximation by performing OAE simulations in a global ocean model at locations that span a variety of dynamical and biogeochemical regimes. We find that the IRF prediction can typically reconstruct the carbon uptake in continuous-release simulations in our model within several percent error. Our simulations elucidate the influences of oceanic variability and deployment duration on carbon uptake efficiency. We discuss the strengths and possible shortcomings of the IRF approach as a basis for quantification and uncertainty assessment of ocean-based CDR, facilitating its potential for adoption as a component of the carbon removal market’s standard approach to Monitoring, Reporting, and Verification (MRV).

Induction of a surface-ocean DIC (dissolved organic carbon) deficit through alkalinity-based or direct CO₂ removal methods has been recognized as a promising approach to meet the projected need for negative emissions. The difficulty of directly measuring the counter-factual CO₂ flux due to rapid spreading of the DIC-deficient plume has put ocean circulation models in the center of the Measurement, Reporting and Verification (MRV) challenge. Confidence in the results of such models is essential for the emerging industry to access carbon credit markets and grow at the required pace, to reach substantial negative emissions by 2050, as envisioned by the Intergovernmental Panel on Climate Change (IPCC). The kinetics and equilibration time of such a DIC deficit have been shown to vary substantially depending on the location and season of the initial induction point. A major component of this variance is the vertical transport and mixing of the DIC-deficient plume; however, air-sea CO₂ gas exchange and carbonate chemistry are also important. Currently, it is poorly understood how much the results of DIC-deficit pulse simulations depend on the models chosen. To help close this knowledge gap, we investigate two global circulation models, the CESM2/MARBL model (1°) and the data-assimilative ECCO-Darwin model (1/3°). We perform pulse injection simulations at twelve locations with both models, matched precisely in terms of injection patch geometry, release year and season. We analyze the differences in CO₂ uptake curves, vertical mixing, gas exchange and carbonate chemistry. We show that in some locations, such as subtropical regions, substantial differences exist between these two models — well beyond the expected intrinsic variation of each model. Furthermore, we demonstrate that the majority of the differences are attributable to the representation of vertical transport, followed by the effect of wind parameterizations. A small amount of difference is attributable to carbonate chemistry parameterization. In some locations, there exists good agreement between the models. In most injection locations, the largest differences between models are found in the first 7 years post alkalinity injection, followed by slow convergence towards the expected theoretical maximums.

Induction of a surface-ocean DIC (dissolved organic carbon) deficit through alkalinity-based or direct CO₂ removal methods has been recognized as a promising approach to meet the projected need for negative emissions. The difficulty of directly measuring the counter-factual CO₂ flux due to rapid spreading of the DIC-deficient plume has put ocean circulation models in the center of the Measurement, Reporting and Verification (MRV) challenge. Confidence in the results of such models is essential for the emerging industry to access carbon credit markets and grow at the required pace, to reach substantial negative emissions by 2050, as envisioned by the Intergovernmental Panel on Climate Change (IPCC). The kinetics and equilibration time of such a DIC deficit have been shown to vary substantially depending on the location and season of the initial induction point. A major component of this variance is the vertical transport and mixing of the DIC-deficient plume; however, air-sea CO₂ gas exchange and carbonate chemistry are also important. Currently, it is poorly understood how much the results of DIC-deficit pulse simulations depend on the models chosen. To help close this knowledge gap, we investigate two global circulation models, the CESM2/MARBL model (1°) and the data-assimilative ECCO-Darwin model (1/3°). We perform pulse injection simulations at twelve locations with both models, matched precisely in terms of injection patch geometry, release year and season. We analyze the differences in CO₂ uptake curves, vertical mixing, gas exchange and carbonate chemistry. We show that in some locations, such as subtropical regions, substantial differences exist between these two models — well beyond the expected intrinsic variation of each model. Furthermore, we demonstrate that the majority of the differences are attributable to the representation of vertical transport, followed by the effect of wind parameterizations. A small amount of difference is attributable to carbonate chemistry parameterization. In some locations, there exists good agreement between the models. In most injection locations, the largest differences between models are found in the first 7 years post alkalinity injection, followed by slow convergence towards the expected theoretical maximums.

St Helena Bay (SHB), a retentive zone in the productive southern Benguela Upwelling System off western South Africa, experiences seasonal hypoxia and episodic anoxic events that threaten local fisheries. To understand the drivers of oxygen variability in SHB, we queried 25 years of dissolved oxygen (DO) observations alongside high‐resolution wind and hydrographic data, and dynamical data from a high‐resolution model. At 70 m in SHB (mid‐bay), upwelling‐favorable winds in spring drove replenishment of cold, oxygenated water. Hypoxia developed in summer, becoming most severe in autumn. Bottom waters in autumn were replenished with warmer, less oxygenated water than in spring—suggesting a seasonal change in source waters upwelled into the bay. Downwelling and deep mixing in winter ventilated mid‐bay bottom waters, which reverted to hypoxic conditions during wind relaxations and reversals. In the nearshore (20 m), hypoxia occurred specifically during periods of upwelling‐favorable wind stress and was most severe in autumn. Using a statistical model, we extended basic hydrographic observations to nitrate and DO concentrations and developed metrics to identify the accumulation of excess nutrients on the shelf and nitrogen‐loss to denitrification, both of which were most prominent in autumn. A correspondence of the biogeochemical properties of hypoxic waters at 20 m to those at 70 m implicates the latter as the source waters upwelled inshore in autumn. We conclude that wind‐driven upwelling drives the replenishment of respired bottom waters in SHB with oxygenated waters, noting that less‐oxygenated water is imported later in the upwelling season, which exacerbates hypoxia.

We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and anticyclonic eddies near Hawai'i in the North Pacific Subtropical Gyre (NPSG) and explored mechanisms that sustain productivity in the cyclone after the initial intensification stage. The top of the nutricline was uplifted into the euphotic zone in the cyclone and depressed in the anticyclone. Subsurface nutrient concentrations and apparent oxygen utilization at the cyclone's inner periphery were higher than expected from isopycnal displacement, suggesting that shallow remineralization of organic material generated excess nutrients in the subsurface. The excess nutrients may provide a supply of subsurface nutrients to sustain productivity in maturing eddies. The shallow remineralization also raises questions regarding the extent to which cyclonic eddies promote deep carbon sequestration in subtropical gyres such as the NPSG. An upward increase in nitrate ¹⁵N/¹⁴N isotope ratios below the euphotic zone, indicative of partial nitrate assimilation, coincided with negative preformed nutrients—potentially signaling heterotrophic bacterial consumption of carbon‐rich (nitrogen‐poor) organic material. The ¹⁵N/¹⁴N of material collected in shallow sediment traps was significantly higher in the cyclone than in the anticyclone and showed correspondence to the ¹⁵N/¹⁴N ratio of the nitrate supply, which is acutely sensitive to sea level anomaly in the region. A number of approaches were applied to estimate the contribution of N₂ fixation to export production. Results among approaches were inconsistent, which we attribute to non‐steady state conditions during our observation period.

To limit global warming to below 2°C by 2100, carbon dioxide removal (CDR) from the atmosphere will be necessary. Ocean alkalinity enhancement (OAE) is a promising approach to achieving CDR at a large scale. However, OAE deployments are subject to slow or incomplete air-sea CO2 exchange, reducing the efficiency of carbon removal, defined as the excess CO2 uptake per mol of alkalinity addition. We used a coupled ocean circulation-biogeochemistry model to generate the first global, time-resolved map of OAE efficiency across four different seasons and investigated its controlling factors. An ensemble of alkalinity pulse injections in the global ocean were simulated with the global 1-degree ocean component of the Community Earth System Model version 2 (CESM2). Alkalinity was added to the surface ocean for 1 month in a total of 690 patches and in 4 different seasons of a year. Each simulation was run for 15 years for each patch and season to compute OAE efficiency, residence time of excess alkalinity retained in the mixed layer, and CO2 re-equilibration timescales - all referenced to the geographic location of the induced perturbation. OAE efficiency showed large spatial and seasonal variations. The highest seasonal mean OAE efficiency achieved after 15 years, ranging from 0.7 to 0.9, were found in the subpolar oceans, the semi-closed regions, such as the Gulf of St. Lawrence and the North Sea, as well as the coastal zones along the Pacific and South Atlantic. The lowest seasonal mean, ranging from 0.3 to 0.5, was found in the high latitudes Atlantic and Southern Ocean where deep water forms. The intermediate values, ranging from 0.5 to 0.7, were found predominantly in the subtropical gyres, as well as western and eastern boundary currents. Seasonally, higher maximum OAE efficiency could generally be achieved when alkalinity is released in the summer rather than in winter. Accurate understanding of the CO2 response curves, as provided by our maps, is critical for choosing suitable OAE deployment sites and is central to the MRV (Measurement, Reporting & Verification) challenge faced by all marine CDR methods.

Limiting global warming to 2 °C by the end of the century requires dramatically reducing CO₂ emissions, and also implementing carbon dioxide removal (CDR) technologies. A promising avenue is marine CDR through ocean alkalinity enhancement (OAE). However, quantifying carbon removal achieved by OAE deployments is challenging because it requires determining air-to-sea CO₂ transfer over large spatiotemporal scales–and there is the possibility that ocean circulation will remove alkalinity from the surface ocean before complete equilibration. This challenge makes it difficult to establish robust accounting frameworks suitable for an effective carbon market. Here, we propose using impulse response functions (IRFs) to address such challenges. We perform model simulations of a short-duration alkalinity release (the “impulse”), compute the resultant air-sea CO₂ flux as a function of time, and generate a characteristic carbon uptake curve for the given location (the IRF). Applying the IRF method requires a linear and time-invariant system. We attempt to meet these conditions by using small alkalinity forcing values and creating an IRF ensemble accounting for seasonal variability. The IRF ensemble is then used to predict carbon uptake for an arbitrary-duration alkalinity release at the same location. We test whether the IRF approach provides a reasonable approximation by performing OAE simulations in a global ocean model at locations that span a variety of dynamical and biogeochemical regimes. We find that the IRF prediction can typically reconstruct the carbon uptake in continuous-release simulations within several percent error. Our simulations elucidate the influences of oceanic variability and deployment duration on carbon uptake efficiency. We discuss the strengths and possible shortcomings of the IRF approach as a basis for quantification and uncertainty assessment of OAE, facilitating its potential for adoption as a component of the carbon removal market’s standard approach to Monitoring, Reporting, and Verification (MRV).