Since the Industrial Revolution, the ocean has absorbed a cumulative ~40% of the anthropogenic carbon (Cant) released into the atmosphere by fossil fuel emissions. Cant accumulation in the upper ocean has driven an increase in the partial pressure of carbon dioxide gas (pCO2) and associated declines in pH and carbonate ion concentration. These chemical changes, collectively referred to as ocean acidification (OA), progressively weaken the ocean’s buffer capacity and reflect the evolution of a positive marine carbon cycle feedback that reduces the efficiency of future Cant uptake and amplifies the influence of natural variability on the carbonate system. This dissertation investigates the spatial and temporal changes in the ocean carbon cycle caused by Cant using a combination of in situ observations, data synthesis products, and output from regional and global ocean models to improve our understanding of the processes governing the ocean carbon sink and its evolving feedbacks. Chapter 1 evaluates the impact of Cant accumulation on multiple OA metrics throughout the water column in the North Pacific Ocean and California Current Large Marine Ecosystem using ship-based observations. Results indicate that the greatest increases in pCO2 occur subsurface, where Cant content is moderate and pCO2 change can exceed overlying surface change by ≥100%. Amplified pCO2 responses in the interior ocean are related to background ocean carbonate chemistry, with the greatest subsurface changes associated with poorly buffered waters that have experienced substantial organic matter remineralization. Chapter 2 evaluates the impact of Cant on the seasonal variability of pCO2 in the surface ocean using output from global ocean biogeochemical models (GOBMs) used by global carbon budgeting efforts to estimate the historical ocean carbon sink strength. Results indicate that dissimilar model representations of surface ocean pCO2 seasonality, particularly during winter, lead to increasing disagreement in annual ocean carbon sink strength estimates over time. Chapter 3 examines how differences in representations of interior ocean Cant and natural carbon influence patterns of amplified subsurface pCO2 change using the same set of GOBMs, in addition to observation-based data products. Results indicate that GOBMs dissimilarly simulate subsurface Cant-induced pCO2 changes, particularly at the depth of maximum winter mixing, when these signals can re-emerge at the surface and bias estimates of the annual ocean carbon sink strength. This research contributes to ongoing international efforts to better constrain the global ocean carbon sink. Discrepancies between observation- and model-based estimates of the modern ocean carbon sink have grown over time, with across-model disagreements compounding in future climate projections. This points to an outstanding need to constrain sources of model discrepancies. This work helps to address this by clarifying: (1) a model’s projected end-of-century ocean carbon sink magnitude is highly dependent on its post-spin-up seasonal and annual mean-state; (2) a more realistic representation of interior ocean carbon distributions and ecosystem processes is needed to achieve a more realistic representation of ocean carbon cycle change and the evolution of its feedbacks.
Arroyo M. C., 2025. The evolution of ocean carbon cycle feedbacks in observations and models. PhD thesis, University of California, Santa Cruz. 175 p. Thesis.
