Ocean-Based Carbon Removal Risks Exacerbating Marine Oxygen Loss

Climate manipulations of atmospheric CO₂ have the inadvertent potential to worsen marine oxygen depletion, warns a study. Ocean warming has already depleted nearly 2% of the world ocean's oxygen budget in the recent decades and had implications for marine ecosystems.

Scientists working with Prof. Dr. Andreas Oschlies of GEOMAR Helmholtz Center used global model simulations to estimate the effect of various marine carbon dioxide removal (mCDR) strategies on dissolved oxygen levels. They found that biotic approaches such as ocean fertilization, large-scale sinking of macroalgae, and artificial upwelling can compel oxygen levels to be 4 to 40 times greater than the O₂ recoveries projected from constrained warming. "What is good for the climate is not necessarily good for the ocean," says Prof. Oschlies.

By contrast, geochemical mCDR strategies like the input of limestone-based alkaline substances have very small impacts on oceanic O₂ levels and represent the benefits of minimized direct emissions. Strikingly, macroalgae aquaculture with removal of harvested biomass yielded net increases in oxygen; model calculations suggest the technique might recover warming-induced oxygen losses by as much as an order of magnitude in a century, but might starve necessary nutrients.

With these compromises in mind, the authors suggest mandatory inclusion of oxygen monitoring in all future mCDR research and implementation to avoid adding more pressure to marine ecosystems through climate solutions.