Simulated 21st century's increase in oceanic suboxia by CO ₂ ‐enhanced biotic carbon export

<jats:p> The primary impacts of anthropogenic CO <jats:sub>2</jats:sub> emissions on marine biogeochemical cycles predicted so far include ocean acidification, global warming induced shifts in biogeographical provinces, and a possible negative feedback on atmospheric CO <jats:sub>2</jats:sub> levels by CO <jats:sub>2</jats:sub> ‐fertilized biological production. Here we report a new potentially significant impact on the oxygen‐minimum zones of the tropical oceans. Using a model of global climate, ocean circulation, and biogeochemical cycling, we extrapolate mesocosm‐derived experimental findings of a <jats:italic>p</jats:italic> CO <jats:sub>2</jats:sub> ‐sensitive increase in biotic carbon‐to‐nitrogen drawdown to the global ocean. For a simulation run from the onset of the industrial revolution until A.D. 2100 under a “business‐as‐usual” scenario for anthropogenic CO <jats:sub>2</jats:sub> emissions, our model predicts a negative feedback on atmospheric CO <jats:sub>2</jats:sub> levels, which amounts to 34 Gt C by the end of this century. While this represents a small alteration of the anthropogenic perturbation of the carbon cycle, the model results reveal a dramatic 50% increase in the suboxic water volume by the end of this century in response to the respiration of excess organic carbon formed at higher CO <jats:sub>2</jats:sub> levels. This is a significant expansion of the marine “dead zones” with severe implications not only for all higher life forms but also for oxygen‐sensitive nutrient recycling and, hence, for oceanic nutrient inventories. </jats:p>