Strong time dependence of ocean acidification mitigation by atmospheric carbon dioxide removal

In Paris in 2015, the global community agreed to limit global warming to well below 2 [Formula: see text]C, aiming at even 1.5 [Formula: see text]C. It is still uncertain whether these targets are sufficient to preserve marine ecosystems and prevent a severe alteration of marine biogeochemical cycles. Here, we show that stringent mitigation strategies consistent with the 1.5 [Formula: see text]C scenario could, indeed, provoke a critical difference for the ocean's carbon cycle and calcium carbonate saturation states. Favorable conditions for calcifying organisms like tropical corals and polar pteropods, both of major importance for large ecosystems, can only be maintained if CO[Formula: see text] emissions fall rapidly between 2025 and 2050, potentially requiring an early deployment of CO[Formula: see text] removal techniques in addition to drastic emissions reduction. Furthermore, this outcome can only be achieved if the terrestrial biosphere remains a carbon sink during the entire 21st century.