Biological Responses to Ocean Acidification Are Changing the Global Ocean Carbon Cycle

Abstract

Increased oceanic uptake of CO₂ due to rising anthropogenic emissions has caused lowered pH levels (ocean acidification) that are hypothesized to diminish biotic calcification and reduce the export of total alkalinity (A_T) as carbonate minerals from the surface ocean or their burial in coastal sediments. This “CO₂-biotic calcification feedback” is a negative feedback on atmospheric CO₂, as elevated levels of surface A_T increase the ocean's capacity to uptake CO₂. We detect signatures of this feedback in the global ocean for the first time using repeat hydrographic measurements and seawater property prediction algorithms. Over the course of the past 30 years, we find an increase in global surface A_T of 0.072 ± 0.023 μmol kg⁻¹ yr⁻¹, which would have caused approximately 20 Tmol of additional A_T to accumulate in the surface ocean. This finding suggests that anthropogenic CO₂ emissions are measurably perturbing the cycling of carbon on a planetary scale by disrupting biological patterns. More observations of A_T would be required to understand the effects of this feedback on a regional basis and to fully characterize its potential to reduce the efficiency of marine carbon dioxide removal technology.