Biological responses to ocean acidification are changing the global ocean carbon cycle

Key Points

● We find evidence of a long-term trend in alkalinity in the surface ocean using measurements and seawater property prediction algorithms. 

● We determine this trend is likely driven by the CO2-biotic calcification feedback, a hypothesized negative feedback on atmospheric CO2. 

● Ocean chemistry is changing globally due to shifting patterns in the ecology of calcifying organisms caused by anthropogenic CO2 emissions.

Plain Language Summary 

Human activities are causing more carbon dioxide (CO2) to be absorbed by the oceans from the atmosphere, leading to decreasing ocean pH levels (ocean acidification). Acidification slows down biotic calcification, the process by which many marine organisms build their shells and skeletons. Lowered biotic calcification is hypothesized to reduce the carbon moved from the ocean’s surface to the deep when these organisms die and sink. This decrease in the amount of sinking shells leads to a buildup in total alkalinity (AT) in surface waters, which helps the ocean absorb more CO2—a natural feedback mechanism that could limit the rise of atmospheric CO2. We have identified signs of this feedback in the global ocean. Our findings show that the AT in the ocean's surface is increasing by 0.072 ± 0.023 µmol per kilogram per year, which would have caused the amount of human-emitted carbon in the ocean to increase by about 0.20 PgC since the 1990s. This shows that the chemistry of the oceans is changing as human-produced CO2 emissions cause shifts in the patterns of life and death of some marine organisms. More data on AT is needed to better quantify this feedback and its impacts.

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 (AT) 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 AT 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 AT of 0.072 ± 0.023 µmol kg-1 yr-1, which would have caused approximately 20 Tmol of additional AT 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 AT 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.