Southern Ocean acidification: a tipping point at 450-ppm atmospheric CO2

Abstract

Southern Ocean acidification via anthropogenic CO(2) uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate ion (CO(3)(2-)) to levels where calcium carbonate (both aragonite and calcite) shells begin to dissolve. Natural seasonal variations in carbonate ion concentrations could either hasten or dampen the future onset of this undersaturation of calcium carbonate. We present a large-scale Southern Ocean observational analysis that examines the seasonal magnitude and variability of CO(3)(2-) and pH. Our analysis shows an intense wintertime minimum in CO(3)(2-) south of the Antarctic Polar Front and when combined with anthropogenic CO(2) uptake is likely to induce aragonite undersaturation when atmospheric CO(2) levels reach approximately 450 ppm. Under the IPCC IS92a scenario, Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038. Some prominent calcifying plankton, in particular the Pteropod species Limacina helicina, have important veliger larval development during winter and will have to experience detrimental carbonate conditions much earlier than previously thought, with possible deleterious flow-on impacts for the wider Southern Ocean marine ecosystem. Our results highlight the critical importance of understanding seasonal carbon dynamics within all calcifying marine ecosystems such as continental shelves and coral reefs, because natural variability may potentially hasten the onset of future ocean acidification.

Fig. 1.

Zonally averaged surface carbon measurements for the Southern Ocean where blue represents wintertime conditions (April–October) and red represents summertime conditions (November–March). Solid lines with circles represent the raw measurements from the Global Ocean Data Analysis Project database (12), and the dotted lines represent the empirical prediction from this study. (A) pH. (B) Carbonate ion (CO₃²⁻, μmol/kg). For reference, the carbonate ion concentration for aragonite saturation is ≈65 μmol/kg.

Fig. 2.

Seasonal estimates of pH and CO₃²⁻ for the Southern Ocean. (Top) Winter and summer distributions of pH. (Middle) Winter and summer distributions of CO₃²⁻. (Bottom) Surface contour map of the seasonal amplitude (winter–summer) from the empirically derived values from this study of pH and carbonate ion (CO₃²⁻, μmol/kg).

Fig. 3.

Observed and predicted Southern Ocean surface acidification conditions for the 21st century. (A) IPCC IS92a atmospheric CO₂ scenario (black) and the average oceanic pCO₂ level south of 60°S from the CSIRO ocean carbon model (blue line). (B and C) Projections for Southern Ocean (south of 60°S) for surface pH and carbonate ion (CO₃²⁻, μmol/kg) for two different methods using the IPCC IS92a atmospheric CO₂ scenario. The observed seasonal cycle is represented in the year 1995 with a box-and-whiskers plot. The concentration of CO₃²⁻ that results in aragonite and calcite saturation is shown by the horizontal dotted lines. The observations were used as the baseline for these two different scenarios. The solid red line represents the average conditions assuming atmospheric equilibrium from the year 1995, and the blue line includes the estimated CO₂ disequilibrium from the CSIRO climate model. The shading for red and blue represents the maximum seasonal variability taken from the observations derived here.

Fig. 4.

Contour plot of the year in which the onset of wintertime undersaturation occurs under equilibrium conditions. Shown are the average location of Southern Ocean fronts (38).