Ben McNeil Climate Change Research Centre, University of New South Wales, Sydney

1. Ben McNeilClimate Change Research Centre,University of New South Wales, Sydney Southern Ocean Acidification: Earlier Impacts via Seasonal Amplification

2. Importance of the Ocean for Atmospheric CO2 CO2 Without Ocean Atmos CO2 (ppm) Oceanic CO2 Absorption Year Observed CO2

3. Unfortunate Consequences of Oceanic Anthropogenic CO2 Absorption CO2 Increase Increase from air-sea exchange CO2 + H2O  HCO3- + H+ CO32- + 2H+ Decrease pH = -log[H+] More CO2 Uptake = More Acidic = Less CO32-

4. Ocean Carbon Chemistry Primer CO2(gas) 280 matm 560 matm CO2 + H2O  H2CO3 H2CO2 H++ HCO3- HCO3- H++ CO32- 8 mmol kg-1 15 mmol kg-1 Carbonic acid 1617 mmol kg-1 1850 mmol kg-1 Bicarbonate 268 mmol kg-1 176 mmol kg-1 Carbonate 1893 mmol kg-1 2040 mmol kg-1 TCO2 (DIC) 100% increase in pCO2 ~ 8% increase in DIC 100% increase pCO2 ~ 33% decrease in CO32-

5. Changing Carbonate Equilibrium in Seawater - Graphically

6. Ω = [Ca2+] [CO32-] /  Aragonite Solubility Coeff Aragonite Saturation State Carbonate Ion Calcium Calcium Carbonate (CaCO3) Cycle Stable CaCO3 Ω >1 Unstable CaCO3 Ω <1

7. Two forms of calcium carbonate - very different stability Calcite (plankton) Aragonite (plankton and corals)

8. Ω < 1 Geochemically Dissolves Aragonite Shells M. Mercenaria reared in Ω= 0.5. T= 0 hours T = 24 hours T = 72 hours Peteropod, Clio pyramidata after 24 hours Orr et al (2005)

9. 1880 1995 Observations Ω 2100 Model McNeil and Matear, 2006 Will Ω <1 this century?

10. Southern Ocean Vulnerable Orr et al (2005)

11. But averages aren’t the problem! Natural Cycles amplify the mean trend Only 0.4m Mean Sea-level rise

12. How do natural carbonate cycles impact annual trend in Southern Ocean acidification? • Estimate the seasonal cycle of carbonate in the Southern Ocean using sparse measurements of Dissolved Inorganic Carbon (DIC) and Alkalinity (ALK) • Combine with future oceanic CO2 uptake to better estimate the onset of Southern Ocean acidification • Any in-situ evidence? • Implications for Southern Ocean marine calcifiers

13. How to Estimate Surface Seasonality in the Southern Ocean? Step 1) Gather all available DIC/ALK surface measurements up to 55m

14. Empirical fits for surface DIC / ALK DIC= f(temp, sal, nitrate, silicate, oxygen) S.E = ±8umol/kg n=1200 ALK=f(temp,sal) S.E= ±9umol/kg • No change to the fit with season • No strong residual pattern indicating potential biases

15. World Ocean Atlas - Hydrographic Data • 96000 stations 100 times the amount of data than carbon

16. First ever seasonal carbon climatology Step 2) Extrapolate using World Ocean Atlas climatology

17. Step 3) Calculate seasonal pH and Carbonate ion from DIC/ALK DIC (umol/kg) ALK (umol/kg)

18. Seasonal Variability in Carbon

19. Independent Winter Analysis

20. IS92a Atmospheric CO2 Scenario Future Projections Over the Annual Cycle

21. Onset of Southern Ocean Aragonite Undersaturation (Ω<1)

22. 450ppm is the tipping point Orr et al (2005)

23. Southern Ocean Acidification Divide Acidification divide set-up by the Polar Front

24. Southern Ocean Pteropods Vulnerable • Pteropods, particularly Limacina species, are an abundant group, with regionally very high densities • Pteropods can make a substantial contribution to both meso-and macrozooplankton communities • Up to 65% of zooplankton biomass • Feed for higher trophic species (fish, whales etc) • Pteropods have long life-cycles (1-3years), vulnerable to peridodic undersaturation • Limacina helicina • Limacina retroversa

25. Known’s and Unknown’s of Ocean Acidification • Flow on effects to fisheries • Yet unknown? • Organisms that require calcium carbonate will be vulnerable • Phytoplankton and corals

26. Conclusions • Considerable seasonal amplification of the carbonate ion in the Southern Ocean • Very low winter-time values coupled with future anthropogenic CO2 will induce ‘corrosive’ seawater conditions decades before previously though (~2030 in the open Southern Ocean) • Suggests that 450ppm is the tipping point for these corrosive conditions • The Ross Sea was found to have a huge seasonal cycle, and the rapid decline in Ω between summer and autumn can not be ruled out in causing the Pteropod die-off collected in sediment traps • The very low winter Ω brings forward the onset of aragonite saturation to the year 2015 in the Ross Sea • Coinciding biological and geochemical time-series studies are needed to understand the implications for Southern Ocean calcifiers