OCEAN ACIDIFICATION

Sammanfattning: Ocean acidification is a consequence of the anthropogenic release of carbon dioxide (CO2) to the atmosphere. When CO2 is dissolved in water it affects the composition of ions and lowers the pH of the water. As the partial CO2 pressure in the atmosphere increases a new balance is reached with the surface waters which causes the pH of the surface oceans to decrease. For small shallow seas this effect may be worse than in the oceans because of the limited vertical mixing of deep water and the limited buffering capability (alkalinity). A decrease in ocean pH may have direct effects on ecosystems and indirect effects where the saturation states of calcium carbonates are the most important. This project aims to model projections of future pH of the Baltic Sea surface waters from several CO2 scenarios suggested by the IPCC. A predictive, deterministic model was constructed to project future changes in pH based on the atmospheric CO2 partial pressure, alkalinity and the equilibrium of ions. From the model the ion concentrations can be calculated and used to calculate projections of the saturation state of calcite and aragonite of the Baltic Sea surface waters. Projections of Baltic Sea pH during the 21st century have been made using the model and the CO2 scenarios A1FI, A2 and B2 described by the IPCC. For these scenarios the projections show a decrease in pH by 0.2-0.4 units. If possible changes in alkalinity, which may be a result of changes in river run off to the Baltic Sea, are included the decrease in pH may be as large as 0.5 units. Projections for calcium carbonate indicate that it is possible that calcite becomes under saturated in spring and autumn during the 2050's but is not likely to become under saturated during summers. Aragonite is in our model projected to become under saturated in all seasons already in the 2040's. The magnitude of our projected changes in pH and saturation states of calcite and aragonite has been reported by SMHI to cause negative effects on calcifying organisms and may have effects on the ecosystem as a whole.