OCEAN ACIDIFICATION: PRESENT STATUS AND FUTURE CONDITIONS IN A HIGH-CO2 WORLD

The uptake of anthropogenic CO₂ by the global ocean induces fundamental changes in seawater chemistry that could have dramatic impacts on biological ecosystems in the upper ocean. Estimates based on the Intergovernmental Panel on Climate Change (IPCC) “business as usual” emission scenarios suggest that atmospheric CO₂ levels could approach 800 ppm near the end of the century. Corresponding biogeochemical models for the ocean indicate that surface water pH will drop from a preindustrial value of about 8.2 to about 7.8 in the IPCC A2 scenario by the end of this century, increasing the ocean’s acidity by about 150% relative to the beginning of the industrial era. In contemporary ocean water, elevated CO₂ will also cause substantial reductions in surface water carbonate ion concentrations, in terms of either absolute changes or fractional changes relative to pre-industrial levels. For carbonate ion, the highest contemporary surface water concentrations are in the tropical and subtropical regions, particularly on the western side of each basin where the waters are warmer and saltier. For most open-ocean surface waters, aragonite undersaturation occurs when CO₃^2- concentrations drop below approximately 66 µmol kg^-1. The model projections indicate that aragonite undersaturation will start to occur by at about 2020 in the Arctic Ocean and 2050 in the Southern Ocean. By 2050 all of the Arctic is undersaturated with respect to aragonite, and by 2095 all of the Southern Ocean and parts of the North Pacific would become undersaturated. For calcite, undersaturation occurs when carbonate ion drops below 42 µmol kg^-1. By 2095 most of the Arctic and some parts of the Bering and Chukchi Seas would become undersaturated with respect to calcite. However, in most of the other ocean basins the surface waters will still be saturated with respect to calcite, but at a level greatly reduced from the present.