Typically, seawater is heavily saturated with dissolved calcium carbonate from eroded limestone. This neutralizes any acid that forms from CO2 and leaves plenty of carbonate for marine creatures to use for shell- and reef-building. But as oceans absorb increasing amounts of CO2 from fossil fuels, their stores of calcium carbonate dip. Over time, this reduces carbonate available for marine creatures. Shell and coral formation slows.
Once seawater is too deficient in carbonate, these creatures find it hard to form shells or corals at all. In fact, existing shells start to dissolve, notes Ben McNeil, a researcher at the University of New South Wales in Australia.
In a recent study, he and a colleague looked at trends in the Southern Ocean. Oceans at the top and bottom of the world might be expected to lead in acidification because cold water soaks up more CO2 than warm water. But the duo also found large seasonal swings in carbonate levels. They traced increases in the water’s relative acidity to strong wintertime winds off Antarctica that bring to the surface cold water from the deep, which has low levels of carbonate.
The challenge, Dr. McNeil says, is that this seasonal peak in acidification comes just as tiny swimming snails – which some call potato chips of the sea – exist as larvae. The tiny zooplankton, called pteropods, need carbonate to build their shells. They represent a vital source of food for many fish. Some pteropods already show signs of dissolving shells, the team reports.
With a business-as-usual emissions scenario, McNeil and his colleague estimate that the Southern Ocean is likely to reach a wintertime tipping point for these creatures when atmospheric CO2 concentrations reach 450 parts per million, versus today’s level of around 383 ppm. That would occur by 2030 and no later than 2038, they estimate. The results appear in the Dec. 9 issue of the Proceedings of the National Academy of Sciences (PNAS).