As carbon dioxide levels rise, surface seawater pH falls, according to this model. By 2250, pH will reach a peak of -0.77 units and drop as the ocean and air reach equilibrium. Credit: Ken Caldeira, 2003.

The oceans have absorbed at least a quarter of the carbon dioxide that humans have pumped into the air since the industrial revolution—taking up a record 2.3 billion tons of manmade CO₂ last year (Khatiwala et al., 2009). But this hidden work has come at a cost: ocean acidification and what’s become known as “the other CO₂ problem.” As carbon accumulates in the oceans, it is lowering the pH of seawater—with far-reaching consequences for marine life and climate.

Carbon dioxide is a greenhouse gas that absorbs Earth’s outgoing infrared radiation, trapping heat in the atmosphere and raising temperatures. Since the 1750s, humans have raised carbon dioxide levels 40 percent, to 385 parts per million (ppm)—mostly by burning fossil fuels. This is the highest that carbon dioxide levels have been in 2.1 million years, and probably longer (Hoenisch et al., 2009). Earth’s average temperature has warmed nearly 1˚ Celsius but we would have seen more warming still if not for the oceans’ role in disposing of excess carbon dioxide. Scientists estimate that CO₂ levels today would be about 450 ppm without the oceans mitigating effect (Doney et al., 2009).

When the concentration of atmospheric CO2 is greater than at the ocean surface, the oceans naturally soak up the added CO2 to maintain equilibrium. Once CO2 dissolves in seawater, it forms carbonic acid—the weak acid that gives soda its fizz. Adding carbonic acid to seawater depletes its concentration of carbonate ion, making it harder for plankton, shellfish and coral to build their chalky shells and skeletons. If seawater grows acidic enough, it may also become corrosive to shells and interfere with animal behavior, respiration and reproduction.

Ocean acidification carries another risk. It may weaken the ocean’s appetite for manmade carbon, leading to more severe warming than climate models predict. Carbonate ion acts as a buffer for CO2 in seawater but as carbonate levels fall so does the oceans’ capacity to neutralize CO2. Since 2000, the proportion of fossil-fuel emissions absorbed by the oceans may have declined by as much as 10% (Khatiwala et al., 2009).

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