Explained: Ocean Acidification

The oceans are becoming increasingly acidic, and scientists believe this is occurring at a greater rate than at any other point in geologic history.

That's devastating news for most marine species, as many of them are sensitive to even minor changes in their watery environment's acidity. It's particularly dangerous for corals, oysters, and other species with delicate carbonate shells or skeletons, which are harmed by even minor changes in the ocean's acid balance, much like acid rain corrodes stone gargoyles and limestone structures.

The Culprit is Carbon

The culprit behind the acidification is the extra carbon dioxide humans have caused to collect in the atmosphere by burning fossil fuels, cutting down forests, and other actions.The oceans have always absorbed and spewed out carbon dioxide, transporting it from the atmosphere to the sea. However, the exchange took thousands or tens of thousands of years to complete.

The slow exchange has been disrupted by humans. Humans have added 400 billion tons of carbon to the atmosphere since the beginning of the Industrial Revolution in the mid-eighteenth century. That's a result of the massive amounts of fossil fuels we've consumed for energy, the forests we've felled, the cement we've made, and so on.

The majority of the carbon, in the form of CO2, remains in the atmosphere, trapping heat and contributing to global warming. Nevertheless, the ocean absorbs nearly a quarter of all CO2 emissions each year. About 30% of all the excess carbon dioxide humans have produced to the atmosphere has percolated down into the oceans during the previous few hundred years.

That’s a good thing for the atmosphere. Without the extra carbon dioxide removal, the planet would have warmed substantially more than it has already. However, this is bad news for the oceans.

Oceans Acidification, Geologically

The oceans had equilibrated to be slightly alkaline in the late 1700s, with a pH of around 8.1—roughly the same acidity level as an egg white. (On the pH scale, more acidic substances are lower.) Lemon juice and vinegar measure a mouth-watering 2 to 3 on the pH scale; perfectly distilled water is about 7.

On geologic time scales, the pH of the ocean has altered. It crept up (become more alkaline) by about 0.2 pH units during cold periods in the planet's past, and it crept down (became more acidic) by roughly the same amount when the world warmed up. But, those changes took tens of thousands of years to occur, giving aquatic organisms plenty of time to respond.

Since the commencement of the Industrial Revolution, the surface waters have dropped by around 0.1 pH unit—a fraction of a second in geologic or evolutionary time. While 0.1 units may not appear to be a big difference, it is: Because the pH scale is logarithmic (like the Richter Scale for earthquakes), that modest change actually means the water is 28 percent acidic.

The Future is Not Bright

The cetaceans are being stressed by this rapid transformation. Scallop shells are softened by it. It prevents crabs, lobsters, and other animals from moulting. Corals are weakened by it. It perplexes fish by interfering with their sense of smell. It could even alter how sound travels through water, making the undersea environment a little noisier.

There will be even more obstacles in the future. Scientists forecast that by 2050, 86 percent of the world's ocean will be warmer and acidic than at any time in history. By 2100, the pH of the surface ocean could drop to below 7.8, a drop of more than 150 percent from today's already-corrosive state—and possibly even more in some of the planet's most vulnerable areas, such as the Arctic Ocean.


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