What Is Ocean Acidification?

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Ocean acidification, or OA, is the process by which increases in dissolved carbon make seawater more acidic. While ocean acidification occurs naturally over geologic timescales, the oceans are currently acidifying at a faster rate than what the planet has ever experienced before. The unprecedented rate of ocean acidification is expected to have devastating consequences on marine life, particularly shellfish and coral reefs. Current efforts to combat ocean acidification are largely focused on slowing the pace of ocean acidification and bolstering the ecosystems capable of dampening ocean acidification’s full effects.

What Causes Ocean Acidification?

Today, the primary cause of ocean acidification is the ongoing release of carbon dioxide into our atmosphere from the burning of fossil fuels. Additional culprits include coastal pollution and deep-sea methane seeps.1 Since the start of the industrial revolution about 200 years ago, when human activities began releasing large amounts of carbon dioxide into Earth’s atmosphere, the ocean’s surface has become about 30% more acidic.2

The process of ocean acidification begins with dissolved carbon dioxide. Like us, many underwater animals undergo cellular respiration to generate energy, releasing carbon dioxide as a byproduct. However, much of the carbon dioxide dissolving into the oceans today comes from the excess of carbon dioxide in the atmosphere above from the burning of fossil fuels.

Once dissolved in seawater, carbon dioxide goes through a series of chemical changes. Dissolved carbon dioxide first combines with water to form carbonic acid. From there, carbonic acid can break apart to generate standalone hydrogen ions. These excess hydrogen ions attach to carbonate ions to form bicarbonate. Eventually, not enough carbonate ions remain to attach to each hydrogen ion that arrives in seawater via dissolved carbon dioxide. Instead, the standalone hydrogen ions accumulate and lower the pH, or increase the acidity, of the surrounding seawater.

In non-acidifying conditions, much of the ocean’s carbonate ions are free to make connections with other ions in the ocean, like calcium ions to form calcium carbonate. For animals that need carbonate to form their calcium carbonate structures, like coral reefs and shell-building animals, the way in which ocean acidification steals carbonate ions to instead produce bicarbonate reduces the pool of carbonate available for essential infrastructure.

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