9.7 Ocean Acidification

CO2 lowers ocean pH

Oceans absorb about 30% of atmospheric CO2. Dissolved CO2 reacts with seawater to form carbonic acid (H2CO3), which dissociates into H+ and bicarbonate ions. Rising H+ concentration lowers ocean pH, a process called ocean acidification. Since the Industrial Revolution, average surface ocean pH has dropped from about 8.2 to 8.1.

Impact on calcifying organisms

Extra H+ ions bind with carbonate ions (CO3²⁻), reducing their availability for organisms that build shells and skeletons from calcium carbonate (CaCO3), such as corals, mollusks, and some plankton. Lower carbonate availability weakens shell/skeleton formation and can dissolve existing structures, threatening coral reefs and shellfish populations.

Ecosystem and economic ripple effects

Because calcifying organisms like coral and plankton form the base of many marine food webs, acidification threatens biodiversity, fisheries, and coastal economies dependent on reefs and shellfish harvesting. Reducing fossil fuel emissions is the primary strategy to slow further acidification.

Key terms

Ocean acidification

The decrease in ocean pH caused by increased absorption of atmospheric CO2, which forms carbonic acid in seawater.

Carbonic acid formation

CO2 + H2O → H2CO3, which dissociates into H+ and HCO3- ions, increasing acidity.

Carbonate ion (CO3²⁻)

An ion needed by calcifying organisms to build calcium carbonate shells; its availability decreases as ocean pH drops.

Calcifying organisms

Species like corals, mollusks, and some plankton that build shells/skeletons from calcium carbonate; highly vulnerable to acidification.

Main cause of ocean acidification

Rising atmospheric CO2 from fossil fuel combustion, about 30% of which is absorbed by oceans.

Try an AP-style question

What causes ocean acidification?

1. Oil spills along coastlines
2. Oceans absorbing CO₂ to form carbonic acid
3. Acid rain falling on the ocean
4. Salt dissolving from rocks