Ocean Acidification (College Board AP® Environmental Science) : Study Guide

What is ocean acidification?

• Ocean acidification is the decrease in pH of ocean water caused by the absorption of carbon dioxide from the atmosphere

  • The oceans absorb about 30% of human-produced CO₂.

Chemical process of ocean acidification

• When CO₂ dissolves in seawater, it reacts with water (H₂O) to form carbonic acid (H₂CO₃)

• Carbonic acid is a weak acid that lowers the pH of the water slightly, making it more acidic

CO₂ + H₂O → H₂CO₃

• Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻)

• The release of H⁺ ions is what directly causes the lowering of pH in the oceans, making the water more acidic

H₂CO₃ → H⁺ + HCO₃^-

• Hydrogen carbonate ions can then dissociate again to form more hydrogen ions and carbonate ions (CO₃^2-)

HCO₃⁻ → H⁺ + CO₃^2-

pH changes and impact

• Since the Industrial Revolution, ocean pH has dropped from 8.2 to 8.1

  • This represents a 30% increase in acidity (due to the logarithmic nature of the pH scale)

Anthropogenic activities & ocean acidification

• Anthropogenic (human-caused) activities significantly increase CO₂ emissions, which in turn accelerates ocean acidification

Key anthropogenic activities

Burning of fossil fuels

• Coal, oil, and natural gas combustion for energy and transportation releases CO₂.

• Example: Power plants and cars are major sources of fossil fuel emissions

Deforestation

• Clearing forests reduces the Earth's ability to sequester CO₂, increasing atmospheric CO₂ levels

Agriculture

• Livestock farming produces methane (CH₄), which eventually oxidizes to CO₂ in the atmosphere (this process takes around 12 years)

Industrial processes

• Cement production releases CO₂ when limestone (CaCO₃) is converted to lime (CaO)

• The cement industry alone contributes about 8% of global CO₂ emissions

Impact of ocean acidification

How does ocean acidification affect marine life?

• Ocean acidification reduces calcium carbonate availability

  • This makes it harder for marine organisms to form shells and skeletons

  • This affects a range of species from coral reefs to plankton and disrupts entire marine food webs

Key impacts of ocean acidification

Impact on coral reefs

• Acidification makes it difficult for corals to form calcium carbonate skeletons

  • This is leading to weaker and slower-growing reefs

  • Example: The Great Barrier Reef has shown reduced calcification rates due to lower pH

• The reaction during which hydrogen carbonate ions dissociate to form hydrogen ions and carbonate ions reverses to buffer the increasing number of hydrogen ions

  • This reduces the availability of carbonate ions for the building of hard exoskeletons

Impact on shell-forming organisms

• Mollusks, crabs, and plankton struggle to form and maintain shells in acidic waters

  • Example: Pacific oyster hatcheries have seen high mortality rates linked to ocean acidification

Disruption of marine food webs

• Pteropods (sea butterflies) are tiny, shelled snails that are found in almost all oceans

  • They are a key food source for fish

• They have thinner shells due to acidification

• Higher mortality due to thinner shells means fewer of them are available as food for fish

• Disruption at this level cascades up the food web, affecting fish and marine mammals