9.7 Ocean Acidification

Ocean acidification is the drop in ocean pH that happens when the ocean absorbs extra CO2 from the atmosphere. In AP Environmental Science, you should connect dissolved CO2 to carbonic acid formation, lower pH, reduced calcium carbonate, and harder shell or skeleton building for corals, shellfish, and other calcifying organisms.

Ocean Acidification APES Summary

In AP Environmental Science, ocean acidification is the decrease in ocean pH caused mainly by increased atmospheric $CO_2$. As the ocean absorbs more $CO_2$, it forms carbonic acid, increases hydrogen ion concentration, and makes seawater more acidic.

The exam usually wants the cause-and-effect chain: burning fossil fuels, vehicle emissions, and deforestation increase atmospheric $CO_2$; the ocean absorbs part of that $CO_2$; pH decreases; calcium carbonate becomes less available; and corals and other calcifying organisms have a harder time forming shells and skeletons.

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Why This Matters for the AP Environmental Science Exam

Ocean acidification connects human CO2 emissions to a measurable chemical change in ocean water and then to real biological effects. On the AP Environmental Science exam, you should be ready to explain the cause-and-effect chain from burning fossil fuels to lower ocean pH to harm to marine calcifiers like coral.

This topic pairs well with data analysis. You may see graphs of atmospheric CO2 versus ocean pH, and you should be able to describe trends, identify relationships, and connect them to the underlying chemistry. Free-response prompts in this unit often ask you to explain causes, describe effects, and propose or evaluate solutions, so practice tying the chemistry to clear environmental consequences.

Key Takeaways

• Ocean acidification is a decrease in ocean pH caused mainly by increased atmospheric CO2 dissolving into seawater.
• The main human activities that drive it are burning fossil fuels, vehicle emissions, and deforestation, all of which raise atmospheric CO2.
• Dissolved CO2 forms carbonic acid, which raises hydrogen ion concentration and lowers pH.
• Lower pH reduces available calcium carbonate, making it harder for coral, shellfish, and other organisms to build shells and skeletons.
• The pH scale is logarithmic, so a small drop in pH means a large increase in acidity.
• Harm to calcifying organisms can ripple through marine food webs.

Acid Formation

When the ocean absorbs CO2 from the atmosphere, the gas reacts with seawater. This raises the hydrogen ion (H+) concentration in the ocean and lowers ocean pH. The basic chemistry can be written as a series of reactions:

$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^- \rightleftharpoons 2H^+ + CO_3^{2-}$

Dissolved CO2 combines with water to form carbonic acid (H2CO3). Carbonic acid then breaks apart, releasing hydrogen ions (H+) and bicarbonate (HCO3-). More H+ in the water means a lower pH, which is what "more acidic" means.

Ocean acidification is driven by the ocean absorbing excess atmospheric CO2. As more CO2 is released into the air, the oceans take up a large part of it and continue to become more acidic. Human activities that raise atmospheric CO2 are the main cause: burning fossil fuels, vehicle emissions, and deforestation.

Over roughly the last 200 years of industrialization, ocean pH has dropped by about 0.1 pH units. Because the pH scale is logarithmic, that 0.1 drop corresponds to about a 30% increase in acidity. Small-looking pH changes represent large chemical shifts.

Impacts of Changing pH

Calcium Carbonate and Calcifying Organisms

As CO2 dissolves and forms carbonic acid, it lowers pH and reduces the amount of calcium carbonate (CaCO3) available in seawater. This is the central problem for many marine organisms.

Calcium carbonate is the building material for the shells and skeletons of corals, snails, clams, and other shellfish. When less calcium carbonate is available, these organisms struggle to build and maintain those structures. Shells and skeletons protect organisms from predators, help maintain shape and buoyancy, and help some organisms regulate internal pH and calcium levels. This is why acidification is so damaging to coral reefs.

Fish Physiology

Acidification can also affect fish behavior and physiology. Many fish rely on their sense of smell to find food, locate mates, and avoid predators. Changes in seawater chemistry can disrupt their ability to detect certain odors, making it harder to respond to their environment.

Aquatic Plants

Some ocean plants, like seagrass and algae, can actually benefit from a CO2-rich environment. Combined with a lack of herbivores, rapid plant growth could contribute to hypoxic, eutrophic conditions in some areas.

At current CO2 emission rates, scientists project that ocean acidity could rise substantially over the coming century, posing a serious challenge to marine organisms and to the human populations that depend on them.

Pteropods (Example)

Pteropods are tiny pelagic snails that form the base of the food chain for many marine animals. In lab studies using seawater set to predicted future acidity levels, pteropod shells dissolved within a relatively short time. If pteropod populations collapsed, the effects would ripple up the food web to animals like whales and salmon that depend on them. This is an example of how harm to calcifiers can spread through a marine food web, not a required AP example.

How to Use This on the AP Environmental Science Exam

Free Response

Build your answers around a clear cause-and-effect chain: human activity raises atmospheric CO2, the ocean absorbs CO2, CO2 forms carbonic acid, pH drops, and calcium carbonate becomes less available, which harms shell-building organisms. If a prompt asks for a solution, focus on reducing CO2 emissions, since the root cause is rising atmospheric CO2.

MCQ

Expect questions that ask you to identify the cause of acidification (atmospheric CO2), name human sources (fossil fuel combustion, vehicle emissions, deforestation), or predict effects on calcifying organisms. Watch for answer choices that confuse acidification with ocean warming, since both come from CO2 but are different problems.

Data Analysis

You may be given graphs showing rising atmospheric CO2 alongside falling ocean pH. Practice describing the trend, stating the relationship between the two variables, and explaining the chemistry that connects them.

Common Trap

Remember that pH is logarithmic. A pH drop that looks small represents a much larger percent change in acidity, so do not dismiss a "small" pH change as unimportant.

Common Misconceptions

• Acidification does not mean the ocean becomes a strong acid. The ocean is slightly basic; acidification means its pH is dropping toward neutral, which is still a major problem for marine life.
• Ocean acidification and ocean warming are not the same thing. Both are tied to rising CO2, but warming is about temperature and acidification is about pH and carbonate chemistry.
• More CO2 in seawater does not directly "burn" or dissolve shells with acid. The problem is that lower pH reduces available calcium carbonate, making it harder to build and maintain shells.
• A 0.1 drop in pH is not trivial. Because pH is logarithmic, it reflects a large increase in hydrogen ion concentration.
• Not every organism is hurt the same way. Some plants and algae can benefit from extra CO2, while calcifying organisms are the most harmed.

Related AP Environmental Science Guides

• 9.1 Stratospheric Ozone Depletion
• 9.3 The Greenhouse Effect
• Major Environmental Disasters
• 9.8 Invasive Species
• Unit 9 Overview: Global Change
• 9.4 Increases in the Greenhouse Gases