How does carbon dioxide make the sea more acidic, and why does that dissolve shells?
Topic 9.7 Ocean Acidification: explain how rising carbon dioxide acidifies the ocean and describe the effects on marine organisms.
A focused answer to APES Topic 9.7, covering how the ocean absorbs carbon dioxide to form carbonic acid, the resulting fall in pH, why acidification harms shell- and skeleton-building organisms (corals, shellfish, plankton), the effect on carbonate availability, the link to food webs, and how it differs from ocean warming, with a worked pH and carbonate reasoning example.
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What this topic is asking
The College Board (Topic 9.7) wants you to explain how rising carbon dioxide acidifies the ocean and describe the effects on marine organisms.
How the ocean acidifies
Why calcifying organisms suffer
Ecological consequences
Why this matters
Ocean acidification pairs with ocean warming (Topic 9.6) as the two great ocean impacts of climate change, and the AP exam stresses that they are different problems, one chemical, one thermal, driven by the same carbon dioxide. It ties Unit 9 to the carbon cycle (Unit 1) and to pH (the acid rain ideas of Unit 7).
Try this
Q1. Identify the acid formed when carbon dioxide dissolves in seawater. [1 point]
- Cue. Carbonic acid.
Q2. Explain why ocean acidification harms corals and shellfish. [2 points]
- Cue. The extra hydrogen ions from the carbonic acid reduce the carbonate ions that corals and shellfish need to build their calcium carbonate shells and skeletons, so these form poorly or dissolve.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Explain how rising atmospheric carbon dioxide makes the ocean more acidic. (b) State what happens to ocean pH. (c) Explain why acidification harms organisms that build shells or skeletons. (d) Describe one ecological consequence of ocean acidification.Show worked answer →
A 4-point FRQ on ocean acidification.
(a) Explain (1 point): the ocean absorbs carbon dioxide from the air; the carbon dioxide reacts with water to form carbonic acid, releasing hydrogen ions.
(b) State (1 point): ocean pH falls (the water becomes more acidic).
(c) Explain (1 point): the extra hydrogen ions reduce the availability of carbonate ions, which organisms need to build calcium carbonate shells and skeletons, so shells form poorly or dissolve.
(d) Describe (1 point): weaker corals and reefs, declining shellfish and plankton, and disruption of food webs that depend on them.
Markers reward the carbon-dioxide-plus-water-makes-carbonic-acid step, the falling pH, the reduced-carbonate reason shells suffer, and a valid ecological consequence.
AP 2018 (style)1 marksSection I (multiple choice). Ocean acidification most directly threatens marine organisms that: (A) live in deep water away from light (B) build shells or skeletons from calcium carbonate (C) feed only on other fish (D) tolerate a wide range of temperatures. Justify your choice.Show worked answer →
A 1-point MCQ on ocean acidification. The answer is (B).
Acidification reduces the carbonate ions that organisms such as corals, shellfish and some plankton need to build calcium carbonate shells and skeletons, so these organisms are most directly threatened. Deep-water (A), fish-eating (C) and temperature-tolerant (D) traits are not the key vulnerability. The trap is overlooking that the harm targets calcifying organisms specifically, through reduced carbonate availability.
Related dot points
- Topic 9.6 Ocean Warming: explain how the ocean absorbs heat and describe the effects of ocean warming on marine ecosystems and sea level.
A focused answer to APES Topic 9.6, covering how the ocean absorbs most of the extra heat from climate change, coral bleaching, thermal expansion and sea-level rise, shifting species ranges, reduced oxygen, effects on currents, and why the ocean buffers but does not escape warming, with a worked thermal-expansion reasoning example.
- Topic 9.5 Global Climate Change: describe the evidence and effects of global climate change and explain the role of positive feedback loops.
A focused answer to APES Topic 9.5, covering the evidence for global climate change, its effects (rising temperatures, melting ice, sea-level rise, extreme weather, shifting species), positive feedback loops (ice-albedo, permafrost methane, water vapor), the difference between weather and climate, and mitigation and adaptation, with a worked sea-level reasoning example.
- Topic 9.3 The Greenhouse Effect: explain the greenhouse effect, identify the major greenhouse gases, and distinguish the natural effect from the enhanced effect.
A focused answer to APES Topic 9.3, covering how the greenhouse effect works, the major greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor, CFCs), the difference between the natural and enhanced greenhouse effect, global warming potential and residence time, with a worked global warming potential calculation.
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 8.2 Human Impacts on Ecosystems: explain how pollution and other human activities disrupt ecosystems and harm organisms.
A focused answer to APES Topic 8.2, covering how pollution, oil spills, plastic, heavy metals and habitat disturbance disrupt ecosystems, the idea of ecological tolerance and indirect effects through food webs, coral reef damage, and ecosystem recovery, with a worked species-loss reasoning example.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)