Climate change and ocean acidification (QCE Marine Science Unit 4)
“Explain how rising carbon dioxide drives ocean warming, coral bleaching, ocean acidification and sea level rise, and describe the consequences for marine ecosystems such as the Great Barrier Reef”
Rising atmospheric carbon dioxide warms the ocean, lowers its pH through acidification, and raises sea level. Warming above the seasonal maximum causes corals to expel their zooxanthellae and bleach. Acidification lowers aragonite saturation, making it harder for corals and shellfish to build skeletons. Together these processes have driven repeated mass bleaching on the Great Barrier Reef and threaten reef survival.
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What this dot point is asking
QCAA wants you to explain the chain of cause and effect from rising atmospheric carbon dioxide to ocean warming, coral bleaching, acidification and sea level rise, and to describe what these do to marine ecosystems. Mechanism and named Great Barrier Reef examples are what earn marks.
The driver: rising carbon dioxide
Burning fossil fuels and clearing land have raised atmospheric carbon dioxide from about 280 parts per million before industrialisation to over 420 parts per million today. The ocean absorbs roughly a quarter to a third of this extra carbon dioxide and most of the extra heat trapped by greenhouse gases, which is why climate change is so much an ocean story.
Ocean warming and coral bleaching
The ocean absorbs heat, so sea surface temperatures are rising. Reef-building corals live close to their upper thermal limit, so even a sustained rise of 1 to 2 degrees C above the usual summer maximum is enough to break the coral-zooxanthellae partnership.
Under heat stress the coral expels its zooxanthellae, the algae that give it both colour and most of its energy. The white coral skeleton shows through the now-transparent tissue, which is coral bleaching. A bleached coral is not dead, but it is starving; if the heat stress passes quickly the algae can return, but prolonged stress kills the coral.
The Great Barrier Reef has suffered repeated mass bleaching events (notably 2016, 2017, 2020, 2022, and again in 2024 and 2025), each driven by marine heatwaves. The back-to-back events leave too little time for reefs to recover, causing long-term decline in coral cover.
Coral bleaching happens because heat stress disrupts photosynthesis in the zooxanthellae, causing them to produce harmful reactive oxygen molecules. The coral expels the algae to protect itself, losing the partner that supplies up to 90 per cent of its energy. The coral then slowly starves unless the algae return, which is why prolonged or repeated bleaching is lethal even though a single short event may be survivable.
Ocean acidification
When carbon dioxide dissolves in seawater it forms carbonic acid, which releases hydrogen ions and lowers the pH of the ocean. Surface ocean pH has already fallen by about 0.1 units since industrialisation, a roughly 30 per cent rise in acidity.
The crucial effect is on aragonite, the form of calcium carbonate that corals, shellfish and some plankton use to build skeletons and shells. More acidic water has a lower aragonite saturation, so these organisms must spend more energy to build and maintain their skeletons, and growth slows. Acidification therefore weakens reefs even where the water is not warm enough to bleach them, and it threatens the plankton at the base of marine food webs.
Sea level rise
Sea level is rising for two reasons: thermal expansion (warmer water takes up more volume) and the melting of land ice (glaciers and ice sheets adding water to the ocean). Rising seas drown low coral cays and reef flats, push seawater into coastal mangroves and freshwater systems, and increase coastal erosion and flooding. Low-lying Queensland coasts and Pacific island nations are especially exposed.
Consequences for marine ecosystems
- Reefs lose coral cover to bleaching and grow more slowly under acidification, shifting toward algae-dominated systems with lower biodiversity.
- Shifting species ranges. As the East Australian Current warms and pushes south, tropical species move into temperate waters, disrupting established communities.
- Food web effects. Acidification and warming hit plankton and shellfish at the base of food webs, with flow-on effects to fisheries.
- Loss of ecosystem services. Degraded reefs protect coasts less well from waves and support fewer fisheries and less tourism.
- Confusing bleaching with acidification
- Bleaching is loss of zooxanthellae from heat stress; acidification is falling pH from dissolved carbon dioxide. They are different processes with different mechanisms.
- Saying bleached coral is dead
- A bleached coral is stressed and starving but can recover if the heat passes; death follows only prolonged or repeated stress.
- Forgetting thermal expansion
- Sea level rises from both melting land ice and the expansion of warming water; melting sea ice does not raise sea level.
- Treating acidification as only a coral problem
- It also weakens shellfish and plankton, threatening whole food webs.
Why this matters
Climate change is the overarching threat that the management strategies in Unit 4 try to address, and it interacts with every other human impact. Because much of the driver (global emissions) lies beyond local control, it is a powerful case for evaluating which management responses can realistically protect reefs, a key IA3 and external exam skill.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice5 marksThe boundary of the Triassic and Jurassic periods was marked by intense volcanic activity, an atmospheric carbon dioxide concentration of approximately 2000 ppm, and a mass extinction in which more than 20% of marine genera disappeared. Species with aragonitic shells were particularly affected. Explain how volcanic activity was responsible for the mass extinction.Show worked answer →
For 5 marks, build the causal chain from volcanism to acidification to the loss of aragonitic species.
Source of CO2. Intense volcanic activity released huge volumes of carbon dioxide, raising atmospheric CO2 to about 2000 ppm.
Ocean uptake and acidification. A large fraction of this CO2 dissolved into the ocean, forming carbonic acid (CO2 + H2O gives H2CO3). This released hydrogen ions, lowering ocean pH - ocean acidification.
Effect on carbonate. The extra hydrogen ions reacted with carbonate ions, reducing the carbonate available and lowering the aragonite saturation state of seawater.
Why aragonitic species died. Organisms that build shells and skeletons from aragonite (a soluble form of calcium carbonate) could no longer calcify efficiently, and their structures began to dissolve, so they were hit hardest.
Outcome. Combined with volcanic warming, the acidified, carbonate-poor ocean drove the extinction of more than 20 per cent of marine genera.
Practice2 marksGraphs show benthic communities at a low CO2 site (pH = 8.1) and a high CO2 site (pH = 7.7). Explain the impact of high CO2 on fish populations.Show worked answer →
For 2 marks, link high CO2 to reduced coral habitat and then to fish (1 mark for each linked idea).
Loss of habitat. High CO2 lowers seawater pH (ocean acidification) and reduces the carbonate available for calcification, so hard, structurally complex corals decline and reefs lose their three-dimensional structure. With less shelter and fewer feeding niches, fish abundance and diversity fall.
Direct physiological effects. Elevated CO2 can also impair fish directly, disrupting acid-base balance and altering sensory behaviour (for example impaired predator avoidance in larvae), further reducing survival. The net effect is smaller, less diverse fish populations at the high CO2 site.
Practice1 markThe graph shows the effect of ocean acidification due to different concentrations of dissolved carbon dioxide on coral types in laboratory and field settings. Use the data in the graph to identify how ocean acidification affects Acropora coral growth.Show worked answer →
For 1 mark, read the trend straight from the graph.
Increasing dissolved CO2 (greater ocean acidification) decreases the growth, i.e. the calcification rate, of Acropora coral. The high CO2 Acropora bars show a lower calcification rate (in grams per square centimetre per year) than the low CO2 Acropora bars, so more acidic, high CO2 conditions slow Acropora growth.