Quotas, mesh sizes and marine parks can all be enforced. Neither warming nor acidification can. Both come from the same cause — extra carbon dioxide in the atmosphere — and both are already reshaping the systems that aquatic food production depends on.
📘 What you need to know
Climate change is long-term change in global temperatures and weather patterns, driven mainly by burning fossil fuels, deforestation and industry.
Ocean acidification is the ongoing fall in ocean pH caused by absorption of excess CO2, which forms carbonic acid.
Acidification causes coral bleaching alongside heat, and weakens the shells of calcium carbonate organisms.
Damage spreads through food webs, fisheries, tourism and coastal protection.
The Great Barrier Reef is the standard case study.
Two problems, one cause
Extra CO2 in the atmosphere does two separate things to the ocean. It traps heat, warming the water. And it dissolves into seawater, where it forms carbonic acid and lowers the pH. Students often merge these; keep them apart and your answers get sharper immediately.
Keep these separate
more CO2 → warmer water • more CO2 → lower pH
What warming does
Habitat ranges shift. Many fish populations are moving towards cooler water, which disrupts the fishing industries built around where they used to be.
Polar ice melts. Ice habitats matter for hunting, avoiding predators and raising young. Polar bears and seals depend on them, and walruses forced ashore crowd together, raising mortality.
Sea level rises. Coastal ecosystems are threatened — the mangrove forests of Bangladesh, for example, which are both habitat and coastal defence.
Storms intensify. More powerful hurricanes physically break reefs apart. Hurricane Irma in 2017 caused widespread coral destruction around the Florida Keys and the Virgin Islands.
Coral bleaching, step by step
Corals live with tiny algae inside their tissues. The algae photosynthesise and feed the coral; the coral gives them shelter. Heat and acidity break that partnership.
The distinction in stage 3 matters. Coral can regain its algae if conditions improve quickly, which is why the frequency of bleaching events matters as much as their severity.
What acidification does to shell builders
More acidic water weakens calcium carbonate shells in oysters, clams, sea urchins and some plankton.
Weakened organisms are more vulnerable to predation, disease and other environmental stress.
Population declines then disrupt marine food webs, because many of these species sit near the base.
Oyster populations in the Pacific Northwest of the USA are in decline partly because of acidification, and oyster farms there struggle with shell degradation and reduced harvests.
Case study: the Great Barrier Reef
Off Queensland, Australia, the Great Barrier Reef is the world’s largest coral reef system and supports both tourism and fishing. It is also the clearest illustration of what these two pressures do together.
🧩 What is happening, and what it costs
Repeated bleaching. Back-to-back events in 2016 and 2017 affected around two-thirds of the reef.
Rising sea temperatures. Even small increases are enough to trigger bleaching, so events are becoming more frequent.
Acidification. Weakens the coral structure itself, making it more likely to erode or break.
Biodiversity. Reefs support enormous numbers of species; losing coral removes both habitat and food.
Fishing. Commercial and recreational fisheries lose stock, and local fishers lose income.
Tourism. Millions of visitors come for vibrant reef; bleached reef draws fewer, and snorkelling and diving revenue falls.
Coastal protection. Reefs buffer storm surge and waves. Weakened reef means Queensland coastal communities face higher flood and property damage risk.
Grouping the consequences this way makes essays easy to structure: two ecological branches, two economic, one that is both.
Every impact here is an ecosystem service being lost. Naming them as services — provisioning (fish), cultural (tourism), regulating (coastal protection) — connects this page straight to the systems parts of the course.
Worked examples
WORKED EXAMPLE 1
Scale of the 2016–17 bleaching
The Great Barrier Reef covers roughly 344 000 km2. If back-to-back bleaching affected about two-thirds of it, estimate the area affected.
Step 1: convert the fractiontwo-thirds = 0.667, or about 67%Step 2: apply it to the area344 000 × 0.667 = 229 000 km2Roughly 230 000 km2 affectedfor context, that is larger than the United Kingdom — comparisons like this earn credit in extended answers
WORKED EXAMPLE 2
Explain how rising CO2 causes coral bleaching [4]
Cover both mechanisms.
Link 1
Rising atmospheric CO2 traps heat, so sea temperature risesLink 2
CO2 also dissolves in seawater, forming carbonic acid and lowering pHLink 3
Both stresses cause the coral to expel the algae living in its tissues
Link 4
Without the algae the coral loses its colour and its main food supplyProlonged stress kills the coralthe two-mechanism structure is the point — an answer about heat alone caps at half marks
WORKED EXAMPLE 3
Why are oyster farms struggling? [3]
Explain using acidification.
Link 1
More dissolved CO2 makes seawater less alkaline, reducing available carbonate ions
Link 2
Oysters build shells from calcium carbonate, so shell formation slows and shells weaken
Link 3
Larvae in particular fail to survive, so harvests fall
Reduced yields for growers in acidifying watersthe Pacific Northwest is the named example to reach for here
💡 Exam tip
Treat warming and acidification as two mechanisms from one cause. Say both.
Describe bleaching as expelling algae, not as coral dying.
Have the Great Barrier Reef facts ready, including the 2016–17 events and two-thirds figure.
Cover ecological and economic consequences. Tourism and coastal protection are marks students leave behind.
Name the vulnerable groups precisely: corals, oysters, clams, sea urchins, calcareous plankton.
Link back to food webs: losing reef structure removes habitat for a huge share of marine species.
⚠ Common mix-up
Saying bleached means dead. Bleached coral can recover if the stress ends soon enough.
Saying the ocean is acidic. It is becoming less alkaline.
Blaming bleaching on heat alone. Acidification contributes as well.
Thinking corals are plants. They are animals that host photosynthetic algae.
Forgetting the human consequences. Fisheries, tourism and flood protection all depend on reefs.
Treating fish moving to cooler water as harmless. It relocates the fishery, often across a national border.
Up next: Nutrients and Productivity in the Oceans — the first HL section, on why some seas are crowded with life and others are close to empty.
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