Ocean acidification at Higher Level: The same ideas as SL, with different examples: the Arctic Ocean, Oregon's oyster hatcheries and the CO₂ vents off Ischia. At HL, expect a seven-mark answer that covers both environmental systems and societies.
Practise this as you read
- Link each step: CO₂, acid, carbonate, shells, food web.
- Balance impacts on ecosystems with impacts on people.
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The ocean pays a price for being a sink: The CO₂ the ocean absorbs does not just sit there: it makes seawater more acidic. This is ocean acidification, measured on the pH scale.
The points to remember
- More CO₂ in the air means more CO₂ dissolves in the ocean.
- CO₂ reacts with water to form carbonic acid, so the water becomes more acidic.
- Surface pH has fallen from about 8.2 to 8.1 since before the industrial age.
- That small fall means about 30% more acidity: the pH scale is not a straight line.
- Seawater is still alkaline (above 7): 'acidification' means moving towards acid.
The chemistry, in one line: CO₂ + H₂O → H₂CO₃ (carbonic acid). More CO₂ in, more acid, lower pH.
Real example: the Arctic Ocean is acidifying fastest of all, because its cold water absorbs more CO₂ and melting sea ice adds fresh water that holds less carbonate.
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Shells and coral skeletons are made of calcium carbonate. To build them, animals take carbonate ions from the water. Acid uses up those carbonate ions.
The points to remember
- Lower pH means fewer carbonate ions in the water.
- Molluscs and corals need them to build calcium carbonate shells and skeletons.
- So shells and skeletons grow more slowly, are thinner, or even dissolve.
- Young animals (larvae) are hit hardest: many die before their shells form.
- Animals affected: corals, oysters, mussels, sea snails, some plankton.
Remember it as: More CO₂, more acid, fewer building blocks, weaker shells.
Normal seawater
- pH about 8.2
- Plenty of carbonate ions
- Shells and skeletons grow
More acidic seawater
- pH about 8.1 or lower
- Fewer carbonate ions
- Shells thin, grow slowly or dissolve
Real example: in 2007-2008 the Whiskey Creek oyster hatchery in Oregon, USA, lost most of its young oysters. Scientists traced it to corrosive, CO₂-rich water welling up from the deep; the young molluscs could not build their shells. The hatchery now treats its water before use.
The harm does not stop at shells. It passes through food webs and whole ecosystems.
The points to remember
- Many species are adapted to a narrow pH range, so a small change harms them.
- Producers such as phytoplankton and corals are sensitive: less food at the base of the food web.
- Corals bleach more easily and recover less well in more acidic water.
- Low pH can reduce reproduction in fish and shellfish and change fish behaviour and senses.
- Fewer producers and habitats: lower biodiversity and resilience; a possible tipping point.
- Some seagrasses and seaweeds may benefit from the extra CO₂.
A double threat for corals: Warmer water causes coral bleaching; acidification slows the growth of coral skeletons. Together they make it much harder for a reef to recover.
Real example: off the island of Ischia, Italy, volcanic vents bubble CO₂ into the sea. Close to the vents, where the pH is low, sea urchins, snails and corals disappear, but seagrass grows well: a window on a more acidic future ocean.
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Ocean acidification is a problem for societies too, especially for people who live from the sea.
The points to remember
- Fisheries and aquaculture (oysters, mussels) can collapse, costing jobs and income.
- Coastal and indigenous communities lose food and may have to import it.
- Coral reefs support tourism, which declines as reefs die.
- Dead reefs protect coasts less from storm waves.
- There is an aesthetic loss, and the species' own right to exist (bio-rights) is harmed.
Environmental systems
- Shells and skeletons
- Food webs, biodiversity
- Resilience of reefs
Societies
- Fisheries and aquaculture
- Food, jobs, income
- Tourism, coastal protection
Three traps: This is not acid rain: do not write about acid deposition, sulfur dioxide or NOx. Describing the chemistry earns only two marks in an essay. And a question on 'systems and societies' needs both sides.
Real example: Washington State's shellfish farms employ about 3200 people and earn about 270 million US dollars a year; in 2012 the state set up a panel to protect them from acidification.
Burning fossil fuels, and cutting down forests, change far more of the ocean than its pH. These are the ocean's abiotic conditions.
The points to remember
- Greenhouse gases warm the atmosphere and the oceans.
- More CO₂ dissolves: acidification, lower pH.
- Warmer water expands and ice melts: sea level rises.
- Melting ice dilutes the sea: salinity falls; more evaporation can raise it locally.
- Warmer water holds less oxygen; changes in temperature and salinity alter currents.
- Oil spills block light, so phytoplankton die and oxygen falls.
- Warmth can boost algal growth locally; dead sea life decomposes, adding CO₂ (or methane deep down).
When the cause is deforestation: Link each step: trees cleared, less CO₂ absorbed, more CO₂ in the air; thawing permafrost adds CO₂ and methane; so global warming and acidification. Bare soil also erodes and sends sediment into rivers and the sea.
Real example: meltwater from Greenland's ice sheet is freshening the North Atlantic, lowering its salinity; scientists are watching whether this slows the ocean currents that carry heat to Europe.
What does not count: Acid rain, NOx or sulfur dioxide changing ocean pH; timber ships polluting the sea; nutrient runoff from taiga soils (they are poor in nutrients). Write 'global warming', not just 'climate change', and say how warming raises sea level.
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How this comes up: Paper 2, Section B (b): explain the potential impact of ocean acidification on environmental systems and societies [7].
Oyster farmers on the Pacific coast of North America and tour operators on the Great Barrier Reef are both worried about the changing chemistry of the ocean.
Explain the potential impact of ocean acidification on environmental systems and societies.
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