From one reef to the whole ocean: Climate change acts at every scale: it can bleach one reef, dry out one grassland, or raise the sea everywhere. It also weakens an ecosystem's resilience.
The points to remember
- Local impacts change one ecosystem: coral bleaching on a reef, desertification of dry grassland.
- Global impacts change the whole planet: rising sea level and changes to ocean circulation.
- Over time, whole biomes shift towards the poles and up mountains.
- Climate change is an indirect impact: it changes conditions rather than removing organisms at once.
- It is a multiplier: it adds stress and makes habitat loss, pollution and overfishing worse.
- Many species cannot adapt or move as fast as the climate is changing, so populations fall.
Local impacts
- Coral bleaching on a reef
- Desertification of dry grassland
- Wildfire in one forest
- Plants moving up one mountain
Global impacts
- Rising sea level
- Changes to ocean circulation
- Ocean acidification
- Biomes shifting towards the poles
Remember it as: Climate change does not knock the house down; it weakens every wall.
Real example: polar bears in Hudson Bay, Canada, hunt seals from the sea ice. The ice now melts earlier each spring, so the bears have less time to feed; their number fell from about 1,200 in 1987 to about 600 in 2021.
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Coral reefs are the clearest local impact. They are also the ecosystem the guide suggests you explore with temperature data.
The points to remember
- Coral bleaching: when water stays too warm for weeks, corals expel the algae living in them.
- The algae feed the coral and give its colour, so a bleached coral starves and may die.
- Dead coral means lost habitat: reef fish and other species decline, so biodiversity falls.
- Ocean acidification: the ocean absorbs CO2, which forms carbonic acid; pH has fallen by about 0.1 since 1750.
- Lower pH leaves fewer carbonate ions, so corals and shellfish struggle to build shells and skeletons.
- Other causes of bleaching: El Niño heat, runoff, sediment, extreme low tides, salinity changes, UV.
Real example: in 2016 a marine heatwave killed about 30% of the corals on Australia's Great Barrier Reef. The reef suffered five mass bleaching events between 2016 and 2024.
Causes of bleaching: what does not count: Rising sea level, scuba divers and overfishing do not cause bleaching. Anything that breaks or kills the coral outright is damage, not bleaching.
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On land, the local impacts are drought, desertification, heat and fire.
The points to remember
- Desertification: less rain and more evaporation, plus overgrazing, turn dry grassland into desert.
- Droughts, including sudden flash droughts, are becoming more frequent in many regions, but not everywhere.
- Human causes: warming raises evaporation and shifts rainfall; also water extraction, deforestation and poor farming.
- Heatwaves close leaf pores and stress plants, so primary productivity falls.
- Wildfires spread further in drier, windier conditions and where dead vegetation (fuel) has built up.
- Fire kills organisms, destroys habitat and exposes soil to erosion.
Real example: in Australia's Black Summer of 2019-2020, after the hottest and driest year on record, fires burned more than 18 million hectares and killed or displaced an estimated 3 billion animals.
Reading a fire chart: If the number of fires stays the same but the area burnt rises, the fires were bigger, not more numerous. Explain it with drier or windier weather or built-up fuel, not just 'global warming'.
Species respond to warming in three ways: they move, they change their timing, or they decline.
The points to remember
- Range shifts: species move towards the poles or uphill to stay in the temperatures they need.
- Mountain and polar species can run out of space: nowhere left to go.
- Timing (phenology): spring comes earlier, so species that depend on each other can fall out of step.
- Food supply: warmer water can cut upwelling of nutrients, so plankton and fish fall, and their predators go hungry.
- Prey may move away to cooler water, or new species and competitors may arrive.
- Reptiles: the temperature of the nest can set the sex of the young; extremes, drought, fire and disease add risk.
Move
- Towards the poles
- Uphill
Change timing
- Earlier spring
- Out of step with food
Decline
- Nowhere to go
- Less food, more disease
Real example: on Raine Island, on the northern Great Barrier Reef, green turtle eggs develop as females in warm sand. A 2018 study found that about 99% of young turtles from the northern reef were female.
Link it to food: In a 'food supply' question, say whether the prey increases or decreases, and why: 'warmer water reduces upwelling, so fewer plankton and fish are available for seals'.
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Global impacts: the sea and its currents: Some impacts reach every coast and ocean. The two the guide names are rising sea level and changes to ocean circulation.
The points to remember
- Sea level rises for two reasons: thermal expansion of warmer water and melting land ice (glaciers, ice sheets).
- Melting sea ice does not raise sea level: it is already floating.
- Global sea level rose by about 0.20 m from 1901 to 2018, and the rise is speeding up.
- Rising seas flood and erode coastal habitats: salt marshes, mangroves, beaches used by turtles, seals and birds.
- Mangroves can keep up with a slow rise, but not a fast one; more sediment can smother corals.
- Ocean circulation: warming and meltwater can weaken currents and upwelling, changing where nutrients and fish are.
Raises sea level
- Warmer water expanding
- Glaciers melting
- Greenland and Antarctic ice sheets melting
Does not raise it
- Floating sea ice melting
- Daily tides (they average out)
Real example: the Sundarbans, the world's largest mangrove forest, on the coast of India and Bangladesh, is home to Bengal tigers. Rising seas push salt water further inland, killing freshwater trees and drowning low islands.
Marine, not land: In a question on marine biodiversity, flooding the land of a low island does not count, unless you link it to coastal or marine habitats such as beaches, reefs or mangroves.
Climate change does not harm every ecosystem in the same way. Some gain, many lose, and how well each copes depends on its resilience.
The points to remember
- In cold places, warming and longer growing seasons can raise productivity: tundra greens, the treeline rises.
- More CO2 can speed up photosynthesis where water and nutrients are not limiting.
- But heat, drought and fire lower productivity elsewhere, and less primary productivity means less for consumers.
- Other human changes to the air also cut productivity: UV from ozone loss, acid rain, smog.
- Resilience is higher with more biodiversity, larger and connected habitats, and genetic diversity.
- Resilience is lower when change is fast and the ecosystem is already stressed by other impacts.
Productivity may rise
- Tundra and boreal forest
- High mountains
- Places where cold limits growth
Productivity falls
- Places hit by drought and heat
- Coral reefs
- Ecosystems already stressed
Real example: satellites show that about a fifth of the Arctic tundra became greener between 1985 and 2016, as shrubs grew taller and spread into warming land.
Weighing benefits: In a 'to what extent' question about one region, give gains (more growth, new species) and losses (lost habitat, extinctions), then a judgement such as 'gains now, losses later'.
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How this comes up: Paper 2, Section B (b) [7]: explain how a change caused by climate change affects the biodiversity of an ecosystem. Seven linked points, each one cause and effect.
Coral reefs cover less than 1% of the ocean floor but are home to about a quarter of all marine species.
Explain how rising sea temperatures and ocean acidification could affect the biodiversity of coral reef ecosystems.
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