Ecosystems are not simply getting warmer. They are being pulled apart at different speeds — corals losing the algae inside them, deserts spreading into farmland, whole biomes creeping towards the poles while the species inside them fail to keep up. This page covers the impacts, the examples that earn marks, and why some ecosystems cope far better than others.
📚 What you need to know
Impacts run from local (coral bleaching, desertification) to global (ocean circulation, sea-level rise).
Coral bleaching: warmer seas make corals expel the algae living in their tissues, so they lose both their colour and their main food supply.
Sea-level rise has two causes: melting land ice and thermal expansion of warming seawater.
Productivity changes regionally — longer growing seasons in the far north, falling yields in much of the tropics.
Resilience depends on biodiversity and on how connected habitats are. Fragmented, species-poor systems recover least well.
Biome shifts: ecosystems move towards the poles and to higher altitudes, some transform, and a few have nowhere left to go.
Coral bleaching
Corals live in partnership with tiny algae called zooxanthellae, which live inside the coral’s tissues. The algae photosynthesise and pass most of the organic compounds they make to the coral, which is where the coral gets most of its energy — and its colour.
When sea temperatures rise beyond a narrow tolerance, that partnership breaks down and the coral expels the algae. The white calcium carbonate skeleton then shows through the transparent tissue, which is why we call it bleaching.
The Great Barrier Reef has now been through several mass bleaching events. It is the shrinking gap between them, more than any single one, that does the damage.
Effects: reef fish and invertebrates lose their habitat, biodiversity falls, and fisheries that depend on the reef decline. A weakened reef is also less able to cope with other stresses such as ocean acidification and storms — lost resilience, in the language of this course.
Desertification
Causes: prolonged drought and higher temperatures increase evaporation and kill vegetation, while unsustainable land use — deforestation, overgrazing, over-cultivation — strips away what is left.
Effects: with no roots holding it, bare soil is blown or washed away. Fertile land is lost, agricultural productivity falls, water tables drop, and communities that depend on farming are displaced. The Sahel region of Africa is the standard example.
Desertification is a feedback story too. Vegetation loss leaves bare soil, bare soil reflects more light and holds less moisture, less moisture means less local rainfall, and less rainfall means less vegetation. The land dries itself out.
Global impacts
Changes to ocean circulation
Cause: melting ice caps and glaciers pour fresh water into the North Atlantic. Fresh water is less dense than salt water, so it disrupts the sinking that drives the circulation.
Effect: altered weather patterns, changes to marine and coastal ecosystems, and shifts in fish migration that hit fisheries.
Example: the slowing of the Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream — a warming world that could paradoxically bring colder winters to parts of Europe.
Sea-level rise
Causes: melting land ice adds water to the ocean, and thermal expansion means the water already there takes up more space as it warms.
Effects: coastal flooding and erosion, loss of mangroves and salt marshes, and saltwater pushing into freshwater habitats and farmland.
Example: low-lying island nations such as the Maldives, where whole communities face becoming uninhabitable.
The classic trap: melting sea ice does not raise sea level, because floating ice already displaces its own weight of water. It is land ice — ice sheets and glaciers — plus thermal expansion that does it. Sea ice loss still matters enormously, but through albedo and habitat, not sea level.
Regional changes in productivity
Region
What is happening
Example
Northern high latitudes
Warmer conditions extend the growing season and open new land to agriculture and forestry
Farming expanding northwards in parts of Canada and Russia
Tropics and subtropics
Heat stress and unpredictable rainfall reduce yields and raise crop failure risk
Shifting monsoon patterns threatening rice production in Southeast Asia
Drylands
Rainfall becomes less reliable and desertification advances
The Sahel, where farming and grazing land is being lost
Climate change has winners as well as losers, and saying so is not denial — it is balance, and examiners reward it. The catch is that the gains are concentrated in a few wealthy northern regions while the losses fall on places least able to absorb them.
What makes an ecosystem resilient
Biodiversity
A species-rich ecosystem has more ways to respond to change. If one species fails, another can fill a similar role, so the system keeps functioning — a tropical rainforest recovers from disturbance far better than a monoculture farm, which can be wiped out by a single pest. Climate change works against this, because habitat loss, altered food webs and extreme weather all reduce biodiversity, which reduces resilience, which makes the next shock worse.
Habitat fragmentation
Connected habitats let species move as conditions change. Fragmented ones trap them. Climate change fragments habitats in several ways: spreading deserts split ecosystems apart, more frequent fires break up forests, melting sea ice divides the hunting range of polar species, and mountain species are pushed into ever smaller patches at higher altitudes.
Biome shifts
A biome shift is a change in the location or type of an ecosystem caused by changing climate. Three things happen:
Movement — forests, grasslands and tundra shift polewards or upslope as species track the conditions they need. Temperate forest is moving north in North America, slowly replacing boreal forest.
Transformation — a biome stays put but changes character. Arctic tundra is turning to shrubland as warmth lets woody plants grow.
Loss — some biomes simply run out of room. Alpine communities move up the mountain until there is no more mountain.
This is why mountain-top and polar species appear so often in extinction-risk lists: their habitat is being squeezed from below with nothing above it.
Worked examples
WORKED EXAMPLE
Explain how rising sea temperatures lead to coral death, and outline two consequences for the reef ecosystem. [5]
Step 1: the stress
Sea temperatures rise above the narrow range corals tolerate.
Step 2: the mechanism
The coral expels the zooxanthellae living in its tissues, so it loses its colour and appears white — bleaching.
Step 3: why that kills
Those algae photosynthesise and supply most of the coral’s energy. Without them the coral starves, and if the heat persists it dies.
Step 4: two consequencesFish and invertebrates lose their habitat, so biodiversity falls; and reef fisheries decline, affecting people who depend on them.Heat → algae expelled → energy supply lost → coral death
WORKED EXAMPLE
Suggest why alpine species face a higher extinction risk from warming than lowland species. [3]
Point 1: how species respond
As temperatures rise, species shift to cooler ground — polewards, or up in altitude.
Point 2: the geometry of a mountain
A mountain narrows towards the summit, so each upward step leaves less habitat area and smaller populations.
Point 3: the dead endSpecies already at the top have nowhere cooler to move to. Populations also become isolated on separate peaks, so they cannot mix or recolonise.Shrinking, fragmented habitat with no escape route
💡 Exam tip
Learn one named example per impact: Great Barrier Reef, the Sahel, the AMOC, the Maldives. Named examples lift an answer a whole band.
Structure impact answers as cause → effect → example. It fits almost every question on this page.
Give both causes of sea-level rise. Leaving out thermal expansion is the most common lost mark here.
Use the word resilience and link it to biodiversity and habitat connectivity.
Acknowledge the gains as well as the losses when a question says “discuss” or “evaluate”.
⚠ Common mix-up
Bleached does not mean dead. The coral is alive and starving; whether it recovers depends on how long the heat lasts.
Corals do not “lose their colour because they die”. The colour comes from the algae, and they leave first.
Melting sea ice does not raise sea level. Land ice and thermal expansion do.
Desertification is not only caused by climate. Overgrazing and deforestation are usually part of the story.
Biomes do not move as neat blocks. Individual species shift at different rates, so new and unstable communities form.
A warmer Arctic does not mean a warmer Europe. A slowing AMOC could cool parts of north-west Europe, which is a genuinely counter-intuitive point worth making.
Up next: Climate Change and Human Societies — why the same hazard produces very different outcomes in different places, and what determines whether a society copes.
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