All subjects · Geography · WGE02 Geographical Investigations

Threats to coastal ecosystems: local and global

Edexcel IAL Geography · WGE02 Geographical Investigations

Coastal ecosystem services are being lost to human activity, both from local pressures and from global climate change (spec 2.3.3, key idea 8).

Practise this in the app: instant marking and a fix for every mistake. Free, no email needed to start.

Practise this lesson

Local threats

Coastal development: ports, hotels, housing and roads replace dunes, marshes and mangroves directly.
Pollution: sewage, agricultural run-off (nutrients cause algal blooms), plastics, oil spills and industrial waste.
Unsustainable fishing and resource extraction: overfishing removes species that keep reefs healthy; blast fishing breaks coral; sand and coral mining remove sediment.
Alien (invasive) species that outcompete native species.
Aquaculture and agriculture: the State of the World's Mangroves 2024 reports that conversion to aquaculture, oil palm and rice explained 43% of mangrove losses from 2000 to 2020.
Tourism: trampling of dunes, anchors and divers damaging coral.

Global threats

Rising water temperatures cause coral bleaching: stressed corals expel the algae that live in their tissues and give them food and colour. NOAA reports that the fourth global bleaching event (early 2023 to mid-2025) exposed 84% of the world's coral reef area to bleaching-level heat stress, in at least 83 countries and territories. Earlier global events were in 1998, 2010 and 2014 to 2017.
Ocean acidification: the ocean absorbs about 30% of the CO2 released into the atmosphere. Surface pH has fallen by about 0.1 since the industrial revolution, about a 30% increase in acidity, leaving fewer carbonate ions for corals and shellfish to build skeletons and shells (NOAA).
Rising sea levels: NASA reports about 10 cm of global rise since 1993, with a rate of 0.59 cm per year in 2024, and the rate has more than doubled since 1993. Marshes and mangroves are drowned if they cannot build up fast enough or migrate inland (coastal squeeze where sea walls or development block them).

Why this matters for livelihoods

Local communities depend on these ecosystems for fish, tourism income, building materials and protection. Loss of a reef or mangrove means lower fish catches, fewer tourists, more storm damage and higher costs of artificial defences. Global threats cannot be fixed locally, but reducing local stresses (pollution, overfishing) can make ecosystems more resilient, so they recover faster from bleaching or storms.

Threat	Scale	Main effect
Coastal development	Local	Direct habitat loss
Pollution and run-off	Local	Algal blooms, poisoned habitats
Overfishing, blast fishing	Local	Damaged reefs, fewer fish
Warming oceans	Global	Coral bleaching (84% of reef area hit, 2023 to 2025)
Acidification	Global	Weaker shells and skeletons
Sea-level rise	Global	Drowning, coastal squeeze

Worked example

Explain how coastal squeeze threatens a salt marsh in front of a sea wall.

Sea level rises, so the marsh would naturally migrate inland to stay at the right height in the tidal frame.
The sea wall blocks this landward movement.
The marsh is squeezed between rising water and the wall, drowns and is lost, along with the wave protection it gave the wall.

Answer: Rising sea plus a fixed barrier: the marsh cannot migrate and is lost.

Key idea: Local threats: development, pollution, overfishing and extraction, alien species, aquaculture. Global: warming (bleaching, 84% of reef area 2023 to 2025), acidification (pH down 0.1, about 30% more acidic), sea-level rise (coastal squeeze).

Check yourself

Coral bleaching is mainly triggered by
- lower sea levels
- unusually high sea temperatures
- colder winters
- increased salinity from rivers
Show answer
unusually high sea temperatures. Heat stress makes corals expel their algae.
pH is logarithmic. NOAA says a fall of 0.1 pH units means about a 30% increase in acidity. If acidity is given an index of 100 before industrialisation, what is the index now?
Show answer
130. 100 x 1.30 = 130.
NASA measured a 2024 rate of 0.59 cm per year. At that constant rate, how much would sea level rise in 10 years, in cm?
Show answer
5.9 cm. 0.59 x 10 = 5.9 cm.
Which is a local, not a global, threat to a reef?
- Ocean acidification
- Rising global sea temperature
- Blast fishing
- Global sea-level rise
Show answer
Blast fishing. Blast fishing is a local activity with local damage.
According to the State of the World's Mangroves 2024, what explained 43% of mangrove losses from 2000 to 2020?
- Conversion to aquaculture, oil palm and rice
- Coral bleaching
- Tsunami damage
- Tourist trampling
Show answer
Conversion to aquaculture, oil palm and rice. Conversion to farming and fish or shrimp ponds was the largest cause.
Coastal squeeze happens when
- longshore drift stops completely
- dunes grow too tall
- rising sea level pushes habitats against fixed defences or development
- a spit joins an island
Show answer
rising sea level pushes habitats against fixed defences or development. Habitats cannot migrate inland.
Explain why ocean acidification threatens coral reefs.
Show answer
The ocean absorbs carbon dioxide, which lowers its pH; this leaves fewer carbonate ions, so corals find it harder to build their calcium carbonate skeletons, which become weaker or even dissolve.. CO2, pH, carbonate.

Practise with instant feedback

Sources

Last updated 8 November 2026. Spotted a mistake? Tell us.