Rising levels of greenhouse gases from burning fossil fuels are wreaking havoc on our oceans. These emissions fuel temperature increases above and below the surface and alter the chemistry of the sea, creating interconnected impacts that affect all marine life, including coral reefs.
Global Threats to Reefs
Warming Sea Temperatures
The oceans absorb 91% of excess atmospheric heat, masking the true impacts of climate change. But they are reaching a saturation point. and the impacts on marine ecosystems and marine life are bubbling to the surface.
This has two major implications for reefs:
1. Coral bleaching
Coral bleaching occurs when corals are exposed to temperatures above their natural range for extended periods of time. When it’s too warm, the symbiotic zooxanthellae in coral tissues can damage the coral polyp. Like a frustrated landlord, the coral evicts the zooxanthellae into the water, leaving its tissues transparent so we can see the bleached white of its skeleton.
Whilst bleached, the coral is still alive, but they can’t survive long in that state.
If the temperatures stay too hot for too long, the coral animal will die, but if the coral polyp is still alive when the temperature cools, it can re-contract the zooxanthellae in the water column. There is a significant bleaching hangover, though, for years after corals will have slower growth rates, reduced reproduction and be more susceptible to diseases.
GLOBAL BLEACHING EVENTS
1997–1998 - 8% of global coral died
2009–2010 & 2014–2017
- 14% of global coral died
2023–2024 - 84% of reefs affected.
1-2 years to fully understand
the impact.
2. Increased storms and frequency
Rising ocean temperatures intensify tropical storms as they cross seas worldwide. In our warming climate, typhoons, hurricanes, and cyclones have become both more powerful and frequent.
Storm waves damage the delicate coral structures and deposit sediment over reefs, which smothers them and blocks the sunlight essential for their survival.
Ocean Acidification
The ocean doesn't just absorb heat — it also takes in carbon dioxide through gas exchange at the surface. This occurs at such a massive scale that it's disrupting the chemical balance that controls seawater pH and calcium carbonate availability.
Coral polyps build their skeletons by extracting minerals from the seawater. As oceans worldwide become increasingly acidic, these minerals become scarcer, depriving corals of essential building materials. This causes corals to grow more slowly and develop thinner skeletons, making them significantly more susceptible to damage from events like storm waves.
Fringing our coastlines, coral reefs are exposed to many different human activities that directly impact their ecosystem health. These mounting pressures erode reef resilience, leaving them deteriorated and more vulnerable to global threats.
Local Threats to Reefs
Marine Tourism
Scuba diving and snorkelling activities can directly damage coral reefs. Whether it’s an errant fin kick, dropping an anchor, using toxic sunscreen or flushing a marine toilet onto a reef, these tourism activities are impacting reefs across 96 countries and territories.
Pollution
Pollution caused by runoff from fertilisers, pesticides and sewage from land can promote algae growth, which competes with coral for space and sunlight. Harmful bacteria and toxic chemicals can cause further damage, inhibiting coral growth and killing some areas of the reef entirely.
Plastic pollution has reached every part of our ocean. Corals are actively consuming plastic particles, even preferring them to their natural diet. When corals fill their stomachs with plastics, they stop eating their regular nutritious food. These tiny plastic pieces become lodged in their digestive systems, causing gradual starvation.
Overfishing & Destructive Fishing
Overfishing devastates coral reef ecosystems. The removal of herbivorous fish species is particularly damaging, as these crucial grazers maintain ecosystem balance by controlling algae growth on reefs.
Practices like dynamite fishing obliterate reef habitats so completely that recovery becomes impossible. Similarly harmful is cyanide fishing — a method used to capture live fish for seafood markets and aquarium trade, which also poisons countless non-target species, including coral, leaving behind a trail of collateral damage.
Coastal Development
Poorly planned coastal development threatens reefs by altering natural water flows, increasing sewage discharge into marine environments, elevating harmful sedimentation levels, and sometimes directly removing reef structures for construction projects. These activities collectively disrupt the delicate balance reef ecosystems require to thrive.
. . . but there is hope >
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