The rainforests of the oceans
Coral reefs have been described as the ‘rainforests of the oceans’.[i] Like the rainforest, reefs are full of life, yet they manage to thrive on very little. Full of colour and movement, reefs are home to around a quarter of marine life. They are important places for fish to feed and breed. Crabs, sea slugs and other molluscs shelter in their rocky architecture.[ii] Although vital to our oceans, corals are highly threatened by humans. Climate change means that warm water corals are at very high risk of severe damage by the end of the century.[iii]
What are corals?
Corals may look like stony outcrops, but in fact they are alive. Small animals called polyps produce a chalk-like substance (calcium carbonate) which forms the reef around them. The polyps use tentacles to catch food from the water. Algae (simple plants) live alongside the polyps and help them to harvest food. Where the sea is shallow, light passes through the water and stimulates the algae, making the reef highly productive. The algae are also responsible for giving corals their colour.
Where can we find corals?
Corals occur in warm tropical waters where the temperature is between 18 and 30 oC. For example, the Great Barrier Reef runs for 1600 miles along the north-eastern coast of Australia. Corals are also found in the Red Sea, Indian Ocean, East Africa and around the Caribbean, Mexico and Florida.
What are cold water corals?
In contrast to the warm water reefs, cold water corals grow slowly in the deeper, darker waters off north and west Scotland, the west of Ireland and the south-west coast of England. Cold water corals can be found as far north as Alaska. Fishing boats dragging equipment along the sea-bed (known as trawling) can smash coral to pieces and so has been banned in some of these areas.
What does climate change mean for coral?
Climate change is a major threat to corals. This is because it poses two very significant risks: rising temperatures and ocean acidification, both of which can cause coral reefs to weaken and die.
Left: A scientist studies corals in the Virgin Islands National Park, photo: NS CC BY 2.0
What causes coral bleaching?
High temperatures are major cause of coral bleaching, when the coral loses its colour and turns white. Although the coral is still alive, it has lost its algae and may eventually starve. Bleaching can happen when the water temperature is 1oC above its normal maximum.[iv] Such high temperatures are likely to be more common, more extreme and longer lasting as a result of climate change. Several global-scale bleaching events have already happened, including one in 2016 which damaged large stretches of the Great Barrier Reef.[v]
If temperatures rise by 1.5 oC as a result of climate change, warm water corals will be at very high risk of severe and irreversible damage.[vi] Currently, we are on course for temperatures to increase by nearly double that amount. Even if countries around the world meet their current pledges under the Paris Agreement on climate change, then temperatures could be 2.8 oC higher by 2100.[vii]
What causes ocean acidification?
Burning fossil fuels to build and heat our homes, to travel and to power factories releases carbon dioxide, a colourless gas, to the atmosphere (see The Climate Change Challenge). About 30% of this gas is taken up by the oceans where it dissolves and breaks apart, making the water less alkaline.[viii] As a result it is harder for coral to build its chalky skeleton and the risk of bleaching increases.[ix]
What other threats do corals face?
Corals are threatened by over-fishing and by fishing in damaging ways. These include trawling, dynamiting reefs with explosives, or poisoning the water to make the fish easier to catch. A second problem is that rivers and heavy rain wash soil off the land into the sea. The soil smothers sea-life and releases nutrients (such as nitrogen and phosphorus) as well as metals, plastics, pesticides and other chemicals into the water. The nutrients stimulate the growth of algae (simple plants and seaweeds) in the sea. Sometimes the algae grow so vigorously that they form large mats, known as algal blooms, that block out the light and reduce the amount of oxygen in the water, causing dead zones. Tourism and shipping cause problems for reefs if they are badly managed. For example, if people collect or damage coral, if sun-screen gets into the water, or if boats pump oil and waste into the sea.
What can be done to protect coral reefs?
Countries around the world have been taking steps to protect reefs from fishing, pollution and other types of damage. This includes setting up marine protected areas (also known as marine conservation zones). These are parts of the sea where damaging activities are either limited or banned altogether. International bodies have called for 30% of the sea to be protected globally, although only around 2.5% to 5% is well protected currently.[x]
As a consumer, you can choose to eat fish and sea-food that has been caught in a sustainable way and to avoid fish (such as the orange roughy) that comes from cold-water coral reefs. You can also help by avoiding sunscreen containing oxybenzone.
While these measures will help, coral reefs will be in danger unless major steps are taken to cut back the fossil fuels that cause climate change.
[i] Coral Reefs: Canaries of the Sea, Rainforests of the Oceans P K Swart, Nature Education Knowledge, 2013, 4(3):5.
[ii] As above.
[iii] Special Report: Global Warming of 1.5 oC – Summary for Policymakers, Intergovernmental Panel on Climate Change, 2018, p. 11.
[v] Global warming and recurrent mass bleaching of corals Terry Hughes, et al, Nature, 2017, 543, pp.373–377.
[vi] Compared to the pre-industrial period. Special Report: Global Warming of 1.5 oC – Summary for Policymakers, Intergovernmental Panel on Climate Change, 2018, pp. 4 & 11.
[vii] Range 2.3 oC to 3.5 oC, CAT Warming Projections: Global Temperature Increase by 2100 Climate Action Tracker December 2019 update.
[viii] Sustainable Development Goals: Goal 14: Conserve and sustainably use the oceans, seas and marine resources, United Nations, Facts & Figures.
[ix] Ocean acidification affects coral growth by reducing skeletal density, Nathaniel Mollica et al, Proceedings of the National Academy of Science of the United States of America, 2018, 115(8): 1754–1759.