Coral reefs are the backbone of ocean biodiversity, as well as industries that support over 500 million people worldwide.
They are, however, some of the most threatened habitats on earth. Vulnerable to temperature changes that destabilise their delicate ecosystems, some scientists estimate that up to 90% of the world’s coral could be gone by 2050 owing to climate change. This would have devastating consequences for ocean plants and animals, as well as the millions of communities who rely on fishing for their sustenance and livelihoods.
‘Global warming has caused unprecedented increases in sea surface temperatures, which kills coral,’ says Dr Melita Samoilys, a research fellow at Oxford’s Department of Zoology. ‘Corals cannot survive even just a few degrees of temperature rise because of their remarkable symbiotic relationship with microscopic algae called zooxanthellae. These zooxanthellae feed the coral and enable them to form into reefs. Zooxanthellae can only survive in a narrow temperature range, so cannot cope with temperature increases caused by climate change. When the water gets too warm, the algae, which allow the coral to survive and give them their beautiful colours, desert the coral – hence “coral bleaching” occurs.’
Coral bleaching, so called because of the ghostly white colour taken on by dead coral, has increased rapidly over the last few years. Dr Samoilys says: ‘Changes in weather patterns, such as El Niño and La Niña, have been connected to climate change and cause a rise in sea surface temperatures. Over the last few years, we have seen summer sea surface temperatures exceed corals’ comfort zone almost every year.’ Intense summer temperatures in 2016 and 2017 alone are blamed for the coral bleaching of nearly half of the Great Barrier Reef, a reef structure so large that it can be seen from outer space.
The mass coral bleaching occurring on enormous scales globally is a startling and accurate indicator of how much our climate is changing
‘Massive coral mortality, such as in the Great Barrier Reef, has a dramatic effect on marine life,’ says Dr Samoilys. ‘Coral reefs are one of the most biodiverse ecosystems on the planet, and their loss has a major impact on other invertebrates, vertebrates and plants. We are seeing a disappearance of fish species that are highly dependent on live coral for food and shelter. Reef sharks, too, are rapidly disappearing in developing countries such as Kenya and Tanzania, where they are at a critical level of depletion. Large predatory fish such as sharks are a critical component of a reef’s food chain and their loss precipitates a cascade of disruption throughout reefs.’
Not only is ocean biodiversity under threat, but so are the coastal communities that rely on the ocean to survive. ‘In the western Indian Ocean alone, we estimate there are 60 million people dependent on coral reefs for their livelihoods,’ says Dr Samoilys. ‘Local fisheries are vital to the economies of developing countries, for food as well as income. Reefs also create a barrier protecting coastlines, which is essential to defend villages against ocean storms. They protect lagoons against seasonal weather like monsoons, which can last several months, allowing fishers to continue to fish in the lagoon. This is critical for their livelihoods.’
Despite this bleak picture for coral reef survival, researchers are continuing to look for ways to save and conserve reefs. ‘When a reef dies, the reef structure is still there, in carbonate rock form. And there are usually a few isolated coral colonies still alive,’ explains Dr Samoilys. ‘What we are working on is trying to understand if it is possible for coral larvae to settle and grow when the reef is no longer functioning – if it has the potential to return.’
Going forward, the future of coral reefs is uncertain. Dr Samoilys says: ‘Reefs’ responses to elevated sea surface temperatures are highly variable and not always predictable; some species are very sensitive, while others are more tolerant. Thus, as we go forward into a future with warmer seas, the community of coral species on a reef will change.
‘The response of reef fishes, too, is also highly variable. Some species that are highly dependent on live coral are likely to go locally extinct. Others may survive a changed ecosystem – these are often larger, more important species in fisheries, which is a good thing for people dependent on fishing. It also means, however, that these species’ populations need to be safeguarded now so they survive into the future.
‘The mass coral bleaching occurring on enormous scales globally is a startling and accurate indicator of how much our climate is changing.’
In light of the increasing damage wrought by climate change, controlling for other factors that affect reefs is becoming even more important. ‘Maximising the resilience of reefs to hot water is a primary objective of conservationists,’ says Dr Samoilys. ‘There is strong evidence that the healthier the general state of the reef, in terms of its fish populations and the surrounding water, the more resilient it will be to climate change.’ This means combating pollution and overfishing, in order to give reefs the healthy environments they need to bounce back.
‘But ultimately the most important thing we can do to preserve coral reefs is to address climate change. Governments need to get serious about changing policies across all sectors to reduce greenhouse gas emissions. Emissions need to be central to all domestic policies; there is no “later” for climate change.’