The latest reports from organizations like the International Union for Conservation of Nature (IUCN) and UN Environment Programme, report that about 50% of the world's coral reefs are considered damaged or at risk of collapse, with some estimates suggesting that we could lose up to 90% of coral reefs by 2050 if current trends continue. This is a staggering figure when considering the ecological and economic importance of coral reefs.

Coral reefs support over 25% of marine life despite occupying less than 1% of the ocean floor. They also support economies through tourism, diving, and fisheries, and they are prominent in supporting the balance of marine biodiversity. Reef sharks, for example, would be lost without them.

The decline in coral health can be attributed to multiple stressors, but global warming has become one of the most significant, especially in the last few decades.

How Global Warming Affects Coral Reefs

Coral bleaching is the most visible effect of global warming on coral reefs. Corals are symbiotic organisms, meaning they live in a close relationship with tiny algae called zooxanthellae. These algae provide corals with food through photosynthesis and give corals their colour. However, when water temperatures rise beyond the level that coral can tolerate, which is usually around 1 to 2°C above the average, corals become stressed. As a result they expel the algae, which causes them to lose their vibrant colours. This is known as "coral bleaching."

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Bleached coral on a dying reef

Bleached corals are not dead, but they are severely weakened and can no longer get sufficient nutrients from photosynthesis. If high temperatures persist for weeks or months, they will die. Over the past few decades, coral bleaching events have been becoming more frequent and severe. For instance, the 2016 mass bleaching event, triggered by an El Niño year combined with rising ocean temperatures, killed large portions of coral across the Great Barrier Reef, one of the largest and most studied coral ecosystems in the world.

Ocean Acidification

Global warming also drives an increase in atmospheric CO₂, much of which is absorbed by the oceans. This excess carbon dioxide lowers the pH of seawater, a phenomenon known as ocean acidification. Acidified waters make it harder for corals to build their calcium carbonate skeletons, which are essential for their structure and growth. Weakened skeletons can result in reduced reef resilience and slower growth rates.

Increased Storm Intensity

Warmer ocean temperatures are also linked to more intense storms, including cyclones and hurricanes. These storms can physically damage coral reefs by breaking apart corals, stirring up sediment that smothers coral, and altering the surrounding environment. The rise in storm frequency and intensity has become an added stressor in many reef ecosystems.

Changes in Water Quality

As global temperatures rise, the combination of higher sea levels and increased run-off from rainfall can exacerbate water quality issues, like pollution and nutrient loading. Coral reefs are highly sensitive to changes in water quality, and excess nutrients (often from agricultural run-off or untreated sewage) can lead to algal blooms, which further reduce the amount of sunlight reaching the corals and deprive them of the oxygen they need to survive.

Loss of Habitat and Biodiversity

Beyond the immediate threats to coral itself, warming oceans are also affecting the entire marine ecosystem. Many species that depend on coral reefs for shelter, food, and breeding grounds are being pushed to migrate to cooler waters, further stressing the biodiversity of reef systems. As the habitats degrade, their fish populations decline, and the entire ecosystem becomes less resilient.

Historical Trends

Over the past 50 years, ocean temperatures have increased significantly. According to NASA, global sea surface temperatures have risen by about 0.13°C per decade since 1880, with a more pronounced rate of warming in recent decades. This warming has contributed to the frequency and severity of coral bleaching events:

1980s: The first major documented coral bleaching event occurred in the early 1980s in the Indian Ocean, though it was relatively limited.

1990s: Bleaching events started to appear more frequently across the Pacific and Indian Oceans. In 1998, one of the most severe global bleaching events affected reefs worldwide, with over 16% of the world’s coral reefs affected.

2000s: The early 2000s saw repeated bleaching events, particularly in the Pacific Ocean. The Great Barrier Reef lost significant portions of its coral cover during this period. This writer saw the local coral fringe reef in French Polynesia, including its fish and complement of invertebrates, die over this ten-year period.

2010s: The decade was marked by some of the most devastating coral bleaching events, notably in 2016 and 2017, where the Great Barrier Reef lost around 50% of its coral cover. These events were directly linked to rising ocean temperatures caused by the combined effects of global warming and El Niño patterns.

2020s: We are currently witnessing continued coral loss, particularly in places like the Great Barrier Reef, the Coral Triangle (the world's most biodiverse marine area), and the Caribbean. Warming continues to outpace coral adaptation, and predictions for the next 30 years are grim without significant global mitigation of greenhouse gas emissions.

What Can Be Done?

The most critical action to safeguard coral reefs in the long term is to address the root cause: global warming. This means a global push to drastically reduce the emissions of carbon dioxide and other greenhouse gases, and limit warming to well below 2°C above pre-industrial levels, as set out in the Paris Agreement. If global warming exceeds 1.5°C, it is projected that many coral reefs will be unable to survive.

Marine Protected Areas (MPAs)

Expanding and enforcing marine protected areas can provide corals with a refuge from human activities like fishing, pollution, and coastal development. These areas give coral ecosystems the best chance to regenerate if and when local conditions improve.

Coral Restoration Projects

There is growing research into coral restoration techniques, such as breeding heat-resistant coral strains or transplanting healthy corals from other regions. However, these efforts are still in their early stages and face challenges related to scale and sustainability, not to mention the ultimate effects on local ecology.

Sustainable Coastal Development

Reducing pollution and run-off from agriculture and coastal cities can improve water quality around coral reefs. This is a crucial step in reducing additional stressors on coral systems as well as contributing to the health of the rest of the marine life along the coasts. 

Raising Awareness and Policy Advocacy

Coral reefs are in severe jeopardy due to the accelerating impacts of global warming and efforts to protect them rely on raising awareness about their importance and advocating for strong policy measures to combat global warming. Educating local communities, stakeholders, and governments is essential for the long-term protection of coral reefs.

The sixth global mass extinction has been brought about by human activities. Marine biodiversity loss in general is largely the result of decades of over-exploitation by fisheries, and now global warming is accelerating the losses. There must be a shift in attitude away from the idea that human expansion is the priority, above all other concerns, towards the management of the biosphere in the interests of sustainability, not of coral alone, (although it may be taken as a key indicator), but to permit our civilization to continue on in good health. 

A new set of priorities must be adopted if the planet’s coral reefs, as well as aquatic ecosystems in general, are to be saved from ecological collapse. In particular, a binding international agreement for the protection of biodiversity in general is what is needed.

(c) Ila France Porcher

author of The True Nature of Sharks