What causes coral bleaching, and how can we stop it?

Coral reefs are one of our planet’s most diverse ecosystems, providing a wide range of ecological and economic benefits. Yet, they are increasingly threatened, and coral bleaching is the most visible sign of their degradation.

The process is linked to climate change and rising sea temperatures. It is not a recent phenomenon but has been observed since the early 1980s, according to the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch. The latest scientific data shows that a new wave of mass bleaching is underway.

So, what are the reasons for coral reef bleaching and how can it be countered?

Coral reefs are underwater habitats created by colonies of small aquatic animals called coral polyps. They use calcium and carbonate to build the reefs. Their distinctive colour comes from symbiotic algae – zooxanthellae – that live inside the corals and form their main energy source.

Healthy coral reefs serve as a home for the polyps themselves, but they also provide shelter, spawning grounds and food for thousands of marine species, creating complex ecosystems.

Australia’s Great Barrier Reef, for example, is home to 400 types of corals, 1,500 kinds of fish, 4,000 mollusc species and six out of seven types of sea turtles, according to the UK’s Natural History Museum (NHM). The Coral Triangle in Southeast Asia is said to be the most biologically diverse ecosystem in the world.

But not just marine species depend on the corals’ constructions. Globally, an estimated one billion people benefit from coral reefs, either directly or indirectly, according to the US Office for Coastal Management. Globally, the total value of coral reefs has been estimated to be as high as £6 trillion ($7,530,000) annually, the NHM states.

This includes the key roles they play in fishing and tourism as well as coastal protection: intact coral reefs can protect shorelines by absorbing 97% of a wave’s energy, acting as buffers for currents, storms and even tsunamis.

Coral bleaching has been observed for the last 40 years. The term refers to the process by which corals lose the symbiotic zooxanthellae algae, NOAA’s Coral Reef Watch explains.

Ocean ecosystem changes, especially rising ocean temperatures, cause coral polyps to become stressed and expel the symbiotic algae. Without the zooxanthellae, coral reefs fade and turn white, as if bleached. If the ocean temperatures stay high or continue to rise, the corals don’t reabsorb the algae and die as a result.

Over the past three decades, sea surface temperature (SST) has consistently been higher than at any other time since reliable observations began in 1880, United States Environmental Protection Agency data shows.

Mass coral bleaching over hundreds or even thousands of kilometres has become one of the most obvious and damaging impacts of global warming, NOAA warns. The US Office for Coastal Management states that the last mass bleaching wave, between 2014 and 2017, encompassed 75% of global reefs.

And ocean temperatures continue to rise. In August 2023, SST reached an unprecedented high of 20.98°C, and by February 2024, this had increased to 21.06°C, according to the European Union’s Copernicus Climate Change Service reports.

On 15 April, NOAA and the International Coral Reef Initiative declared a new global mass coral bleaching event. This is attributed to both climate change and the El Niño climate pattern pushing water temperatures up to record levels, Reuters reports. The new bleaching wave affects many reefs in the Atlantic, Pacific and Indian oceans.

One of the reefs affected by this is the Great Barrier Reef. Close to 40% of its many hundreds of individual reefs are experiencing either very high (over 60%) or extreme (over 90%) bleaching, mostly in the south of the reef area.

Marine biologists monitoring Kenya’s coast told Reuters they estimate at least 60% of coral they’ve observed is bleached, threatening local artisan fishermen’s livelihoods and food security – not just locally but on a wider scale.

While in India’s Gulf of Mannar Marine Biosphere Reserve, early signs of mass coral bleaching have also been spotted. During the last major wave in the mid-2010s, the Gulf of Mannar lost nearly half of its coral cover, which dropped from 39% to 23%. The new bleaching event is mainly affecting porite corals, half of which have been bleached.

And, in Indonesia, the coral reefs around the Gili Matra islands in the south of the archipelago have almost disappeared. Only 10% of the total area covered by coral ecosystems in Gili survives. By the mid-2040s, scientists expect more than half of Indonesia’s reefs to be affected by bleaching, The Conversation reports.

As the majority of coral degradation is a result of human activity, action to reduce CO2 and other emissions remains the single most important countermeasure to stop the underlying drivers of coral bleaching.

But wider action will also be needed.

For example, Indonesia has a number of bleach-resistant coral reefs in conservation areas. The issue is not expected to affect them until the mid-2040s, so they could provide some refuge if correctly protected and supported, The Conversation points out.

This includes restoring and growing new reefs and monitoring protected areas to prevent irresponsible fishing practices, pollution from agricultural run-off and unsustainable tourism. Promoting sustainable economic activity will also be key, not only in Indonesia but worldwide.

Last year, the International Coral Reef Initiative launched a blended finance initiative with the Global Fund for Coral Reefs and the UN High-Level Climate Champions. The aim is to mobilize $12 billion to raise the resilience of 125,000 km2 of tropical coral reefs and transition to “blue economy” practices that make sustainable use of the ocean ecosystem.

Protecting the marine environment and developing the blue economy are the focus of the World Economic Forum’s Friends of Ocean Action initiative. It aims to conserve and use marine resources responsibly to enable sustainable development.

Meanwhile, UpLink partnered with the Kingdom of Saudi Arabia's Ministry of Economy and Planning, WAVE, a collective action platform powered by the Future Investment Initiative Institute, Ocean Action Agenda and 10 more ecosystem partners to launch the Regenerative Blue Economy Challenge. This challenge is calling for solutions to reduce marine pollution and promote ecosystem protection, restoration, and rewilding. The winning innovations will benefit from exclusive networking opportunities, targeted support, visibility, and the chance to share part of the CHF 300,000 prize. To learn more and submit your solution, visit UpLink.