Highest Ocean Warming in 400 Years Poses ‘Existential Threat’ to Australia’s Great Barrier Reef, Researchers Say

The ocean heat extremes in 2017, 2020, and 2024 – the highest in four centuries – were caused by human influence on the climate system, new research suggests.

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This generation will likely witness the demise of Australia’s iconic Great Barrier Reef unless rapid, coordinated, and ambitious action is taken globally to reverse climate change, a new study has warned.

Scientists came to this conclusion after looking at recent sea surface temperature trends. They found that the extreme ocean heat recorded between January and March in 2017, 2020, and 2024 was the highest in 400 years. These extremes, which were caused by anthropogenic influence on the climate system, pose an “existential threat” to the reef’s ecosystem. Located off the coast of Queensland, the reef is the largest in the world. It covers an area of about 344,400 square kilometres (133,000 square miles), more than the UK and Ireland combined.

Coral bleaching occurs as a heat stress response from rising ocean temperatures, which drives algae away from coral reefs, causing reefs to lose their vibrant colours. While a bleaching event is not directly linked to corals’ death, more frequent and intense heat stresses make corals more vulnerable to diseases, slowing down their recovery and limiting their ability to spawn.

Scientists at universities across Australia said in a paper published Wednesday in the scientific journal Nature that at this rate, the reef is at risk of experiencing “near-annual coral bleaching.” This, they said, would “further threaten the ecological function and outstanding universal value of one of Earth’s greatest natural wonders.”

Recent large-scale marine heatwaves have led to more frequent and severe mass bleaching events around the world, with reefs’ natural recovery processes struggling to keep up.

In March, the Great Barrier Reef Authority said that aerial surveys on over 300 inshore, midshelf, and offshore reefs confirmed that “widespread, often called mass, coral bleaching event is unfolding” across the reef. A month later, the US National Oceanic and Atmospheric Administration’s (NOAA) and the International Coral Reef Initiative (ICRI) confirmed that the world is undergoing its fourth global coral bleaching event. It is the second in the past ten years. Among the 53 regions affected are Florida, Eastern Tropical Pacific nations including Mexico, El Salvador, Costa Rica, Panama, and Colombia, and Australia. 

According to the most recent report by the Global Coral Reef Monitoring Network (GCRMN), the world has lost approximately 14% of corals since 2009. 

Coral reefs are extremely important ecosystems that exist in more than 100 countries and territories and support at least 25% of marine species. They are integral to sustaining Earth’s vast and interconnected web of marine biodiversity and provide ecosystem services valued up to $9.9 trillion annually. They are sometimes referred to as “rainforests of the sea” for their ability to act as carbon sinks by absorbing the excess carbon dioxide in the water. 

Worrying Trends

Until June, sea surface temperatures broke records every month for 15 consecutive months. While the hot streak came to an end last month, warming is expected to continue.

According to the new study, even if all current international climate mitigation pledges are met, global mean surface temperature is still estimated to increase anywhere between 1.9C and 3.2C above pre-industrial levels by the end of the century. But even if we were to limit global warming to 1.5C, the critical threshold established in 2015 as part of the Paris Agreement, 70-90% of the world’s reefs would still “likely” be lost globally.

So far, the world has warmed by 1.2C compared to pre-industrial times. Nevertheless, IPCC data suggests that 20-40% of the global human population live in regions that, by the decade 2006–2015, had already experienced warming of more than 1.5C in at least one season. According to the UN body, every 0.5C (0.9F) of global warming will cause discernible increases in the frequency and severity of heat extremes, heavy rainfall events, and regional droughts.

Featured image: Tourism and Events Queensland.