Great Barrier Reef coral bleaching: unprecedented heat over 400 years

The Great Barrier Reef is immense and extraordinary. Yet repeated mass coral bleaching events, caused by elevated ocean temperatures, are putting the coral colonies that underpin the reef’s survival at risk.

Our research, published today in Nature, offers a new long-term view of the sea surface temperatures responsible for coral bleaching. It finds that recent ocean surface heat is without precedent over the last 400 years. It also confirms that humans are responsible.

These findings provide sobering evidence that global warming, driven by human activity, will keep harming the Great Barrier Reef.

There is still reason for hope. However, we must confront a difficult reality: unless humanity changes its present trajectory, our generation is likely to see the loss of one of Earth’s greatest natural wonders.

A unique ecosystem: the Great Barrier Reef

The Great Barrier Reef is the world’s largest coral reef system. It supports an exceptional range of biodiversity, including more than 400 coral types, 1,500 fish species and 4,000 mollusc types, alongside threatened turtles and dugongs.

But mass coral bleaching during the past three decades has severely affected the reef. Bleaching happens when corals are so stressed by heat that they expel the tiny organisms living within their tissues. These organisms provide some of a coral’s colour and help fuel its metabolism.

Corals can recover following mild bleaching events. However, many corals died during the latest events.

The Great Barrier Reef has endured five mass bleaching events across the past nine summers. Were these events unusual, or were they within the natural variability experienced by the reef in earlier centuries? Our research aimed to address this question.

A story spanning 400 years

Coral can itself reveal what occurred in the past.

As corals develop, the chemistry of their skeletons records the ocean conditions at that time, including temperature. Large, boulder-like corals called Porites are particularly valuable records of the past, as they can survive for centuries.

Our study examined how Coral Sea surface temperatures, including those around the reef, have changed over the previous four centuries. We concentrated on January–March, the reef’s warmest three-month period.

We began by bringing together a network of continuous, high-quality coral records from the area. Coral climate scientists analysed these records, which contain thousands of measurements from Porites corals throughout the western tropical Pacific.

Using these records, we reconstructed mean Coral Sea surface temperatures between 1618 and 1995, then calibrated them against modern temperature records from 1900 to 2024. The findings were alarming.

Between 1960 and 2024, annual average summer temperatures rose by 0.12°C per decade.

Moreover, average sea surface temperatures in 2016, 2017, 2020, 2022 and 2024 ranked among the six warmest experienced by the region in four centuries.

Humans are clearly responsible

We next investigated how far rising Coral Sea temperatures could be linked to human influence.

For this, we used published computer-model simulations of Earth’s climate, both with and without human influences such as greenhouse gases produced by burning fossil fuels.

What did this show? In simulations without human influence, Coral Sea surface temperatures from January–March have stayed comparatively stable since 1900. When human impacts are included, the region warms gradually in the early 1900s and then quickly after the 1960s.

Put simply, our analysis using the world’s leading climate models shows that the exceptionally high sea temperatures of recent years would be virtually impossible without human-caused global warming.

The outlook is even more concerning. Recent climate projections indicate that warming will intensify, even after allowing for international emissions-reduction commitments. This means the reef could face coral bleaching almost every year.

Successive bleaching events are likely to be catastrophic for the Great Barrier Reef, as they prevent corals from having sufficient opportunity to recover between events.

Even if global warming remains below the Paris Agreement target of 1.5°C above pre-industrial temperatures, 70% to 90% of corals worldwide could be lost.

We must remain focused

The Australian government has a vital responsibility in managing the threats facing the Great Barrier Reef. This devastation is occurring in its own backyard and during its watch.

However, what is unfolding on the Great Barrier Reef must also serve as an international warning. The fourth global mass coral bleaching event took place this year, and the Great Barrier Reef is far from the only reef under threat.

Every fraction of a degree of warming avoided offers greater hope for coral reefs. This is why the world must remain committed to ambitious measures that cut greenhouse gas emissions.

At the very minimum, emissions-reduction targets must be achieved. The necessary solutions already exist, and leaders must put them into practice.

Our research provides society with scientific evidence of what is at risk if action is not taken.

The future of one of Earth’s most extraordinary ecosystems rests with all of us.

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The authors gratefully acknowledge the contributions of Andrew King, Ariella Arzey, David Karoly, Janice Lough, Tom DeCarlo and Brad Linsley, as well as the producers of the coral data that made this study possible.

Ben Henley, Lecturer, School of Agriculture, Food and Ecosystem Sciences, The University of Melbourne; Helen McGregor, Professor, Environmental Futures & Securing Antarctica’s Environmental Future, School of Earth, Atmospheric and Life Sciences, University of Wollongong, and Ove Hoegh-Guldberg, Professor, School of the Environment, The University of Queensland

This article is republished from The Conversation under a Creative Commons licence. Read the original article.