World’s biggest coral survey confirms sharp decline in Great Barrier Reef after heatwave
- Written by The Conversation
Official analysis of 124 reefs on the Great Barrier Reef shows coral cover has dropped sharply after a record-breaking marine heatwave in 2024, prompting grave fears over the trajectory of the natural wonder.
Over the past few years, fast-growing corals had pushed the Great Barrier Reef’s coral cover to record highs. But those corals were known to be extremely vulnerable and one bad summer away from losing those gains.
Our new report by the Australian Institute of Marine Science (AIMS) shows these fears have been realised. The percentage of living hard coral covering the Great Barrier Reef’s surface dropped in each region we surveyed.
The recent extreme highs and lows in coral cover are a troubling phenomenon. It raises the prospect that the Great Barrier Reef may reach a point from which it cannot recover.
Another global marine heatwave
In healthy corals, tiny algae produce both the coral’s main food source and its vibrant colours. When the water gets too warm, the algae are expelled and the coral’s tissue becomes transparent – revealing the white limestone skeleton beneath. This is called coral bleaching.
Coral can recover if temperatures are reduced and the relationship with the algae is restored, but it’s a stressful and difficult process. And if recovery takes too long, the coral will die.
In June 2023, a marine heatwave bleached coral reefs from the Caribbean to the Indian and Pacific Oceans.
It reached Australia’s east coast in February 2024, causing extensive coral bleaching. Aerial surveys showed three quarters of 1,080 reefs assessed had some bleaching. On 40% of these reefs, more than half the corals were white.
In the aftermath, in-water surveys measured how much coral died in the northern, central and southern Great Barrier Reef. The worst damage lined up with the highest levels of heat stress.
Sharp declines in coral cover
AIMS has surveyed reefs of the Great Barrier Reef each year since 1986, in a project known as the Long-Term Monitoring Program. It is the most extensive record of coral status on any reef ecosystem in the world.
One component of the surveys involves towing an expert observer behind a boat around the full perimeter of each reef. The observer records the amount of live, bleached and dead coral. These observations are then averaged for each location, and for each of the three regions of the Great Barrier Reef.
After each monitoring season we report on the percentage of living hard coral covering the Great Barrier Reef’s surface. It’s a coarse but robust, reliable indicator of the state of the Great Barrier Reef.
Coral losses this year were not uniform across the Great Barrier Reef. On the northern Great Barrier Reef, from Cape York to Cooktown, average coral cover dropped by about a quarter between 2024 and 2025 (from 39.8% to 30%). The largest declines on individual reefs (up to 70% loss) occurred near Lizard Island.
Reefs with stable or increasing coral cover were mostly found in the central region, from Cooktown to Proserpine. However, there was still a region-wide decline of 14% (from 33.2% to 28.6%), and reefs near Cairns lost between 17-60% of their 2024 coral cover.
In the southern reef (Proserpine to Gladstone) coral cover declined by almost a third. In the summer of 2024, southern reefs experienced the highest levels of heat stress ever recorded, resulting in substantial coral loss (from 38.9% to 26.9%).
The declines in the north and south were the largest in a single year since monitoring began 39 years ago.
Despite these losses, the Great Barrier Reef still has more coral than many other reefs worldwide, and remains a major tourist attraction. It’s possible to find areas that still look good in an ecosystem this huge, but that doesn’t mean the large-scale average hasn’t dropped.
More frequent bleaching events
Mass coral bleaching is becoming more frequent as the world warms.
Before the 1990s, mass bleaching was extremely rare. That changed in 1998 with the first major event, followed by another in 2002.
Back-to-back bleaching events occurred for the first time in 2016 and 2017. Since then, bleaching has struck the Great Barrier Reef in 2020, 2022, 2024, and again this year. The impacts of this year’s bleaching event will be revealed following the next round of surveys.
The time between these events is shrinking, giving corals less time to recover. Cyclones and crown-of-thorns starfish are also continuing to cause widespread coral loss.
You’ll see in the following charts how the percentage of coral cover has changed over time. The vertical yellow lines show the mass coral bleaching events increasing in frequency.
Confronting questions
The coral reefs of the future are unlikely to look like those of the past. The loss of biodiversity seems inevitable.
But will the reefs of the future still sustain the half a billion people that depend on them for food and income? Will they continue to protect coastlines from increasing storm activity and rising sea levels? These are confronting questions.
Effective management and research into reef adaptation and recovery interventions may bridge the gap until meaningful climate action is achieved. But above all, the key to securing a future for coral reefs is reducing greenhouse gas emissions.
