Global report finds marine heatwaves are driving coral decline

The largest, most comprehensive report on the status of global coral reef health was released this week by the Global Coral Reef Monitoring Network (GCRMN), indicating a global trend of declining coral cover.

The Status of Coral Reefs of the World: 2020 shows 14 per cent of the world’s coral have died since 2009 – equating to 11,700km² of coral – and marine heat waves are driving this loss.

More than 300 scientists from around the globe contributed to the report, drawing on almost two million individual observations across 12,000 coral reef sites in 73 countries. It’s the sixth edition of the report and the first since 2008.

The report looks at trends over a long period, from 1978 to 2019. It shows climate-related disturbances are occurring more frequently, and the window for recovery is getting shorter, significantly reducing the opportunity for reefs to fully regain coral cover.

There’s been a global decline in coral cover and rise in reef algae, which grows during periods of stress and can compete with coral for space. Today, there is also 20% more algae on the world’s reefs than a decade ago.

Lead editor and Australian Institute of Marine Science Chief Research Officer Dr David Souter said elevated sea surface temperatures, driven by climate change, was the most significant threat to corals world-wide, and in Australia.

“When sea surface temperatures spike, corals become stressed and bleach which can cause widespread mortality and rapid declines in coral cover. This indicates the greatest threat to coral reefs is heat stress, a phenomenon likely to happen more frequently as the planet continues to warm,” he said.

How are Australia’s coral reefs fairing?

Australia’s coral reefs – including the iconic Great Barrier Reef and Ningaloo Reef – have endured a turbulent two decades with a series of disturbances and recovery periods.  On average, Australian coral reefs have lost one-quarter of their coral since 2007.

The greatest cause of reduced coral cover in Australia is coral bleaching, with high sea surface temperatures occurring every year since 2012 on both the east and west coasts. This is consistent with the global trend.

“In Australia, the greatest decline happened when many of our coral reefs experienced repeated coral bleaching events,” Dr Souter said.

In Western Australia, more than half of coral reefs have been affected by coral bleaching since 2010. Compared to the Great Barrier Reef, Western Australia’s coral reefs do not have from the added challenges posed by poor water quality and crown-of-thorns starfish outbreaks. Overall, average hard coral cover on Western Australian coral reefs was greater than on the Great Barrier Reef.

For the Great Barrier Reef – which constitutes 85 per cent of Australia’s coral reefs – about a quarter of hard coral cover was lost between 1994 and 2019. Over this 25-year period, algae on the Great Barrier Reef almost doubled, coinciding with fluctuations of decline in coral cover.

Between 2007 and 2014, prolonged thermal stress, crown-of-thorn starfish outbreaks, several severe tropical cyclones (Hamish and Yasi) and significant flooding in 2010/11 saw a 23% decline in the amount of coral on the Great Barrier Reef.

“While these reefs had a period of recovery following these disturbances, gains were erased by the back-to-back severe coral bleaching events in 2016 and 2017, which brought hard coral cover on the Great Barrier Reef to its lowest level,” he said.

“The problem is these critical recovery opportunities are becoming much shorter, giving less time for reefs to build back coral cover before the next disturbance strikes.

“This report sees this trend of struggle not only in Australia, but globally.”

The report included 97,000 observations collected from 372 sites in Australia since 1994.

Australia’s monitoring has been one of the longest and consistent compared to other countries, reflecting Australia’s commitment to the long-term health and management of coral reefs. AIMS’ monitoring programs on both the Great Barrier Reef and in Western Australia were a significant contributor of data, with the longest time series of 24 years.  

The report does not include the mass coral bleaching event in 2020 and the recovery in 2021, recently reported by the LTMP.

Are coral reefs showing resilience?

Despite the global loss and downward trend, coral reef resilience is evident. Coral reefs have shown periods of recovery between 2002 and 2009, 2019, and most recently 2021. But recovery is stymied by repeated disturbances.

“It’s natural for coral reefs to be impacted by disturbances, for example a cyclone, and then for that reef to have a break where it can fully recover,” he said.

“We know coral reefs can recover in as little as 10 years if conditions allow, but recovery windows this long are becoming increasingly rare.”

The report showed more than three-quarters of 135 long-term monitoring sites on Australian reefs that had suffered a significant coral loss, had not fully recovered to their pre-disturbance coral cover.

AIMS CEO Dr Paul Hardisty said while the report was sobering, there was a path forward.

“There are clear, unsettling trends toward coral loss, and we can expect these to continue as warming persists,” he said.

“Despite this, some reefs have shown a remarkable ability to bounce back, which offers hope for the future recovery of degraded reefs.

“A clear message from the study is that climate change is the biggest threat to the world’s reefs, and we must all do our part by urgently curbing global greenhouse gas emissions and mitigating local pressures.”  

AIMS is a leader in coral reef science – including restoration and adaptation – and has been developing new ground-breaking approaches to increase the resilience of coral reefs against growing pressures from climate change.