Scientists respond to global mass bleaching declaration

The fourth recorded global coral bleaching event has been declared by US National Oceanic and Atmospheric Administration (NOAA) in a timely reminder of the threats posed by climate change.

Warnings about the likelihood of serious mass bleaching have persistently sounded since an El Niño weather phenomenon began in mid-2023. That event is noteworthy for its increased natural warming of oceans.

But while El Niño comes and goes, one persistent change to the Pacific has been the warming of waters due to the rise of carbon emissions from human activities. The ocean is the biggest carbon absorber on the planet, which can lead to an overall increase in sea temperatures.

When combined with El Niño, it creates a challenging, even deadly environment for corals.

NOAA keeps an eye on global ocean trends. Its satellite measurements from January 2023 to April 2024 placed parts of the world’s oceans, including Australia’s east and south-east coasts, on the highest bleaching alert levels.

The long-term outlook for global reefs is poor. Intergovernmental Panel on Climate Change reporting predicts most coral reefs would be wiped out if global average temperatures reach 2°C above pre-industrial levels.

Eyewitnesses on the reef describe “patchy, but definitely confronting, and severe” extent of bleaching

Emma Camp, a marine biologist in the Future Reefs Team at the University of Technology Sydney, was on Heron Island in the Great Barrier Reef when the NOAA declaration arrived.

“There’s definitely extensive bleaching, still,” Camp tells Cosmos. “Large areas that are just completely white.

“Speaking with the scientists that have been on the island and tracking [bleaching] the whole time, they are seeing some corals showing some signs of recovery, which is promising.

“But equally there are some corals that have already transitioned to mortality.”

Bleaching occurs when, under environmental stress, a coral ejects symbiotic algae that are embedded within their structure, causing them to appear white.

Stony corals consist of thousands of polyps, which secrete hard, chalk-coloured calcium carbonate to form the stony ‘skeletons’ upon which they live. The algae are the food source for these polyps, generating glucose via photosynthesis that feeds the adjacent invertebrates.

When the algae are ejected due to heat stress, the clock begins ticking before polyp mortality eventually occurs.

“I think one of the most alarming things has been the depth at which the bleaching occurs. We’re seeing corals that are below 10m [depth] bleaching.

“Typically we think that the shallows are most at risk of the bleaching because it’s warming in the shallow waters, but even down at those deeper depths, we’re seeing a variety of growth types.”

Worst summer for Great Barrier Reef

The Great Barrier Reef experienced its most severe summer on record, according to the federal government’s Great Barrier Reef Marine Park Authority. In its 2023-24 snapshot, it listed the “widespread” bleaching events, two tropical cyclones and several flood events as the main impacts on the reef’s health.

Predatorial crown-of-thorns starfish outbreaks have also made life more difficult for reefs.

While it’s only the fourth time that a global bleaching event has been reported, it’s the fifth time in nine summers that the reef has been affected by one.

Marine scientists are uniform in their concerns for the reef, particularly pointing to climate change driven by excessive carbon emissions as the main driver of decline.

“Climate change remains the biggest challenge facing our environment and the reef today and will continue to be in coming decades,” says Bruce Taylor, the CSIRO’s Great Barrier Reef research coordinator. “The current bleaching is a very clear and serious reminder that we need to keep working quickly and collaboratively on ways to help the reef withstand these pressures.”

President of the Australian Academy of Science, Chennupati Jagadish noted “the frequency of marine heatwaves and mass bleaching on the GBR weaken its ability to recover and makes the entire ecosystem less resilient to a changing climate”, as well as the impact reef decline has on Australian communities that rely on reef fisheries for economic activity as well as providing food.

It’s a point that Camp emphasised too, but on a global scale.

“We just really need people to realise how important reefs are,” she says. “Even if you don’t live by a reef, over 1bn people directly rely on reefs for something … they require it to survive.”