It’s not just hotter temps – changing currents, disease threaten world’s corals

Shifting ocean circulation patterns driven by climate change are potentially pushing the world’s coral reefs to the brink, a new study warns.

Amid the current global coral bleaching event – the fourth on record – a pair of marine scientists have used a ‘hotspot analysis’ to assess places in the world’s reef oceans where surface temperatures have shot at least 1 degree above averages for record warm months.

Prominent sites identified using this technique include reef areas close to the equator, such as the Caribbean, the Pacific and Atlantic coastlines of Mexico and Central America, Pacific island nations like Fiji and Kiribati, and the east coast of New Guinea.

The research was led by marine biologist Thomas Goreau, who is president of the Global Reef Alliance, based in Massachusetts. He is concerned shifts in ocean circulation are contributing to coral bleaching across the planet while increasing heat distribution towards polar regions.

His work, compiled with colleague Raymond Hayes, is published today in the journal Oxford Open Climate Change. The same hotspot method was previously used by the pair to predict mass bleaching in coral reefs as recently as 2020.

Their new analysis found that ocean currents are continuing to warm, with accelerated movement of warm tropical waters towards the north and southern polar regions. Goreau says this, combined with a reduction of ‘vertical’ mixing with cold deep water and increases in polar ocean temperatures, will accelerate ice melt, with significant knock-on consequences.

And he warns that such feedback processes are not included in climate projections by the Intergovernmental Panel on Climate Change.

“Coral reefs, the most vulnerable of all ecosystems, began to bleach and die from high temperatures starting in the 1980s,” Goreau says.

“Most coral around the world has been killed, and survivors can’t take more warming.

“The sudden rise in global temperature during 2023 further imperils coral reefs, and indicates large-scale changes in ocean circulation are underway, causing positive feedback that amplify global warming, which are not included in IPCC models.”

Caribbean corals under threat on two fronts

The bad news for places like the Caribbean follows reports earlier in May that find coral diseases were spreading.

The University of Florida researchers identified the onset of Stony Coral Tissue Loss Disease, which is thought to be caused by a marine pathogen and has been spotted in 18 nations or territories.

Its destructive effect culls vulnerable corals and fosters an environment for other marine organisms like macroalgae, cyanobacteria and fire coral to take over. These species are less suitable for regional biodiversity.

“Macroalgae doesn’t support as much biodiversity because it doesn’t create a hard habitat,” says Sara Swaminathan, an environmental engineer at the University of Florida. “It might be a positive for herbivores but not for other organisms that need places to settle and grow, hide, or mate.”

While fish communities did not appear to be at risk of collapse due to the disease, some populations of non-reef fish were predicted to be halved.

Dead coral, living healthy elkhorn coral (acropora palmata), both diseased and healthy symmetrical brain corals (pseudodiploria strigosa), and a spotfin butterflyfish in waters off belize
Dead coral, living healthy elkhorn coral (Acropora palmata), both diseased and healthy symmetrical brain corals (Pseudodiploria strigosa), and a spotfin butterflyfish in waters off Belize. Credit: Sara Swaminathan

GBR resilient in some parts, but global warming still looms as existential threat

The Great Barrier Reef has been a hot-button topic in Australia.

It is up against a suite of threats including persistent bleaching events across 5 of the past 8 years, and predatorial crown of thorns starfish infestations.

However, a recent joint investigation by scientists from the University of Queensland and the University of Exeter (UK) identified deep-water mesophotic corals, which exist at depths of 30-50m, may be insulated from increasing marine heatwaves.

Cold water at these depths appears to protect these layers of the GBR, which could also be a finding relevant to corals in similar environments around the world, however local factors may mean these specific regions off the Queensland coast will benefit more. Jennifer McWhorter, a physical oceanographer affiliated to both universities and the US National Oceanographic and Atmospheric Administration, describes her team’s findings as a source of “hope and a warning”.

“That some reefs are resilient to current levels of climate change, and a warning that this resilience has its limits,” McWhorter says.

That resilience looks set to be scrubbed out if global average temperatures exceed 3°C above pre-industrial levels – a devastating scenario that would mean national governments fail to meet the targets of the Paris Climate Agreement by some margin. Currently, the agreement aspires to keep global temperatures to “well below” 2°C.

“Coral reefs are the canary in the coal mine, warning us of the many species and ecosystems affected by climate change. Coral bleaching is a dramatic sign of the impact humans are having on the planet,” McWhorter says.

“Some shallow-water species are not found in deeper areas – so mesophotic reefs can’t provide refuges for them as shallow reefs are degraded. And, as our study shows, mesophotic corals are themselves threatened if global warming continues.”

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The Ultramarine project – focussing on research and innovation in our marine environments – is supported by Minderoo Foundation.

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