Ocean Heatwaves That Instantly Kill Coral Are Getting Worse, Scientists Warn

Marine heatwaves present “a distinct biological phenomenon” from bleaching events, and could threaten the livelihoods of half a billion people.
​The Great Barrier Reef. Image: Mark Tipple
The Great Barrier Reef. Image: Mark Tipple

Humans have long been complicit in the degradation of coral reefs through extensive land-use, pollution, and climate change. Soaring temperatures make corals more likely to bleach (or expel their symbiotic algae) and the acidifying oceans eat away at coral skeletons.

But climate change is exacerbating a threat much worse than bleaching for corals; one that causes immediate death.

A study published Thursday in Current Biology warns that bleaching is not the only existential threat to corals on a heating planet. Reefs will also have to weather extreme marine heatwaves, which is “a distinct biological phenomenon from bleaching events,” according to the study’s authors, led by William Leggat, a coral reef expert at the University of Newcastle in Australia..


“Now, we see there is also a temperature at which the coral animal itself suffers from heat-induced mortality,” explained co-author Tracy Ainsworth, a marine biologist at the University of New South Wales, in an email. “This isn’t starvation, this is the animal itself undergoing mortality directly from the heat of the water.”

Ainsworth and her colleagues began to examine the isolated effects of rapid temperature spikes on coral reefs in the wake of a marine heatwave that struck Australia’s Great Barrier Reef in 2016. The event exposed about a third of the reef to temperatures that “exceeded the threshold for coral mortality,” the team said, resulting in 90 percent of the reef experiencing some degree of bleaching.

The researchers selected two coral species that were particularly hard hit by the 2016 heatwave: Pocillopora damicornis and Acropora aspera. In laboratory conditions, they exposed the corals to simulations of the extreme and rapid temperature increases experienced by the reefs in the wild.

They found that when severe warming hits the ocean, coral colonies can completely collapse in just days. The corals lose their tissues, completely exposing their skeletons. Then, microbes come in and cover them, creating a film that rapidly dissolves their skeleton.

In other words, severe heatwaves don’t kill reefs slowly. Instead, the colony immediately dies, the skeleton dissolves, and the whole structure collapses.


“We must act on climate change and alter the trajectory for coral reefs,” Ainsworth said. “We now should also be considering how marine heatwaves may influence local scale interventions. Where corals don’t survive heat stress events, the capacity for acclimation is removed.”

Failure to curb global heating, which is amplifying these extreme heatwaves, would be devastating not only to reef ecosystems around the world, the team said, but also to the estimated half-billion people who directly rely on coral reefs for their sustenance and livelihood.

“It is critically important that while global scale actions are implemented, that local scale actions that protect reefs are quickly tested and applied in the places that are needed,” Ainsworth noted. “This is particularly important for the regions that rely on reefs for food.”

The new study adds to the already massive body of research establishing the dire consequences of the climate crisis for the planet and its lifeforms—including humans. To curb the damage to reefs in the future, it will be necessary to drastically reduce greenhouse gas emissions that are driving global heating, and ensure surviving reefs are adequately protected from human pressures.

Update: This article has been updated to include comments from marine biologist Tracy Ainsworth.