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

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 extreme marine heatwaves present “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.

“Our study provides compelling evidence for the urgent need for society to execute global and local efforts to mitigate climate change for the protection of coral reef ecosystems,” the team said in the paper.

Leggat and his 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 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, 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.

"This work provides very clear evidence that the intense heatwave conditions, which are now becoming a feature of bleaching events, are far more severe and are changing how we understand the impact of climate change on coral reefs and the consequences of severe heatwave events," said Leggat in a statement.

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 food and livelihood.

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.

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