Global heat wave affected half of world’s coral reefs

(CN) — Almost a decade ago, half the world’s coral reefs suffered greatly in a climate catastrophe and scientists are continuing to understand its ramifications with another possible mass bleaching event underway.

A survey published Tuesday in the journal Nature Communications analyzed the wide-reaching extent of “Third Global Coral Bleaching Event,” finding more than two-thirds of the 15,000 coral reefs surveyed indicated at least moderate coral bleaching between 2014 and 2017.

Coral bleaching occurs when the animal element of the coral, known as the polyp, releases its symbiotic algae as a stress response. Without the vital algae to produce nutrients through photosynthesis, the once vibrant coral turns white, or “bleached.”

Extrapolating from the gathered data, researchers from the Smithsonian Tropical Research Institute, the James Cook University in Australia and Coral Reef Watch at the U.S. National Oceanic and Atmospheric Administration, used satellite-derived heat stress measures to hypothesize how much bleaching happened globally. The team also utilized satellite images of ocean water temperature from Coral Reef Watch observations and worldwide aerial surveys.

According to the scientists, more than 50% of all coral reefs experienced significant bleaching, with 15% mortality, during the three years of the event.

And based on the survey results, the researchers concluded how much heat stress that ultimately pushes corals into a potentially fatal response depends on where the reefs are located.

“Our best predictor to date of bleaching risk is an accumulated temperature stress threshold that is relative to the climate in the region where the reefs are found,” Sean Connolly, a senior scientist at the Smithsonian Tropical Research Institute and a lead author on the paper, said in an email. “Essentially, it measures by how much, and for how long, temperatures have exceeded the long-term average temperature for the warmest month of the year at that location.”

Connolly said the analysis quantifies the probability of a reef experiencing moderate or severe bleaching depending upon the measure of accumulated temperature stress, through a predictor, called “Degree Heating-Weeks.”

“Because DHW is a relative temperature stress measure, it means that reefs growing in very warm environments, such as the Red Sea or Persian Gulf, will on average withstand higher temperatures before bleaching than reefs in, say, Okinawa or Hawaii,” he said.

Entire coral reefs can become a pale, dying skeleton through bleaching, at risk of starvation and disease. The most common stressors related to bleaching are the rise in ocean temperatures, pollution, runoff and other climate change factors.

Currently, coral reefs are experiencing another heating event that started in 2023 — hardly enough time, after the 2014-2017 bleaching the scientists studied, for the organisms to recover.

“A good rule of thumb is that it takes something on the order of a decade for a reef to recover to pre-disturbance coral cover, when mortality from the disturbance is severe,” Connolly said. “That said, the reef is not likely to recover to its pre-bleaching state, but rather experience some shift towards faster-growing, highly fecund species.”

Connolly noted that much older species, such as the century-old corals that perished in a 2016 bleaching event on the Great Barrier Reef near Australia, cannot recover before another event occurs, but some may survive if given a reprieve.

“Mass coral bleaching is driven by climate change that occurs at a global scale,” he said.  “While corals are living organisms, and like all living organisms, given time, they adapt to changing environments. Indeed, corals living in places that have been hotter over geological time tend to cope with higher temperatures before they bleach or die, compared to corals from regions that have not been as hot.”

Coral reef restorations focusing on to propagating temperature-resistant corals in nurseries and then out planting them on reefs can increase coral cover in the short term but doesn’t solve the underlying problem of more heat.

“Adaptation takes time,” said Connolly.  “And the fact that bleaching events are becoming progressively worse, at least through this 2014-2017 global event, tells us that the tropical shallow seas where reefs flourish are heating up faster than many coral species have been able to adapt.”

For coral species to have the time to adapt, Connolly said, the rate of ocean warming needs to be substantially slowed or stopped.

Curbing the ascent of greenhouse gas concentrations in the atmosphere — the main driver of rapidly increasing ocean temperatures — is likely the best possible solution for coral futures.

“It will be up to the world’s policymakers, and (in democratic nations) the people who choose them, to decide what it is they value,” Connolly said. “And based on those values to decide whether and how they may wish to influence that trajectory.”