After 18 months of record ocean temperatures, the planet’s reefs are in the midst of the greatest heat stress ever recorded.
Latest figures from the US government’s Coral Reef Watch, shared with The Guardian, show that 73% of the world’s corals in the Pacific, Atlantic and Indian Oceans have been hit by so much heat that they are starting to bleach.
This is the fourth global mass bleaching event starting in February 2023 – the second in 10 years, and the most common ever.
After watching their beloved reefs struggle to survive, some coral scientists are now calling for a major rethink of how reefs should be protected as temperatures rise even further in the coming decades.
“We’re coming out of a period where we’ve been making predictions for decades,” said Prof. Tracy Ainsworth, vice president of the International Coral Reef Society.
“Now we’re at a point where we hoped we wouldn’t be. Now we’re asking ourselves, what do we do now?”
Three articles were published in the scientific journal Nature Climate Change on Monday calling on the coral conservation and scientific community to collectively rethink.
Related: ‘Most of it was dead’: Scientists discover one of the Great Barrier Reef’s worst coral bleaching events
“I would call it a bit of soul-searching,” said Prof. Tiffany Morrison, co-author of one of the papers sharply critical of widespread programs, many of them corporate-backed, to grow corals in nurseries and then transplant them onto reefs.
“When everyone realized how big the impact of climate was on coral reefs, the first reaction was to just do something and intervene, because people were so upset.”
Last year, many replanted corals in Florida and the Caribbean died when the region experienced extreme heat stress.
“We need a fundamental rethink,” said Prof David Bellwood, a colleague of Morrison’s at James Cook University in Australia.
“The stakes are too high. Right now, coral restoration is at best psychological relief and cosmetic protection, and at worst a dangerous distraction from climate action.”
Coral bleaching is a process in which the coral animal sheds the algae that live in its tissue. These algae cause the animal to lose its color and a large part of its nutrients.
Without the algae, the white skeleton of the coral is visible through the translucent flesh, giving it a bleached appearance.
Mass coral bleaching over large areas, first noted in the Caribbean in the 1980s, is being caused by rising ocean temperatures.
Some corals also display fluorescent colors under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching.
Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme heat waves in the ocean can kill corals outright.
Coral bleaching can also have sub-lethal effects, such as increased susceptibility to disease and reduced growth and reproductive rates.
Scientists say the periods between bleaching events are becoming too short to give reefs a chance to recover.
Coral reefs are considered one of the planet’s ecosystems most threatened by global warming. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries.
The world’s largest coral reef, Australia’s Great Barrier Reef, has experienced seven mass coral bleaching events since 1998, five of which have occurred in the past decade.
Critical coral
Coral reefs provide food for millions of people around the world. They also provide the raw material that ultimately forms much of the sand on beaches and protects coastlines from wave damage.
When corals are in too warm water, they excrete algae in their tissues. These algae provide color and a large portion of the nutrients.
Dr Derek Manzello, Director of Coral Reef Watch, said the number of reefs affected by heat stress from the current global event is continuing to increase and that this “has absolutely caused almost everyone involved in reef science and restoration to think carefully about future activities and best practices”.
The current global event is affecting reefs in 70 countries and its full impact may never be fully understood.
The world’s largest coral reef, the Great Barrier Reef, has likely also experienced its worst coral bleaching, but government scientists may not know how many corals have died until next year.
Whether an individual coral survives bleaching depends on the species and the extreme heat and duration.
In another scientific paper, Professor Michael Webster of New York University put forward a radical idea that, he said, would have been far too controversial for a scientific paper a decade ago.
Coral reefs exist in tropical waters around the world, but are adapted to local conditions. Conservationists should consider introducing corals that evolved in very hot areas to reefs where the current mix of corals struggles to survive, Webster said.
“It’s incredibly controversial and we may never get there, but we’re in a situation where we’re asking ourselves whether we’re going to have reefs in a lot of places. Is it worth asking that question now?”
According to Webster, coral reefs have a greater chance of surviving in the coming decades if they have a high diversity of coral species.
“Reducing carbon emissions should be our ultimate goal, but we have centuries in which coral systems like reefs will be in trouble.”
Cautious interventions
Morrison is cautious about interventions like Webster’s.
A wide range of scientific solutions for coral reefs are currently being developed worldwide, ranging from cloud whitening to shadow reefs and selectively breeding corals for better heat resistance.
“We’re investing too much money and hope in these speculative coral bioengineering and genetic engineering solutions,” Morrison said. “We don’t know if they’re scalable and, if they are, whether we can afford to scale them.”
Many interventions face a philosophical question. Who decides which species to save or adapt, or what actions to take? These decisions can shape what reefs look like in the future – decisions made by humans, not nature.
“There are very few people looking at unintended consequences and there are no governance systems in place to manage that,” Morrison said.
“But number one: we have to reduce fossil fuels.”
Freak out
Members of the International Coral Reef Society wrote in May that scientists should “rethink this challenge” of protecting reefs.
Related: Global warming is causing coral reefs to experience the worst global mass bleaching event ever recorded
Because efforts to reduce global greenhouse gas emissions have been too slow, governments and communities have had to redouble their efforts to reduce other stressors on coral, such as overfishing and local water pollution, the association said.
Tim McClanahan, reef ecologist and director of marine sciences at the US-based Wildlife Conservation Society, admitted that “people are panicking” about the current bleaching.
He said there was little evidence that large-scale coral restoration projects had restored reefs and that in places like Florida coral nurseries had been destroyed by the heat.
“I think they are ignoring past experience and not recognizing the science,” he said.
“I worry that one problem we have with NGOs is that we are not very good at admitting our mistakes. I find there is a tendency to act without consulting the literature.”
McClanahan said in a third paper in Nature Climate Change that predicting the future of coral reefs needs to be more sophisticated.
Rather than just factoring in heat, modeling should take into account how reefs respond differently to heat stress, depending on local conditions such as the mix of coral species or how well protected they are. The prognosis for some reefs may not be so bad, he argued.
McClanahan has been working on reefs for 40 years and says he’s seen them change from pristine wonders to shadows of their former selves.
“Yes, the reefs are screwed — in deep trouble. We are already experiencing very harsh conditions for corals,” he said.
“In the 90s I was in mourning, but now I want to know how we are dealing with the situation we are in. We are not dealing with it very well and we have this fatalistic view.
“We should panic. That is not an unreasonable reaction, but we need to sit back and be a little bit more intelligent”