New research shows that years of unrestrained global warming have made their mark on one of the world's natural wonders, Australia's Great Barrier Reef. Human-made climate compromised the capacity of the Great Barrier Reef's corals to recover from any damage. It's a finding that calls the future of the Reef into question.
“Dead corals don’t make babies,” says lead author , director of the at (JCU) in a press statement. “The number of new corals settling on the Great Barrier Reef declined by 89 percent following the unprecedented loss of adult corals from global warming in 2016 and 2017.”
Coral are animals, marine invertebrates that through the generations have created a haven for some of the most diverse species of life on Earth. An individual coral is called a polyp, and these polyps grow on top of each other through asexual reproduction. Over thousands of years (usually around 10,000), these polyps can grow into a significant structure known as a fringing reef. They move very slowly in search of more water and nutrients, sometimes forming what's known as a barrier reef in the process.
The Great Barrier Reef has formed over 20,000 years, and over time it has slowly changed and grown with new coral. Scientists estimate that the current structure of the Great Barrier Reef, which is actually 2,900 individual reefs creating an ecosystem unique to the planet, has been in place for between 6,000 and 8,000 years.
But recent changes have fundamentally weakened the Reef's ability to protect itself and grow. The Reef has undergone years of heat stress, significantly worsening the Reef's ability to replenish itself with nutrients.
“The number of coral larvae that are produced each year, and where they travel to before settling on a reef, are vital components of the resilience of the Great Barrier Reef. Our study shows that reef resilience is now severely compromised by global warming,” says co-author .
Reef resilience is a scientific term which refers to coral's ability to recover from natural challenges, like storms or changes in water levels. The Great Barrier Reef has faced four or five major natural challenges in its history which scientists classify as "death events." It's always been strong enough to recover.
“The biggest decline in replenishment, a 93% drop compared to previous years, occurred in the dominant branching and table coral, Acropora. As adults these corals provide most of the three-dimensional coral habitat that support thousands of other species,” Baird continues.
“The mix of baby coral species has shifted, and that in turn will affect the future mix of adults, as a slower than normal recovery unfolds over the next decade or longer.”
Reef resilience stems in large part from tiny, single-celled organisms . Zooxanthellae live inside coral, and have a symbiotic relationship with them, providing nutrients in exchange for protection. Both sexually and asexually reproducing coral acquire their zooxanthellae in their youth as larve. Most zooxanthellae cannot withstand even the slightest change in temperature and with warming waters die immediately.
Even with the damage done, there would be reason to be cautiously optimistic about the Great Barrier Reef's resilience if it was allowed to grow unabated. “We expect coral recruitment will gradually recover over the next five to ten years, as surviving corals grow and more of them reach sexual maturity, assuming of course that we don’t see another mass bleaching event in the coming decade,” says Professor Hughes.
However, most scientists see such an idyllic scenario as unlikely. The Reef has faced mass bleaching events in 1998, 2002, and back-to-back in 2016 and 2017 which it was part of a .
“It’s highly unlikely that we could escape a fifth or sixth event in the coming decade,” says co-author .
Previous natural events can't compare to these mass bleachings in terms of damage done. Pratchett brings up cyclones. Cyclones like have caused serious damage to the Great Barrier Reef over the last decade. "When one part was damaged by a cyclone, the surrounding reefs provided the larvae for recovery, he says. "But now, the scale of severe damage from heat extremes in 2016 and 2017 was nearly 1500km (932 miles)—vastly larger than a cyclone track.”
While some southern reefs are still in good condition, Prachett says, they're too far away and slow-moving to help out their northern neighbors.
“We used to think that the Great Barrier Reef was too big to fail – until now,” he says.
There are a number of suggestions on how to strengthen coral in the future. Some scientists have suggested using CRISPR gene-editing to manipulate coral. But that would dance around the main issues at hand, Hughes says.
“There’s only one way to fix this problem,” he says. “And that’s to tackle the root cause of global heating by reducing net greenhouse gas emissions to zero as quickly as possible.”