Rising sea temperatures have destroyed the ability of the Great Barrier Reef to recede, the researchers say, highlighting for the first time a 90% drop in new corals as successive heat waves bleached the World Heritage site.
After unprecedented loss of reef ranges – the largest living structure in the world – in successive heat waves in 2016 and 2017, the number of new corals measured one year later was discovered by a team of scientists 89% lower than historical levels . .
Coral reefs account for less than 1 percent of the Earth's marine environment but are home to about 25 percent of marine life, serving as a nursery for many species of fish and habitats for birds, sharks, dolphins and porpoises.
The study measured how many adult corals survived along the 2,300-kilometer reef off the coast of Queensland following consecutive summers of exceptionally hot seas that bleached and killed numerous species of coral.
He discovered a "crash" in coral replacement compared to levels measured in years prior to a mass bleaching event.
The population of a species – Acropora, a branched coral that supports thousands of marine species – has dropped by 93%.
"We never thought we would see disturbances on a scale to affect recruitment to this point," said Andrew Baird, co-author of the study published in the journal. Nature.
The team estimated that it would take five to ten years for coral reef production to fully recover – but only if there is no other bleaching event.
Atmospheric temperatures have risen by about one degree Celsius since the middle of the last century, and corals are particularly sensitive to fluctuating heat levels.
Last year, a panel of international climate experts warned that coral structures – including the Great Barrier Reef – probably would not survive a 2C increase.
"VERY GREAT TO FAIL, SO FAR"
Bleaching occurs when warmer ambient temperatures cause corals to expel small photosynthetic algae, draining them from their color.
The Great Barrier Reef underwent four mass bleaching events in recorded history – all in the past two decades.
"Dead corals do not make babies," said Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University in Australia.
"Baby fever means the recovery will be slower, and the change in species means that the reefs will not recover to the same setting as before," he said.
"The question is: will the recovery be halted by another massive coral bleaching event due to the escalation of global warming?"
Hughes said there may be some hope, as evidence suggests that some species of coral are more resistant to temperature fluctuations than others.
The team found that while bleaching occurred in 2016 and 2017, much higher heat exposure was required to cause the same level of bleaching the second time – meaning the reef was naturally adapting to harbor more coral varieties resistant to heat.
"So the reef is now moving rapidly to a new setting, with a greater proportion of species that are bleach resistant, or that are able to return faster," said Hughes.
The number of species, however, would probably be greatly reduced, according to previous research.
Co-author Morgan Pratchett, however, warned that there was a limit to how much heating the reef could carry.
"It is highly unlikely that we can escape a fifth or sixth (bleaching) event in the next decade," she said.
"We used to think the Great Barrier Reef was too big to fail – until now."