Half of the corals in Australia’s Great Reef have perished in the past 25 years, scientists said on Wednesday. They warn that global warming is irreversibly disrupting this underwater ecosystem.
A study published in the journal Proceedings of the Royal Society sounds the alarm on the extent of the decline of all types of corals since the mid-1990s at the site in northeast Australia, listed in 1981 World Heritage by Unesco.
The largest species of coral – especially table-shaped and branched ones – have been the most affected, some of which have disappeared from the northernmost part of the Great Reef.
“They are 80% or 90% gone from 25 years ago,” James Cook University professor Terry Hughes, one of the study’s authors, told AFP. “Losing these huge three-dimensional corals will change the entire ecosystem, they provide the nooks and crannies that many fish and creatures take refuge in.”
Besides its invaluable natural or scientific point of view, it is estimated that the coral reef, which stretches over 2,300 kilometers in length, generates four billion dollars in revenue for the Australian tourism sector.
The Great Barrier could lose its World Heritage status, due to its degradation which is largely due to the recurrence of episodes of coral bleaching, which is the consequence of climatic upheavals.
Fifth episode in 2020
Bleaching is a wasting phenomenon which results in discoloration. It is caused by the rise in water temperature which causes the expulsion of symbiotic algae that give the coral its color and nutrients.
Reefs can recover if the water cools, but they can also die if the phenomenon persists. The Barrier is also threatened by agricultural runoff, by economic development and by the purple acanthaster, a starfish that eats corals.
The north of this ecosystem had already suffered in 2016 and 2017 two unprecedented episodes of bleaching of its corals and last year Australia reviewed the outlook for this set, now considering them “very bad”.
Before that, two other episodes had been recorded in 1998 and 2002. A fifth was observed in 2020, but the damage has not yet been fully assessed.
“The vitality of a coral population is characterized by the presence of millions of corals of all sizes, including the largest which produce most of the larvae,” said co-author Andy Dietzel of the James Cook University.
Partial recovery possible
Mr Hughes said he expects corals to continue to die unless countries around the world meet commitments made in the 2015 Paris Agreement to contain rising global temperatures below 2 degrees compared to their pre-industrial levels.
“The fastest growing species take a decade to half recover,” said Hughes. “But the chances of having decades between the sixth, seventh and eighth bleaching episodes are close to zero as temperatures continue to rise.”
If temperatures stabilize over the course of the century below the targets set by the Paris Agreement, the reefs may partially recover.
“We do not think that they will recover in the diversity that we have known historically”, relativizes Mr. Hughes however. If the increase is three or four degrees, then it will be necessary to “forget”, he continues.