The Great Barrier Reef off the east coast of Australia, the world’s largest coral reef, has once again experienced a devastating coral bleaching event. Not only is this the fourth such event in seven years, it also comes at an unexpected time. The Pacific Ocean is undergoing a La Niña phase that should bring cooler waters to the coast of eastern Australia – experts had hoped this year’s reefs would recover from coral bleaching in recent years. According to the Great Barrier Reef Marine Park Authority, which oversees the protected area, bleached corals can be seen in all four sectors of the reef from aircraft observations. These do not die spontaneously, but such events have a significant impact on growth, reproduction, and the ability of organisms to resist infection. Recurrent severe coral bleaching threatens to kill large areas of coral reefs.
As early as December 2021, experts warned of the potential for coral bleaching in light of rising water temperatures. Corals bleach because high water temperatures force them to expel the single-celled algae that live within them. These provide a large part of their strength, so losing their symbionts weakens them, although the algae returns after a while. It is unclear how severe coral bleaching will be, that is, how long it will last and how many affected areas will be. A lot depends on the water temperatures. The Australian Bureau of Meteorology assumes the water will be 1.5 degrees warmer than normal across large areas of the reef by the end of April.
So far, the consequences at the regional level have varied. The worst affected areas are near the coast in the middle of the reef, where experts have already recorded dead corals. Bleaching is milder in the far north and south, and according to the Great Barrier Reef Marine Park Authority, some of the corals here look perfectly healthy so far. Water temperatures are also expected to be lower in the south over the next few weeks compared to the rest of the reef.
Experts clearly attribute both the accumulation of coral bleaching and the unprecedented bleaching in a La Niña year to climate change. Australia is one of the regions where the effects of global warming are most noticeable at the moment. For example, marine heat waves, phases of unusually high water temperatures, Currently around the continent more frequently and longer. In the long term, the last hope for ecosystem survival is for experts to track down particularly heat-tolerant corals and use them to repopulate areas of the reef that have been destroyed.
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