Green sea turtles and tropical fishes at the Great Barrier Reef, Australia, 2022 (photography)

Swimming with green sea turtles and tropical fishes at the Great Barrier Reef, Australia. Australia's Great Barrier Reef has shrunk by 50% in the last 30 years. It affects all coral species, at all depths, and in all sizes. The coral population has decreased in the last three decades by more than 50%. If action is not taken quickly to slow global warming, in ten years all its corals could be lost. Climate change is driving an increase in the frequency of reef disturbances, such as marine heat waves, caused largely by greenhouse gas emissions. Global warming has already bleached two-thirds of the Great Barrier Reef. This bleaching normally takes place when pollutants, excessive sunlight or unusually hot water cause the corals to expel symbiotic algae from their tissues, turning them white. The process does not necessarily kill them immediately. If conditions improve—for example, if the water cools again—many corals can be recolonized by the algae and recover. But without the algae, which are their largest food source, the corals become weak and more susceptible to disease. If conditions don't improve quickly, they die.n.