>>31165201Because scientists have discovered an unprecedented die-off in the world's largest reef, the Great Barrier Reef, prompting the Australian government to issue its highest response level.
Diver surveys based off Cape York, Australia's northeastern tip, found up to 50 percent mortality in the reef from coral bleaching. Death among the organisms that build the reef's structure is most likely linked to rising temperatures in the ocean, the government announced.
"The corals in the remote far north of the reef experienced extremely hot and still conditions this summer, and were effectively bathed in warm water for months, creating heat stress that they could no longer cope with," Russell Reichelt, the chairman of the Great Barrier Reef Marine Park Authority, said in a statement.
Bleaching occurs when warm ocean water stresses corals to the point that they expel the tiny algae, known as zooxanthellae, that normally live inside their tissues. The algae provide the corals with most of their food, as well as their color. If the heat stress is lessened soon enough, the coral can recover. If not the, organisms will die.
What's happening in Australia is part of a global trend. Over the last year, about 12 percent of the world's reefs have bleached, due to El Niño and climate change. Scientists have predicted that nearly half of these reefs (more than 4,600 square miles or 12,000 square kilometers, or more than five percent of reefs) could disappear forever. That warming trend is expected to continue through the year, leading to what may be the longest global coral bleaching event in history.
