Half of the world's coral reefs（珊瑚礁）have died in the last 30 years. Now scientists are racing to ensure that the rest survive.Even if global warming were to stop right now, scientists predict that more than 90% of corals will die by 2050. If no major steps are taken to address the problem，the reefs may be headed for total extinction（灭绝）. (78)The planet's health depends on the survival of coral reefs. They are often described as "the rainforests of the sea",because they provide shelter for a wide variety of sea life.In addition, the reefs serve as barriers that protect coastlines from the full force of powerful storms. Corals are used in medical research for cures to diseases.They are key to local economies as well, since the reefs attract tourists, the fishing industry, and other businesses, bringing in billions of dollars. (79) Corals are particularly sensitive to changes in temperature. A rise of just 1 to 2 degrees can force the corals to drive out the algae（水藻）.Then the corals turn white in a process called “bleaching”. Corals can recover from short-term bleaching, but long-term bleaching can cause permanent damage.In 1998,when sea surface temperatures were the highest in recorded history, coral reefs around the world suffered the most severe bleaching. It is estimated that even under the best of conditions, many of these coral reefs will need decades to recover. Although reefs face other threats from pollution, industrial activities, and overfishing, it is global climate change that most concerns scientists. Scientists remain hopeful that it's not too late to save the reefs, and some are moving ahead on experiments to accomplish that goal.