Between 70 and 90 percent of coral reefs in the Indian Ocean have been destroyed by human activity, marine biologists say, and unless action is taken the remaining reefs will likely be destroyed as well. Sewage dumping, soil erosion, and dynamiting by fishers are all taking their toll, but the most serious problem facing the reefs is global warming. When ocean temperatures rise, algae that live in reefs and support other life evacuate.