Australian Broadcasting Corporation: ELEANOR HALL: The tipping point for catastrophic climate change is an issue of dispute between politicians and environmentalists. But on the climate vulnerable Great Barrier Reef, researchers have been surprised to discover that a tipping point for coral growth has already been reached. In the journal Science this morning they reveal that it was reached 18 years ago, as Nonee Walsh reports. NONEE WALSH: At The Australian Institute of Marine Science in Townsville, …