Australian officials have confirmed that the Great Barrier Reef is experiencing “very widespread” coral bleaching — the reef’s third mass-bleaching event in five years. The Great Barrier Reef Marine Park Authority said the damage is the result of “prolonged thermal stress” due to high ocean temperatures in February and March.

Scientists have so far assessed more than 800 reefs in the Great Barrier system, covering 132,800 square miles, The Guardian reported. The researchers reported that bleaching has happened across the reef, including in some southern areas that had little or no damage during the 2016 and 2017 mass-bleaching events. Inshore and offshore reefs south of Cairns showed the most severe bleaching, with at least 80 percent of the coral in that area affected. Most of the key tourism reefs, located in the northern and central parts of the system, have experienced only moderate bleaching.

Bleached corals are not dead. Rather, when water temperatures get too warm, corals expel the colorful algae living in their tissues, causing them to go white. Most of the Great Barrier Reef’s mildly or moderately bleached corals will recover, scientists say. Severely bleached areas, however, will experience higher mortality.

In a press statement, the Great Barrier Reef Marine Park Authority called climate change “the single greatest challenge” facing the reef. The Australian government has pledged to invest $1.9 billion into protecting the natural landmark, including funding scientific research into reef resiliency, water quality programs, and improved reef monitoring.