A new study on the Great Barrier Reef says half its corals have died over the past 20 years—and the clock is ticking, NBC News reports. Researchers in Queensland, Australia, found a drastic reduction in the size of reef coral colonies between 1995 and 2017, across nearly all species of all sizes in both deep and shallow water. "We used to think the Great Barrier Reef is protected by its sheer size, but our results show that even the world's largest and relatively well-protected reef system is increasingly compromised and in decline," says study co-author Terry Hughes. Among the worst-affected are branching and table-shaped corals, which provide fish and other marine life with a home. In fact, over 25% of all fish species live inside reefs.
These two species "were the worst affected by record-breaking temperatures that triggered mass bleaching in 2016 and 2017," adds Hughes. He also tells the Guardian that he's "very concerned" about the "shrinking gap" between such events. For the record, bleaching occurs when stressed corals expel their algae; the coral then die or turn white, a sign they're on the edge. In this case, scientists point to climate change: "It's hard to have a crystal ball and say a date" when the Great Barrier Reef will die, a professor not involved in the study tells the Washington Post. "Scientists are always trying to be careful, but if we don't act meaningfully in the next five years, we will not have vital and vibrant coral reefs as a legacy for future generations." (Read more Great Barrier Reef stories.)