An aerial survey of the Great Barrier Reef last week showed widespread coral bleaching for the second consecutive year, an indication that water temperatures stayed too warm for coral to survive, Australian officials said.
"We are seeing a decrease in the stress tolerance of these corals," said Neal Cantin, of the Australian Institute of Marine Sciences. "This is the first time the Great Barrier Reef has not had a few years between bleaching events to recover. Many coral species appear to be more susceptible to bleaching after more than 12 months of sustained above-average ocean temperatures."
Bleaching occurs when coral, invertebrates that live mostly in tropical waters, release the colorful algae that live in their tissues and expose their white, calcium carbonate skeletons. Bleached coral can recover if the water cools, but if high temperatures persist for months, the coral will die.
Eventually the reef will degrade, leaving fish without habitats and coastlines less protected from storm surges.
Officials with the Great Barrier Reef Marine Park Authority and the Australian Institute of Marine Science found severe bleaching in the central part of the Great Barrier Reef Thursday during a six-hour flight between Townsville and Cairns. The area was spared the severe widespread bleaching seen last year.
"How this event unfolds will depend very much on local weather conditions over the next few weeks," said David Wachenfeld, director of reef recovery for the Marine Park Authority.
Wachenfeld emphasized that it's unlikely that all the bleached coral found Thursday will die.
"As we saw last year bleaching and mortality can be highly variable across the 344,000 square kilometer (133,000 square mile) Marine Park — an area bigger than Italy," he said.
The first global bleaching event occurred in 1998, when 16 percent of corals died. The problem spiraled dramatically in 2015-2016 amid an extended El Nino natural weather phenomenon that warmed Pacific waters near the equator and triggered the most widespread bleaching ever documented. This third global bleaching event, as it is known, continues today, even after El Nino ended.
The Associated Press contributed to this report.