The longest and most widespread coral bleaching event on record is abating. As the powerful 2015-2016 El Niño faded, the Atlantic, Pacific and Indian oceans remained warmer than average, but they at least cooled to levels that may enable some reefs to start recovering from extreme ocean heat, according to an update from U.S. coral reef experts.
Some of the less-affected reefs are bouncing back, but areas that were hit repeatedly lost so much coral that it could take decades or centuries for the reefs to recover, and only if greenhouse gas emissions are cut to slow global warming. Scientists won't know the full scope of the damage until they compile scientific reports from the far-flung reefs.
The National Oceanic and Atmospheric Administration (NOAA) declared the start of the most recent global coral bleaching in 2015. As heat-trapping greenhouse gases and a strong El Niño combined to drive the global temperature to new records nearly every month, all of the world's coral reefs experienced above-normal temperatures and 70 percent were exposed to conditions that can cause bleaching.
U.S. reefs were among the hardest hit, NOAA said in a statement, with two years of severe bleaching in Florida and Hawaii and three in the Commonwealth of the Northern Mariana Islands. There was also well-documented extensive mortality to the northern third of the Great Barrier Reef in Australia during 2015 and 2016. In the central Pacific, the tiny atoll of Kiribati lost between 80 and 90 percent of its protective coral ring.
"It's an improvement, but I won't say that everything is rosy," said NOAA coral reef watch coordinator Mark Eakin. "For those places that have seen a lot of mortality, it takes 10 to 15 years to start getting back a good amount of coral growth. But that requires 10 to 15 years without another stress event. And that's becoming very problematic. Overall warming has pushed corals closer to their critical heat threshold," he said.
