Corals can survive without their algae companions for a while and recover from moderate levels of bleaching. But each episode weakens coral and makes it more susceptible to other impacts, including disease and damage from pollution, storms, sediment, recreation and runoff from land areas. In the past two years, 38 percent of the world's coral reefs have been exposed to water temperatures above the critical threshold, about half at levels that can cause significant mortality, according to an April bulletin from NOAA.
Even with advance warning, some coral experts were stunned by the extent, duration and intensity of the current bleaching, which has already killed and damaged parts of reef complexes in every tropical ocean basin. The bleaching generally hasn't been as severe as in 1998, but it has affected more reefs and has caused mass bleaching in several reefs that have never been affected before, according to Eakin.
Coral-threatening conditions are expected to continue and spread to new areas for the rest of the year and possibly into 2017. Parts of some reefs are likely to be wiped out. Others will be so damaged that they won't recover for decades, if ever. The planet's current warming trajectory puts half the world's corals at risk of dying by 2050 and 90 percent by 2100.
New observations from the past few weeks reinforce the warnings. Near Jarvis Island, part of the Pacific Remote Islands Marine National Monument, about 95 percent of the coral colonies have died, NOAA researchers recently reported. That came after a year of ocean temperatures running up to 4 degrees Celsius above average. More coral mortality is expected on other reefs in the months ahead.
"Bleaching events in Hawaii and Florida have now occurred for two boreal summers in a row. Back-to-back bleaching events in both of these places is unprecedented," said Ilsa Kuffner, a research scientist with the U.S. Geological Survey's Coastal and Marine Geology Program. "Adding a third bleaching event this summer would again take us into unknown territory."