The majority of coral reefs could be experiencing mass bleaching events by 2045, says an international team of scientists.
High sea temperatures make light toxic to the algae, zooxanthellae, that live within the corals, providing food and giving corals their bright colors. When the algae are expelled, or survive only in low densities, the corals can starve and eventually die. Bleaching events caused a reported 16 percent loss of the world's coral reefs in 1998.
And, says the team, if carbon emissions stay on the current path, three quarters of the world's reefs will experience bleaching conditions annually by 2045. Some equatorial Pacific islands, like Tokelau, need urgent attention now.
"Coral reefs in parts of the western Indian Ocean, French Polynesia and the southern Great Barrier Reef, have been identified as temporary refugia from rising sea surface temperatures," says Ruben van Hooidonk of the University of Miami and NOAA's Atlantic Oceanographic and Meteorological Laboratory.
"These locations are not projected to experience bleaching events annually until five or more years later than the median year of 2040, with one reef location in the Austral Islands of French Polynesia protected from the onset of annual coral bleaching conditions until 2056."
A cut in carbon emissions could delay annual bleaching events by more than two decades in nearly a quarter (23 percent) of the world's reef areas, the research shows.
"Our projections indicate that nearly all coral reef locations would experience annual bleaching later than 2040 under scenarios with lower greenhouse gas emissions." says Jeffrey Maynard of the Centre de Recherches Insulaires et Observatoire de l'Environnement (CRIOBE) in French Polynesia.
"For 394 reef locations (of 1,707 used in the study) this amounts to at least two more decades in which some reefs might conceivably be able to improve their capacity to adapt to the projected changes."