Continued warming of world's oceans could spell ongoing disaster for coral reefs worldwide and result in major reef-bleaching events this year, scientists say.
Some bleaching was already seen in 2014, and more is expected in the warming waters of the tropical Pacific and Indian Oceans this year, the U.S. National Oceanic and Atmospheric Administration (NOAA) is warning.
Bleaching occurs when changes in conditions, including temperature, light or the availability of nutrients, stresses the coral, which causes them to expel symbiotic algae living in their tissues -- coral's main source of food -- and turn white.
Without the algae, large swathes of coral reef often die, something seen in past bleaching events.
NOAA's Coral Reef Watch uses climate modeling and satellite observations to monitor the state of the health of the world's coral reefs and issues a weekly report forecasting possible bleaching up to 4 months into the future.
"The outlook shows a pattern over the next 4 months that is similar to what we saw during global coral bleaching events in 1998 and 2010," says Mark Eakin, Coral Reef Watch coordinator.
He espressed concern that 2015 could bring a third such global coral bleaching event.
The biggest concern through May is in the western section of the South Pacific ocean and the Indian ocean, NOAA says, noting that in some parts of the Pacific ocean, thermal stress has increased to the point of causing bleaching in Nauru, Kiribati. The effect has also reached the Solomon Islands.
In the vicinity of the Indian Ocean, similar increases in thermal stress are approaching levels that can cause bleaching in Mauritius, Madagascar, Seychelles, as well as some sections of western Australia and Indonesia, the agency says.
"You are seeing this move with the sun," Eakin says, explaining the heating is taking place as the sun is overhead warming ocean waters to temperatures above that which corals will tolerate.
Some experts worry about some reefs that have managed to recover from previous events in 1998 and 2010 but face new threats this year with ongoing rising temperatures.
"All those places that have recovered will be right back where they were 15 to 20 years [ago]," says University of North Carolina researcher John Bruno, who has been studying Caribbean reefs. The concern with global warming, he warns, is that "if we start seeing bleaching events every three years or every five years we won't start see any recovery."
Eakin agrees that's the principle worry, warning that by midcentury, temperatures that can result in coral bleaching could become the norm.
