Rising heat, more bleaching seen for Great Barrier Reef

Some corals in the Great Barrier Reef are known to be resilient when subjected to rises in temperature, but a study out today warned that this protective mechanism could soon disappear.
If ocean surface temperatures rise by as little as 0.5 degrees Celsius over what they are at present, the massive coral bleaching event underway in the famed Australian reef could spread dramatically, said the findings in the journal Science.
The reason has to do with an innate response to the stress of warming waters that corals have shown in the past, which scientists studied by analyzing 27 years of satellite records for the Great Barrier Reef.

"When corals are exposed to a pre-stress period in the weeks before bleaching, as temperatures start to climb, this acts like a practice run and prepares the coral," said lead author Tracy Ainsworth from ARC Centre of Excellence for Coral Reef Studies at James Cook University.
"Corals that are exposed to this pattern are then less stressed and more tolerant when bleaching does occur."
But if sea surface temperatures rise more than two degrees Celsius above a given region's monthly average temperature -- calculated over the past three decades -- this protective mechanism could be lost and more corals may be damaged.

Most of the corals that have been protected "will begin to experience single and repetitive bleaching events when sea surface temperatures are approximately 0.5 Celsius higher than present -- which is expected to occur within four decades based on historical warming rates," said the study.

Currently, about three-fourths of corals in the Great Barrier Reef benefit from the protective scenario.
But if sea surface temperatures increase, only about 22 per cent would be protected and far more deadly bleaching could be expected.
"When corals lose the practice run, there is no break, or 'relaxing' for the corals as summer stress develops," said co-author Scott Heron, from Coral Reef Watch at the US National Oceanic and Atmospheric Administration (NOAA).

"In future summers, bleaching events will occur more often and, without the practice run, become even more severe -- with a greater risk for coral mortality and a fast decline in coral cover across reefs."
Bleaching occurs when abnormal environmental conditions, such as warmer sea temperatures, cause corals to expel tiny photosynthetic algae, draining them of their color.
Last month, aerial surveys of the Great Barrier Reef revealed the worst bleaching on record along a 600-mile stretch of the World Heritage-listed site's pristine north.