Warming seas caused Singapore's corals to suffer the longest bleaching incident on record last year, but the good news is that the casualties from the six-month episode may be lower than expected.
Fifteen to 20 per cent of the corals here died because of the bleaching, the National Parks Board (NParks) estimates.
This is comparable to and lower than the mortality rates for two other major bleaching episodes in 2010 and 1998, even though last year's event was the longest.
Corals depend on symbiotic algae, called zooxanthellae, to make food. Bleaching occurs when abnormally high sea temperatures cause the zooxanthellae living in the corals to leave, turning them white.
Last year, scientists observed that the corals fringing Singapore's southern coast started bleaching in early June. The bleaching incident was considered over only last month, when sea surface temperatures went back to the normal 29.6 deg C.
Surface sea-water temperatures in Singapore reached a high of 31.56 deg C on May 9, according to data from the United States National Oceanic and Atmospheric Administration.
In 2010, bleaching started in June and ended in September. Mortality associated with bleaching then was between 5 and 10 per cent, said NParks.
The 1998 incident was the most severe in terms of coral mortality, with 25 per cent of corals dying due to the bleaching. That year, it lasted from June to August.
All three years during which coral bleaching was observed were El Nino years. This refers to the weather phenomenon associated with warm weather.
Marine biologist Huang Danwei, from the National University of Singapore's Reef Ecology Lab, believes the mortality rate for last year's bleaching episode was lower as corals may have adapted to deal with thermal stress.
"This is encouraging since they may be able to withstand the increased frequency and intensity of thermal stress, though this is on the assumption that other impacts on coral reefs, such as sedimentation, do not worsen," said Assistant Professor Huang.
Mr Stephen Beng, chairman of the Nature Society (Singapore)'s marine conservation group, said the health of Singapore's coral reefs should not be taken for granted, even though they appear to be able to recover from thermal stress.
"Other than warming seas, coral reefs face other stressors, such as pollution and coastal development that can affect salinity and water quality. It is important to monitor these other factors to ensure they do not impact our marine life," he said.
Coral reefs help protect Singapore's coastlines and wetlands from the increasingly extreme weather as the climate changes, said Mr Beng.