Ů��С��Ƶ

The world’s corals are in hot water… literally. Prolonged increases in ocean temperatures caused by this year’s severe El Niño are intensifying the loss of corals around the planet.

“We are currently experiencing the longest global coral-bleaching event ever observed,” says , head of NOAA’s Coral Reef Watch in College Park, Maryland. Eakin predicts that the bleaching, which started in 2014, will probably last well into 2017.

Bleaching occurs when corals respond to stress by expelling the symbiotic algae they rely on to provide them with food and energy. This causes them to pale to a ghostly white colour, often leading to disease or death.

Since 2014, some 32 per cent of the world’s reefs have seen temperatures that cause bleaching, and off the US coast, nearly three-quarters of reefs have been exposed. Eakin says that 60 per cent of corals worldwide may be affected by the end of the bleaching event. The extent of the resulting die-off is yet to be seen.

Eakin compares the continuous pressure that reefs have been under year after year to a boxing bout. “What used to be a one-round fight is turning into a two- and three-round fight,” he says.

Global events

Rob Ruzicka, a coral-reef ecologist at the Florida Fish and Wildlife Conservation Commission in Tallahassee, has studied the fallout from previous global bleaching of coral during the 1998 El Niño and in 2010.

Following the 1998 event, Ruzicka found that on some reefs in the Florida Keys, the seabed went from 14 to 7 per cent coral cover – a level at which it remains today.

This time round, he hasn’t seen as much die-off. “The corals that are here now might be more resistant because they already endured an El Niño. Warming is happening more and more, so they may have adapted,” says Ruzicka. “It’s unprecedented how long the waters have been warm, but we don’t know if this is the worst event yet. We’re still diagnosing the patient.”

Of particular concern are losses of older corals, which release more eggs and sperm during reproduction than young ones, and bleaching in nurseries of corals established to replace those that die off.

Apart from replacing lost corals, can anything be done?

Small-scale tests have shown that introducing cool water around corals can help their colour return, which Eakin says shows promise for local interventions.

“The most important thing we can do is make sure that on a global scale we’re working towards a reduction in climate change,” he says. “That’s the main source of the bleaching on coral.”

Read more: Coral comeback: Reefs have secret weapon against climate change