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It’s even worse than we thought. An unexpected coral bleaching event in the South China Sea shows that reefs can heat up substantially more than the surrounding ocean, making them more vulnerable to climate change.

The finding suggests that efforts to limit global warming to 2 °C under the Paris Agreement may not be sufficient to save the world’s tropical reefs.

In June 2015, the South China Sea warmed by 2 °C in response to a normal El Niño weather pattern. The moderate temperature rise was not expected to cause significant coral damage.

However, at Dongsha Atoll in the northern part of the sea, the sea surface temperature soared to 6 °C above average, killing 40 per cent of the coral.

This temperature blow-out occurred because the atoll’s shallow water was able to heat up more than the surrounding ocean, research led by Thomas DeCarlo at the University of Western Australia shows. This amplified the El Niño effect.

In addition, unusually weak winds during the same period slowed the spread of heat into the surrounding ocean, so that it became trapped within the atoll.

“Ocean temperatures are already warming due to climate change,” says DeCarlo. “But what we’ve shown is that on top of that, local weather anomalies or processes like reduced wind can drive reef temperatures even higher,” he says. “That compounds the risk that corals are facing.”

The findings fit with emerging research from other coral reefs, says Bill Leggat at James Cook University in Townsville, Australia. “The evidence suggests that we’re going to get these local conditions pushing corals above bleaching levels a lot more often.”

Triple threat

Sea surface temperatures around the world have been increasing by an average of 0.07 °C per decade over the last century due to human-made climate change. This has increased the risk that El Niño and local weather anomalies will tip reef temperatures into the danger zone.

Tropical coral reefs are sensitive to small temperature anomalies – as little as 1 °C, DeCarlo says. Warmer waters strip away the colourful photosynthesising algae that nourish corals, leading to bleaching and often death.

To examine whether previous El Niño events have caused bleaching at Dongsha Atoll, the team analysed 22 coral skeleton cores. They looked for stress bands along the cores indicative of earlier bleaching events.

The results showed that less than half the coral was bleached during El Niño events in 1983, 1998 and 2007. But during the 2015 El Niño, 100 per cent of the coral was bleached. This implies that the 2015 bleaching event was the most severe to hit Dongsha Atoll in at least the past 40 years, and possibly much longer, DeCarlo says.

Most future projections for coral reefs in a 2°C global warming scenario only take into account background ocean warming, rather than local weather effects on reefs, says DeCarlo. “They may be overly optimistic.”

“The only hope now is to minimise carbon dioxide emissions as much as possible and try to protect reefs as best as we can on a local scale,” says Leggat.