A 2°C rise in the sea surface temperature of the South China Sea (SCS) in June 2015 was amplified to produce a 6°C rise on the Dongsha Atoll, killing approximately 40% of the resident coral community according to a study in Scientific Reports this week.

Thomas DeCarlo, Anne Cohen and colleagues documented the consequences of a 2°C rise in sea temperatures in the SCS in response to the developing Pacific El Nino on the Dongsha Atoll, a coral reef in the northern SCS. On its own, this short-lived warming was unlikely to cause widespread damage to coral reefs in the region. However, the authors found that an anomalous high-pressure system reduced wind speeds and surface wave heights across the northern SCS, adding up to 4°C to the 2°C open-ocean anomaly. They found that over the course of six weeks between June and July, 33-40% of coral on the atoll were killed.

Using computerized tomography scans, the authors confirmed that coral colonies on the atoll had survived prior temperature rises associated with El Nino events in 1983, 1998, and 2007. This would suggest that the 2015 event was the most severe to hit the Dongsha Atoll in at least the past 40 years.

The authors argue that as most projections of coral-reef futures rely on estimates of open-ocean warming, these may be overly optimistic for many shallow coral-reef ecosystems.