When researchers dove into the South China Sea to find out what one of the strongest El Niño effect on record was doing to the sea’s reefs, they found that nearly two-fifths of the coral had died in just six weeks.

The culprit is believed to be the extreme overheating of the water. At the Dongsha Atoll, roughly halfway between Hong Kong and the Philippines and the site where the coral damage was observed, researchers recorded temperatures 6 degrees Celsius warmer than average for that time of year.

Their findings were published Thursday in Nature’s Scientific Reports journal.

Reefs such as those at Dongsha Atoll constitute one of the most vulnerable ecosystems in the world, according to Thomas DeCarlo, one of the study’s researchers at Massachusetts-based Woods Hole Oceanographic Institution.

When coral is subjected to unusual temperatures, it expels the algae that live on it, and which give the reefs their vivid colors. This usually turns the coral completely white — a sign it is dying or dead, and a phenomenon often referred to as “coral bleaching.” Even when conditions revert to normal, it takes decades for the coral to recover. “If there are repeated thermal stress events before the time needed for recovery, the coral could continue to decline,” DeCarlo told Sixth Tone.

Researchers found that the mean temperature in the South China Sea was 2 degrees higher during the summer of 2015, when their study was carried out. But at Dongsha, the huge crescent-shaped coral reef experienced an anomalous high-pressure system that created weak winds and smaller waves, conditions that contributed to raising water temperatures by up to an additional 4 degrees.

Chinese researchers say the dramatic decline of coral reefs in the South China Sea can be traced back to the 1960s. Apart from climate change, they blame human activities such as trawling for bottom-dwelling fish, and other factors.

Coral reefs serve as a habitat for about one-fourth of all marine species. Through fishing, millions of people in the region depend on the reefs’ ecosystems for their livelihoods, but these are also the people who are destroying the coral. According to Qiu Yaowen, a professor at the South China Sea Institute of Oceanology at the Chinese Academy of Sciences, humans have been the party most responsible for the reefs’ degradation. “Many factors have led to the decline of coral reefs,” Qiu said, “and climate change has not been the biggest one.”

However, many reefs are reported to have been bleached during the El Niño of 2015. The world’s most well-known coral, at Australia’s Great Barrier Reef, suffered too. Scientists estimate that two-thirds of the coral there died or is dying, according to another study published in Nature last week.

The El Niño effect, which occurs when warm ocean waters disrupt normal weather patterns, also caused unusually heavy rains and floods across China in 2016.

“We are quite confident that high temperatures were the main cause,” DeCarlo said, referring to the dying coral in Dongsha. “The bleaching began at the same time as the [high] temperatures.”

A worldwide temperature change of 2 degrees warmer is widely accepted as the threshold for avoiding devastating ecological consequences. However, for DeCarlo and his fellow researchers, because of localized spikes in temperature such as those observed at Dongsha, that limit is still dangerously high.

Editor: Kevin Schoenmakers.

(Header image: Dying coral in Yalong Bay near Sanya, Hainan province, Jan. 30, 2007. Yuan Yi/IC)