Whiteout. Bleaching, from rising water temperatures spawned by El Niño, is a growing hazard to coral reef populations.

Coral Reefs Feel the Heat

Nusa Dua, Indonesia--Contrary to conventional wisdom, warmer ocean waters are a greater threat to coral reefs than local environmental insults. That assessment comes from a new scientific report* released here on October 23 at the 9th International Coral Reef Symposium. The study documents a sudden and steep jump in damage stemming from the 1997-98 El Niño event.

The new analysis was done by the Global Coral Reef Monitoring Network, an affiliation of marine scientists and government agencies that 2 years ago produced its first snapshot of the health of reefs. It's the first that contains a complete picture of the physical damage caused by warmer than usual sea temperatures during 1997 and 1998. The study says prior to 1998, an estimated 11% of the world's known reefs had been destroyed by human activities. Barely 1 year later, another 16% had been "severely damaged" by the El Niño event. Although some of these reefs will recover over time, others have already slipped into the destroyed category.

"Coming up to 1998, we thought direct human impacts were the biggest threat to reefs," says Clive Wilkinson, editor of the report and a marine biologist at the Australian Institute of Marine Science in Cape Ferguson, referring to such events as pollution and destructive fishing practices. "Now, we've got to reset our agenda to focus on climate change as well." Climate change models predict not only a steady rise in baseline sea temperatures but possibly more frequent and ferocious El Niños that would cause more bleaching, says Wilkinson.

Even small temperature changes can have dramatic effects on reefs. Exceeding a threshold of around 30°C triggers bleaching, in which coral expel zooxanthellae, the symbiotic algae that give the coral its color. Coral appear able to survive short-term bleaching if there are no other stresses, such as pollution or destructive fishing. But they can die if subjected to high temperatures for long periods of time or if already stressed.

Previous bouts of bleaching have been limited to particular areas, such as the Caribbean. But the 1997-98 El Niño event, which lasted for over a year, produced record-high sea surface temperatures throughout the Indian and western Pacific oceans; this, in turn, led to the most extensive coral bleaching ever seen. "Some of those reefs will come back, but others won't," Wilkinson says.

The news is not entirely bleak. Some reefs have recovered from the 1997-98 event more quickly than researchers expected, says Terry Done, a senior research scientist at the Australian Institute of Marine Science and the current president of the International Society for Reef Studies. In some cases, coral apparently survived deep within reef structures; in others, new coral was recruited from nearby deeper, cooler water. Several groups are also reporting that certain species of coral can apparently adapt to higher temperatures.

* "Status of Coral Reefs of the World: 2000."

Related sites

The 9th International Coral Reef Symposium

NOAA's Coral Health and Monitoring Program