Bleach Alert!

New tool gives advance warning of coral-stressing heat

Bleached staghorn coral near Mariana Islands, Guam. Photo by David Burdick, NOAA

Barb DeLuisi, Fall 2008

Warm water can kill coral, and ESRL researchers are hoping a new tool to forecast warm water episodes can help reef managers and conservationists protect vulnerable reef ecosystems. Increases in ocean temperature due to global warming have brought renewed attention to the issue of coral bleaching.

Coral reefs are important and productive ocean ecosystems, providing shelter and protection for diverse sea life. Large coral structures are formed from colonies of hundreds or thousands of tiny coral polyps, which use carbon dioxide to build limestone shells. Microscopic algae called zooxanthellae inhabit coral polyps and give the coral its vibrant color, but unusually warm water temperatures can damage that symbiosis. In some areas, a prolonged temperature increase as small as 1ºC can trigger coral to expel zooxanthellae, resulting in bleaching and eventual death.

With NOAA's Coral Reef Watch program, ESRL scientists have developed the new Experimental Seasonal Coral Bleaching Outlook Tool, to provide advance warning of warm-water events. Corals can recover from mild bleaching, especially if the reef ecosystem is generally healthy. With advance warning of potential temperature stress, decision makers can reduce other stressors, by limiting diving and fishing, removing coral predators, treating coral disease, or reducing coastal runoff.

NOAA Coral Reef Watch was already using sea surface temperature data from environmental satellites to alert managers and scientists around the world of the current risk for coral bleaching. Robin Webb, ESRL/PSD, decided to combine the satellite data with sea surface temperature forecasts. "NOAA scientists have developed and improved experimental sea surface temperature forecasting products for climate outlooks," said Webb. "The bleaching outlook translates that enormous effort into a management product that can help protect living marine ecosystems." The experimental forecasts predict the likelihood of seasonal bleaching up to three months in advance, and forecasts are generated by an experimental sea surface temperature model developed by PSD's Cecile Penland and Ludmilla Matrosova. The first outlook was issued in July for August through October, and it noted a risk of widespread bleaching in the Northwestern Hawaiian Islands, with less severe bleaching in the Caribbean.

As part of the Global Earth Observation System of Systems (GEOSS), ESRL is also partnering across NOAA, with other federal agencies, and with international researchers to develop a worldwide monitoring network that will issue outlooks for coral reefs.