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Dead in the Water

A snorkler installs temperature and salinity logging devices in a coral reef

Georgia Tech Graduate Student Pamela Grothe installs temperature and salinity logging devices on a Christmas Island coral reef to monitor conditions through the 2015-2016 El Niño event.
Photo: Alyssa Atwood

El Niño conditions in the Pacific Ocean have seriously damaged coral reefs, including those on Christmas Island, which may be the epicenter for what could be a global coral bleaching event. Georgia Tech researchers who visited the island reported that 50 to 90 percent of corals they saw were bleached and as many as 30 percent were dead at some sites.

“This El Niño event is driving one of the three largest global-scale bleaching events on record,” said Kim Cobb, a professor in Georgia Tech’s School of Earth and Atmospheric Sciences who has studied long-term El Niño conditions. “Ocean temperatures exceeded the threshold for healthy corals back in the summer.”

Bleaching is an outward sign of stress in the corals, which release the symbiotic algae that normally help provide them with energy to sustain their metabolism. The loss of these algae turns the coral colonies white and makes them more vulnerable to disease and death. Bleached corals can recover if water temperatures return to normal.

Cobb and other researchers measured water temperatures of 31 degrees Celsius (88 degrees Fahrenheit), well above normal water temperatures of 27 degrees Celsius (81 degrees Fahrenheit). “There’s an astounding amount of warming at this particular site,” Cobb said. “These reefs are under dramatic stress, which is leading to severe coral loss. It will take years for these reefs to recover.”

Cobb said the disaster could provide a unique opportunity for studying the long-term ecological impacts of major bleaching events, which could become more frequent as the Earth warms.

The El Niño Southern Oscillation (ENSO) is a cycle of warm and cold temperatures that occurs naturally in the central Pacific approximately every two to seven years. By studying fossil coral records from Christmas Island, Cobb and her research team have seen evidence of these cycles dating back at least 7,000 years. However, there is increasing evidence that El Niño events have changed in the past few decades.

The research was supported by the National Science Foundation.​ 

— John Toon

Kim Cobb

Kim Cobb is a ­professor in the School of Earth and Atmospheric Sciences. Her research focuses on bio-geochemistry, the dynamics of weather and climate, oceanography, and paleoclimate.

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