Wednesday, December 9, 2015

Bones of Coral Reefs

Fig.1. Coral depends upon algae for survival (Credit:via GIPHY)

In the middle of the Pacific Ocean, just off the small island of Kiritimati, corals are dying off at an astonishing rate. The small atoll, also called Christmas Island, suffers from high water temperatures caused by the current El Niño that have reached 4℃ above normal. Although these temperatures might be nice for a beachgoer, they do not bode well for the survival of the atoll’s extensive coral reefs.

Kim Cobb, a professor at the Georgia Institute of Technology, has studied the systems for 18 years. A recent visit, however, has her worried. Between 50 and 90 percent of the corals were bleached and as many as 30 percent were already dead in some areas. If the water temperatures remain high, even more could die.

Coral bleaching is a stress response that can be triggered by a number of factors - such as sustained low tide or pollution - but wide-scale events are more commonly attributed to disease or high water temperatures. A temperature increase of only 1 ℃ above average for at least four weeks can set off the expulsion of symbiotic micro-algae from coral tissues. These algae give corals their color and provide up to 90 percent of their nutrients. This loss turns the corals stark white and opens them up to disease or death if the temperatures do not drop. Cobb calls the bleaching in the Pacific - one of the largest on record - the epicenter for a global coral bleaching event. If temperatures increase rapidly beyond just the Pacific, reefs around the world may look like those of Kiritimati in as little as fifty years.

Fig.2. Healthy coral reefs (left) are home to over 4,000 species of fish. Coral bleaching (right) threatens these vibrant ecosystems (Credit: Left-Fascinating Universe, 2011;Right-Acropora, 2011)

Fossil Corals and Ice Cores

Instead of looking towards the future, some scientists have chosen to look back in time to understand bleaching events. Researchers with the University of Bristol, the University of St. Andrews, and the University of California Irvine recently published a study in the journal Science on possible coral bleaching millennia ago. They examined fossil corals to determine if deep-sea water circulation was influenced by ancient increases in atmospheric CO2 or temperature.

Between 18,000 and 11,000 years ago, Earth’s climate system went through deglaciation. Global temperatures increased with rising atmospheric CO2, and sea levels went up as ice sheets melted. Analysis of ice core records has shown that the gradual rise in CO2 was punctuated by three abrupt increases in both atmospheric concentrations and temperatures. In order to determine if these changes related to shifts in deep-sea circulation, researchers mapped deep-sea fossil corals with ice core dates.

The fossils were precisely dated using radiometric uranium-series dating that provided a scale directly comparable to ice core records. Measurements revealed two massive events in which the water was homogenized and enriched in radiocarbon, coinciding with the second and third global increases of atmospheric CO2.

Lead author Dr. Tianyu Chen at the University of Bristol says that the abrupt warming of the North Atlantic Region affected deep water formation and circulation. The researchers also think that this analysis provides a good proxy to understand coral bleaching in the paleo record. High and sustained temperatures, shown by ocean turnover in the fossil corals, means bleaching was likely.

Fig.3. The next couple of decades could put corals in hot water (Credit:via GIPHY)

Preserving Coral for the Future

Returning to the challenge at hand, scientists are working to preserve the astonishing biodiversity kept in reefs. Although coral covers well less than one percent of the ocean floor, coral reefs support 25 percent of all marine life. Additionally, coral reefs provide shoreline protection from storms and act as an important source of livelihood and subsistence for coastal populations.

Researchers at the Smithsonian Conservation Biology Institute have developed techniques to store coral sperm and stem cells that, when frozen, could be viable for more than a century. The preserved biodiversity may one day be used to bolster small populations or even re-populate certain areas. Beyond storing genetic diversity, researchers at the University of Hawaii and the Australian Institute for Marine Sciences have turned to breeding more heat- and acid-tolerant coral species for changing ocean conditions.

Although Georgia Tech’s Kim Cobb is working to demonstrate a clearer relationship between stronger El Niño and greenhouse gases, the danger of warmer waters is obvious. Kiritimati may be the starting point for a worldwide event of which coral reefs might not survive. Gene banks, coral fossil research, and breeding efforts may mitigate the damage, but action to address increasing ocean temperatures is necessary to protect corals against bleaching.

Chen, T., et al. (2015, September 25). Synchronous centennial abrupt events in the ocean and atmosphere during the last deglaciation. Science, vol. 349 (6255): 1537-1541. doi: 10.1126/science.aac6159

Hollier, D. (2015, October 19). Creating corals that can survive climate change. Washington Post. Retrieved from

Toon, J. (2015, December 01). El Niño warming causes significant coral damage in central Pacific. Research News, Georgia Institute of Technology. Retrieved from

See this awesome video on coral bleaching that has more to say than its gifs. 


A ring-shaped coral reef or island formed by underwater volcanoes. Lava from eruptions piles on the seafloor and forms an oceanic island. Coral builds a reef around the island and as the volcanic island erodes, the fringe reef turns into a barrier reef (National Geographic).
El Niño
The warm phase of the El Niño Southern Oscillation (ENSO) that is associated with warm ocean water in the Pacific. The cool phase of ENSO is La Niña, and both parts of the cycle are related to temperature and precipitation changes around the world.
coral bleaching
The loss of symbiotic algae in coral tissues through expulsion. This turns the coral completely white and makes it more vulnerable to disease and death.
The transition from glacial conditions to warmer interglacials.

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