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Kim Cobb first noticed the whitish hue of the corals around Kiritimati, a large atoll located south of Hawaii, while on a research trip in July of 2015. Though 18 years had passed without any bleaching, a strong El Niño was now blowing its warm breath across the Pacific, draining color from the reef's spiky, tubular, and bulbous corals, leaving them pallid and skeletal.

Last year was one of unusual climatic activity, with El Niño causing extreme weather around the world. The year 2015 was also dubbed the hottest on record, according to the National Oceanic and Atmospheric Administration and the National Aeronautics and Space Administration.

"My research site is in the very middle of the Pacific Ocean," says Cobb, a Georgia Institute of Technology climate scientist who studies the effects of specific El Niño and La Niña climate patterns on coral reefs. "And this El Niño event has laid the maximum ocean warming smack in the middle of my research site."

If the globe were an archery target with the warmest ocean temperatures in the center and the coolest on the outer edge, Kiritimati, or Christmas Island as it's sometimes called, would be the bullseye. It is the broiling center of a problem affecting reefs globally: A strong El Niño, exacerbated by climate change, is killing off our coral. Christmas Island could also provide a glimpse at what the reefs of the future might look like, should ocean temperatures continue to rise.


Upon diving into Christmas Island's crystalline waters last November, Cobb and her research team found that 50 to 90 percent of the corals had undergone some level of bleaching. As many as 30 percent were already dead.

"It was absolutely jaw dropping," says Cobb, who has been visiting Christmas Island since the last intense El Niño, in 1997. "From having personally never witnessed any bleaching on this reef, to seeing whole traps of the reef being completely dead, to whole species wiped out. It was far worse than I expected."

As with many an El Niño before it, the Pacific Ocean began heating up in the summer of 2015, reaching its peak before the year's end. During an El Niño, the strong trade winds that usually blow from east to west across the Pacific weaken and waft in the opposite direction. The ocean pushes against the diminished winds, which kicks off a subsurface wave that moves eastward, bringing the warm surface waters with it. As the eastern Pacific heats up, the trade winds weaken further. This feedback loop continues until the winds collapse, and El Niño cycles vanish with the arrival of spring.

"We're expecting the maximum death and destruction."

It's important to remember that El Niño is a pervasive and natural component of the climate system. It is one stage within a large climate pattern known the El Niño-Southern Oscillation, or ENSO, that increases and decreases the temperatures in the Pacific Ocean in somewhat predictable cycles, thereby affecting climate worldwide.

There have only been two "super" El Niños until now. The first occurred in 1982–83, and the second in 1997–98. The El Niño we're currently experiencing has exceeded both in terms of temperature.

The current El Niño set a record in November for the warmest waters documented in the equatorial Pacific over a one-week period, according to Weather Underground. For one week in November, temperatures rose 5.4 degrees Fahrenheit above the region's average. The previous record of 5.04 degrees Fahrenheit above average took place in November of 1997. At the height of warming, Christmas Island's surrounding waters were 88 degrees Fahrenheit, seven degrees higher than average.

The million-dollar question, then, is whether El Niños are intensifying due to climate change, and in turn aggravating coral bleaching? To that, Cobb has a grim answer: "Our work is showing yes."


Cobb says the answer is in the data, and the data demonstrates that El Niños have been strengthening in the recent past. In 2013, after examining fossilized coral cores—long tubes of coral skeleton drilled out of reefs or large pieces of coral—Cobb published a paper showing that El Niños were more intense and variable in the 20th century than over the last 7,000 years.

Cobb's research is one data point in a growing list that make the connection between strengthening El Niño events and global warming. She says a number of recent studies all point to mounting carbon dioxide levels as a key component.

Divers approaching a reef that is 90 percent dead. (Photo: Kim Cobb)

Divers approaching a reef that is 90 percent dead. (Photo: Kim Cobb)

Also in 2013, for example, Shang-Ping Xie, a climate researcher at the University of California–San Diego's Scripps Institution of Oceanography, analyzed tree ring patterns from the tropics and mid-latitudes. His results, published in Nature Climate Change, depicted unusually high El Niño-Southern Oscillation activity toward the end of the 1900s. One year later, another study led by Wenjun Cai, a climate modeler at Australia's Commonwealth Scientific and Industrial Research Organisation, found that climate change could double the frequency of super El Niño events.

It's not exactly clear how this will play out for coral reefs. But Xie foresees three major changes: First, oceans around the world will continue to warm. This uptick in temperature leads to increased warming in the equatorial Pacific, which in turn intensifies El Niños. All of this is hard on coral life, which need sustained, warm (but not hot) temperatures to remain unbleached and healthy.


At the start of October 2015, the NOAA confirmed that the third global coral bleaching event—the first occurred during a strong El Niño in 1998; a second came in 2010—was cooking the oceans. The bleaching began in the North Pacific in 2014, and has spread around the world. The areas hit hardest include the Hawaiian Islands, the Caribbean, and parts of the southeast Pacific and Indian Ocean.

Coral reefs provide nursery grounds, food, and shelter for over 25 percent of all marine life. With an estimated quarter of reefs already damaged, any additional threats, such as climate change, impede the benefits they deliver humanity—coastal protection, tourism, fishing revenue, and splendor.

When stressed, whether from changes in temperature, light, or nutrients, corals release their symbiotic algae that live in their tissue. The loss of algae turns corals white and makes them more susceptible to disease. Severe bleaching can devastate reef systems, weakening their overall capacity to recover. A reef hosts a huge diversity of marine life; some fish rely on coral tissue as food, and many have an aversion to degraded reefs. As reefs weaken, so does the food web they support.

"It's the multiple interacting impacts that give the key to disaster," says Jennifer Smith, a marine ecologist at Scripps Institution of Oceanography. "If we turned the ocean up by four degrees tomorrow, corals wouldn't make it. But, if that temperature increase happens over 100 years ... I think reefs are pretty good at handling single stress events."

Smith names the Palmyra Atoll reefs, which she has frequented and studied, as an example of a place that looks almost exactly the same as it did 2,000 years ago. Far from any local human population, Palmyra's reefs retain 80 to 90 percent of corals, despite past bleaching and ocean acidification.

Even if corals are heavily bleached during super El Niños, Smith believes "we might have a different suite of corals in the future. These organisms have been around for many hundreds of thousands of years and will find a way to adapt or acclimate."

Meanwhile, Cobb will return to Christmas Island later this month, where she'll be joined by University of Victoria marine ecologist Julia Baum, to record the level of bleaching and coral mortality. She's not expecting a promising reversal: "Judging by how distressed the reefs were during our last visit," Cobb says, "we're expecting the maximum death and destruction."


Lead Photo: Healthy coral reef in the summer of 2013. (Photo: Kim Cobb)

Catastrophic Consequences of Climate Change is Pacific Standard's year-long investigation into the devastating effects of climate change—and how scholars, legislators, and citizen-activists can help stave off its most dire consequences.