Longest Coral Bleaching Event on Record Continues to Hammer Reefs

Even as the El Niño of 2015-16 winds down, coral reefs remain threatened by the longest episode of global-scale bleaching on record. NOAA announced in October 2015 that the third global bleaching event had begun, with reefs from the Florida Keys to Fiji suffering widespread damage over the past year. In February, NOAA scientists announced that the bleaching event was the longest on record. The event is a result of widespread ocean warming related to long-term climate change as well as regional warmings triggered by El Niño, which began in early 2015. The prolonged nature of the bleaching event is especially worrisome because it allows for multiple years of damage during seasonal peaks in upper-ocean temperature, giving the reefs less time to recover. “We may be looking at a 2- to 2½-year-long event,” said Mark Eakin, coordinator of NOAA’s Coral Reef Watch program. “Some areas have already seen bleaching two years in a row.”

In its 2015 annual summary of conditions across U.S. coral reefs, issued on March 7, 2016, NOAA projected that the global bleaching event will likely extend into 2017. Since 2014, the report noted, 100% of all U.S. coral reef areas have experienced at least some level of thermal stress associated with unusually warm waters, with 41% experiencing Alert Level 2 thermal stress (typically associated with widespread bleaching and mortality). Record-breaking events occurred in 2015 near Hawaii, American Samoa, the Northern Mariana Islands, and Florida. Reefs near Hawaii were hard hit by bleaching in both 2014 and 2015.


Figure 1. Bleached coral at Lizard Island, north of Cooktown, Australia, captured by the XL Catlin Seaview Survey in March 2016. The global insurance firm XL Catlin is working with scientific institutions around the world to carry out the ongoing survey, which has collected more than 700,000 panoramic images along nearly one million kilometers. Image credit: XL Catlin Seaview Survey, via globalcoralbleaching.org.

Major damage at Great Barrier Reef
Severe bleaching is now under way across the northern reaches of the Great Barrier Reef, which is a UNESCO World Heritage Site and one of the world’s great treasuries of marine diversity. At some reefs off far northeast Australia near the tip of Cape York, up to 50 percent of coral have already died. The Great Barrier Reef Marine Park Authority classified the unfolding event as Level R3, the most dire, because of severe regional bleaching over the last few days. (R3 can also be triggered if less-severe bleaching is particularly widespread.) “To put it in simple, stark terms, multiple areas of the reef are now dead and dying,” said David Suggett (University of Technology Sydney) in a Conversation essay published last week. South of the hardest-hit area, clouds and heavy rain had tamped down ocean temperatures and reduced the extent of bleaching, according to a March 21 update from the park authority.

“The damage this year seems more localized but much more intense than 1998 and 2002,” Justin Marshall (University of Queensland) told me in an email, referring to past events across the Great Barrier Reef. The focus toward the north this year, he said, is “likely due to [an] unfortunate stack of factors there—the south got more wind and rain for a bit, and that seems to have shielded it a bit.” Marshall is chief investigator of Coral Watch, a nonprofit that fosters public awareness and entrains students, visitors, and others in helping to monitor and protect the reefs.

After conducting an aerial survey north of Cairns that extended some 600 kilometers (270 miles), veteran researcher Terry Hughes (James Cook University) saw evidence of severe bleaching across all but four of the 520 reefs surveyed. Hughes told Australia’s ABC News that he expects roughly half of the bleached coral across the northern Great Barrier Reef to die over the next month. This week his group plans to extend the surveys southward toward Townsville. “How many 100s of reefs blitzed?” he wondered aloud in a tweet, after calling last week’s expedition “the saddest reef trip of my life.” Both Hughes and Marshall are interviewed in this ABC video.


Figure 2. High-alert areas for coral bleaching across the Pacific Ocean for the week beginning March 26, 2016, based on satellite-derived sea surface temperature data. The areas currently at highest risk are concentrated along and south of the Equator, as upper-ocean temperatures are now close to their seasonal peak in the Southern Hemisphere. In addition, warmer-than-usual waters directly related to El Niño have enhanced the risk in the central and eastern equatorial Pacific. Image credit: NOAA Coral Reef Watch


Figure 3. The sequence of events involved in coral bleaching. Image credit: Great Barrier Reef Marine Park Authority.

How warm water damages reefs
Bleaching is a sure sign of a coral reef in trouble. The brilliant colors associated with coral reefs are produced not only by the reefs themselves (which are animals related to jellyfish or sea anemone) but also by the microscopic algae called zooxanthellae that coexist with the reefs. As the coral send polyps upward and outward from their hard skeletons (Figure 3), the zooxanthellae lodged in the coral tissue carry out photosynthesis and provide nutrients to the coral. In return, the coral provides shelter and carbon dioxide to the algae. Because the algae need sunlight to photosynthesize, coral reefs are located just below the ocean surface in warm tropical and subtropical waters. Stresses such as unusually high water temperatures, increased water acidity, or pollution can disturb the symbiotic relationship between the corals and the algae that live inside them. If water temperatures are more than 1-2°C above their typical warm-season highs for an extended period, the algae may began to photosynthesize too quickly for the coral to handle. To protect its own tissue, the coral may expel the algae, and the grayish-white skeletons of the coral become visible through the now-translucent tissue of the polyps. The more severe and prolonged the bleaching, the more difficult it is for the reef to recover.

Third time is no charm
The global bleaching that struck during and after the “super” El Niño of 1997-98 was a shocker--the first global-scale mass bleaching ever recorded. Although the world’s oceans had been gradually warming for decades, the 1997-98 El Niño was the first time that large sections of coral were exposed to temperatures warm enough to cause extensive bleaching. Some 16% of the world’s reefs died as a result of this mass bleaching. It was followed in 2010 by the second mass global bleaching, again triggered by El Niño on top of long-term warming. About a third of all carbon dioxide emitted by human activity goes into the oceans, where it can degrade coral in another way. Coral growth rates in the Great Barrier Reef have dropped by some 40% in the last 40 years, apparently in large part due to the acidifying influence of human-produced carbon dioxide absorbed by the oceans.

Less than a thousandth of the ocean floor is covered by reefs, but these areas play host to a spectacular variety of life--an estimated quarter of all marine species known to exist--and they help support the ocean-based livelihoods of half a billion people. Reefs are under a variety of threats in addition to long-term human-caused warming, including diseases and overfishing. In the Caribbean alone, surveys indicate that reef habitats have declined by more than 50% since the 1970s. While some types of coral may be able to grow upward quickly enough to keep up with modest sea level rise, not all species will be able to adapt to fast-warming temperatures. We may end up seeing a few hardy coral species proliferate in our warming climate, while many others, including some of the most delicate and beautiful creatures on Earth, struggle--and sometimes fail--to adapt to our new and evolving normal. An interesting article by Meehan Crist in The Atlantic touches on some of the efforts being made to preserve and strengthen reefs and the ecosystems they support, including the development of artificial reefs.

We’ll have a new post by Tuesday afternoon.

Bob Henson   


Figure 4. Departures from the 1971-2000 seasonal average in sea surface temperature for February 1998 (top) and February 2016 (bottom), just after the peaks of the “super” El Niño events of 1997-98 and 2015-16. Image credit: IRI