We’ve all felt the gloom and doom of the threat of a treacherous storm on the horizon.  Right now the coral reef community is bracing for a different but no less grim kind of storm: coral bleaching.  The National Oceanic and Atmospheric Administration (NOAA) has just issued a “level one alert”, which indicates massive coral bleaching expectations for widespread reefs.  We are in the midst of the third ever global coral bleaching event, and many scientists are calling it the worst in history.

So what is ‘coral bleaching’ exactly?  

The process of coral bleaching is explained in this infographic by NOAA.

The process of coral bleaching is explained in this infographic by NOAA.  Click to expand.

The term ‘bleaching’ is used because that’s exactly what happens; the corals turn white, bleached of all color.  To understand how this happens, one must look at the basic biology of a coral.  A coral is not a single organism, but rather a symbiotic relationship of hundreds or thousands of coral polyps (individual animals), and algae, called zooxanthellae, which give the corals their brilliant colors.  The algae is responsible for conducting photosynthesis to supply the coral polyps with oxygen as well as glucose for energy.  This algae is very sensitive to environmental stressors, particularly fluctuating water temperature.  When ocean waters are abnormally warm, the algae are expelled, leaving a clear polyp on top of a white calcium carbonate skeleton.  A bleached coral is still alive, and corals have the ability to recover from isolated or short bleaching events.  However, longer periods of bleaching or repeated instances of bleaching will cause coral mortality.

What is causing this bleaching event?

An underwater “heatwave” in many parts of the world’s oceans, coupled with record global temperatures driven by strong El Niño conditions, are the causes of this bleaching event.  The two previous global bleaching events occurred in 1998 and in 2010, and the climate conditions surrounding those events are very similar to what is happening right now.

Though unfortunate, the current bleaching event was no surprise to scientists.  It was predicted months ago by the NOAA Coral Reef Watch program’s outlook forecasts.  Climate models, among other tools, are used to determine the environmental conditions to be expected in the reefs in the near future.  Since bleaching is linked to thermal stress, scientists knew it was likely to occur once abnormally warm ocean temperatures were predicted for the coming months.  Many of the areas that are experiencing bleaching right now also experienced it last summer, making this a multi-year global event.

Coral reefs of the United States seem to be getting hit particularly hard by this event.  NOAA issued a statement predicting that “by the end of 2015, almost 95 percent of U.S. coral reefs will have been exposed to ocean conditions that can cause corals to bleach.”  It’s important to note that this does not mean 95 percent of U.S. corals will undoubtedly bleach, but rather that they will be under conditions that make it extremely likely that they will.

The outlook for reefs.

Parts of South Florida, including the Keys, experienced some bleaching last year.  Reports of bleaching started appearing again in August of this year.  Fortunately the worst seems to be over in South Florida, and the bleaching is declining.  However, the stressors that caused the bleaching to happen in the Keys are now occurring in other parts of the Caribbean, which is still warming up.  Reefs in Haiti, the Dominican Republic, Puerto Rico, and the U.S. Virgin Islands are all at great risk for the next few weeks.  The climate outlook shows bleaching conditions disappearing by January in the Caribbean.

Expansive colonies of bleached corals. Photo by Ray Berkelmans.

Expansive colonies of bleached corals. Photo by Ray Berkelmans.

Bleaching occurred in the north Pacific as well last summer, which devastated huge tracts of reefs around Hawaii.  This bleaching event, the second of its kind in the area, left one and a half square miles of corals bleached, according to NOAA’s Randy Kosaki.  Sadly, those bleached corals did not recover and are all now dead.  The Hawaiian reefs are considered to be one of the greatest areas of risk, and the current bleaching observations are only growing in frequency and intensity.  It is expected to continue in this fashion well into December.

The concern with this event is that the strong El Niño conditions that are predicted by climate models for this year and into the new year could cause a resurge of bleaching events to spread globally once again in 2016.  The repeated and prolonged exposure to bleaching is what causes corals to be unable to recover.  This does not bode well, particularly in areas like Hawaii where bleaching already occurred last year, and it is happening again now.

A 4-month outlook for the current global bleaching event. Credit: NOAA.

A 4-month outlook for the current global bleaching event. Click to expand. Credit: NOAA.

What can we do?

NOAA Coral Reef Conservation Program acting program manager Jennifer Koss issued the following statement regarding action steps: “We need to act locally and think globally to address these bleaching events. Locally produced threats to coral, such as pollution from the land and unsustainable fishing practices, stress the health of corals and decrease the likelihood that corals can either resist bleaching, or recover from it.  To solve the long-term, global problem, however, we need to better understand how to reduce the unnatural carbon dioxide levels that are the major driver of the warming.”

Pollution cause imbalances of nutrient levels in the water, and overfishing can disrupt the organization of the food web of a coral reef ecosystem, which ultimately degrades the health of the reef.  Koss is suggesting that local management focusing on these issues can at least help avoid making the reefs weaker than they already are.  But the underlying cause of the bleaching events is climate change, which is on a global scale.

Coral reefs cover only 0.1% of the ocean, but they are important sources of food and habitat for over 25% of the world’s marine life.  The loss of coral reefs would trickle down to affect people whose livelihoods depend upon them; roughly 500 million people.  We stand to lose more than just a pretty underwater landscape.

In the long term, controlling pollution and overfishing alone won’t save corals.  But if extreme cuts are made to carbon emissions, in addition to responsible marine management, we could see a rebound of corals by the middle of the 21st century.



CIMH 2015. Caribbean Coral Reef Watch Bulletin Vol 1, Issue 5. Accessed October 9th, 2015 at http://rcc.cimh.edu.bb/files/2015/10/Caribbean-CRW-Vol1_Issue_5_Oct.pdf

Mathiesen, Karl.  World’s oceans facing biggest coral die-off in history, scientists warn.  The Guardian.  Web.  8 October 2015.  http://goo.gl/TZxjr9

NOAA declares third ever global coral bleaching event.  National Oceanic and Atmospheric Administration. Web.  8 October 2015.  http://goo.gl/ceJSuZ

What is coral bleaching?  National Oceanic and Atmospheric Administration.  Web.  http://goo.gl/c2FvB1