Hot topic: Coral bleaching

With the worst-ever coral bleaching event upon us, underwater wonders like the Great Barrier Reef are at risk. However, will a new experiment provide a solution?

By James Draven
Published 15 Nov 2016, 15:00 GMT, Updated 8 Jul 2021, 09:37 BST
Coral bleaching — before and after.

Coral bleaching – before and after.

Photograph by XL Catlin Seaview Survey

Skeletal branches of blanched coral are tangled on the seabed, like antlers fly-tipped from a ransacked hunting lodge. That might sound appealing to the sort of traveller who takes Edgar Allan Poe as holiday reading but most of us expect to see the Great Barrier Reef in living Technicolor. For the uninitiated, the third-ever global coral bleaching event has been sweeping reef systems around the globe for a couple of years now, with dire consequences for a number of holiday hotspots.

Coral bleaching is commonly caused by the water in which it lives becoming too hot, causing the coral to turn white — hence the name. A strong El Niño phenomenon — a periodic surface temperature fluctuation around the equatorial Pacific Ocean — coupled with unusually warm ocean temperatures caused by climate change, have triggered the worst coral bleaching event in history.

Now for the science part: corals get their colour from zooxanthella, a single-cell algae with which they share a symbiotic relationship. The zooxanthella nourishes its host with carbohydrates from photosynthesis, and even produces a pigment that absorbs UV rays, thought to act as a sunscreen to the coral. When corals become stressed, however, their relationship sours. As Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration's (NOAA) Coral Reef Watch, explains: "High temperatures cause photosynthesis to run too fast, causing over-worked chloroplasts to release compounds toxic to the coral." The corals eject their unruly tenants to avoid being poisoned, leaving them an eerie white.

This doesn't just mean diving trips will be less colourful. When coral remains in its bleached state for a sustained period of time, it dies, and although coral reefs constitute just 0.1% of the ocean floor, they harbour a staggering 25% of all marine species. The knock-on effect from losing such a significant chunk of our ecosystem would have devastating repercussions.

Coral reefs also provide half a billion people living in coastal communities with their main source of food protein and income, through fishing and tourism. Research even suggests corals have a role to play in breakthrough cancer treatments.

This summer it was reported that 93% of the Great Barrier Reef has been affected by bleaching. But the devastation doesn't end there. Coral bleaching has also struck Florida, the Caribbean, Hawaii, Japan's Ryukyu Islands, the Red Sea and every other major reef region. A total of 60% of coral colonies assessed in the Maldives have been bleached; 80% of Kiribati's are dead.

Already at risk from rising sea levels, the Maldives really can't afford to lose its reefs, which act as buffers against storm damage and weather events.

In the Caribbean island of Bonaire, Fabien Cousteau, grandson of legendary oceanographer Jacques, is experimenting with 3D printing to build artificial coral. These sandstone and limestone structures will be virtually indistinguishable from the real thing, and it's hoped they'll attract fledgling coral polyps to build upon them and repair reef damage.

The NOAA's Mark Eakin is less optimistic: "It's a bit like 3D-printing trees for a dead forest. Artificial reefs are exactly that: artificial."

Q&A: Coral bleaching

Is bleaching getting worse?

The NOAA's Mark Eakin says: "Coral bleaching has been increasing in frequency and intensity since it was first observed in the early '80s. Global-scale events in 1998 and the ongoing 2014-16 event show global warming is increasing the damage to corals."

How can we help?
Ditch fossil fuels. Buy an electric car, put up solar panels, and try to offset your carbon emissions if you're going to fly. Prevention of pollution and overfishing is essential — the reefs need clean water to survive and require a delicate ecological balance.

Published in the November 2016 issue of National Geographic Traveller (UK)

Follow us on social media 


Explore Nat Geo

  • Animals
  • Environment
  • History & Culture
  • Science
  • Travel
  • Photography
  • Space
  • Adventure
  • Video

About us


  • Magazines
  • Disney+

Follow us

Copyright © 1996-2015 National Geographic Society. Copyright © 2015-2023 National Geographic Partners, LLC. All rights reserved