Mar 17, 2017

Here's How Our Fossil Fuel Addiction is Killing Coral Reefs

Alastair Bland
Bleached staghorn coral off the coast of Queensland. Photo: Matt Kieffer
Bleached staghorn coral off the coast of Queensland. Photo: Matt Kieffer

In the space of geologic seconds, coral reefs that took millions of years to grow are dying – and it’s our fault. Humans have badly abused reefs for decades, if not centuries.


Carbon dioxide emissions are probably the greatest destructive force looming in the future of coral reefs. Though less directly harmful to a reef than, say, pumping cyanide into a crevice to find Nemo or Dory for your saltwater fish tank, CO2 emissions are driving worldwide environmental changes – namely global warming and ocean acidification – that, in turn, are making ocean conditions intolerable for the organisms that build coral.

These animals – called polyps – secrete calcium carbonate, which hardens around them into protective casings. Over centuries and millennia, uncountable generations of polyps build large structures of coral – but they don’t do it alone. They share a symbiotic relationship with photosynthetic algae called zooxanthellae, which live in the tissue of the polyps and pass on energy from the sun to their hosts. The organisms depend on one another.

They also require specific environmental conditions, and they like water temperatures somewhere between roughly the mid-70s and mid-80s. Prolonged exposure to water that is just a few degrees too warm (or too cold, for that matter) causes the polyps to expel the algae en masse from the reef. Since the algae are very colorful, this makes the coral turn a ghostly white – a phenomenon called coral bleaching that scientists have been observing for decades.

Some coral bleaching occurs naturally. However, thanks to global warming, which is causing ocean temperatures to rise, bleaching has rapidly accelerated. In 1998, an El Niño-related warming event impacted coral across the globe, but it hit the reefs of the Indian Ocean especially hard. Here, 80 percent of the coral was bleached. A 2005 bleaching event affected 50 percent of the Caribbean’s coral. In January, 2010, coral reefs in the Florida Keys got bleached – though in this case, unusually cold water was the culprit. Bleaching doesn’t necessarily kill the affected coral, though often it does. A fifth of the Indian Ocean’s coral died in 1998 after the severe bleaching. In 2014, another massive bleaching event began and persisted through 2015 and 2016.  

The other crisis being driven by CO2 emissions is ocean acidification. For every two to three molecules of CO2 that enter the atmosphere, one molecule dissolves into the ocean, where it reacts with water to create carbonic acid. Carbonic acid exists naturally in water just as CO2 floats naturally in the air. But just as gas-guzzling human activity is overwhelming the atmosphere with unhealthy quantities of CO2, the acidity of the ocean is now increasing to problematic levels that directly harm many invertebrates.