Earlier this week, we learned that Earth's coral reefs are in the midst of a massive dieoff, triggered by abnormally hot temperatures in ocean basins worldwide. It's hardly the first time in recent history that we've witnessed a widespread coral bleaching event, and it won't be the last. In several decades, coral reefs could literally be extinct — a striking casualty of global climate change.
The colourful marine organisms we call coral are part animal, part plant, part mineral. Polyps, the squishy animals that live inside coral, secrete calcium carbonate exoskeletons to protect their microscopic photosynthetic partners (called zooxanthellae). In exchange for free housing, zooxanthellae offer their hosts sugary meals. This relationship has worked beautifully for thousands of years, but now, rising sea temperatures are causing it to unravel on global scale.
Threat of coral bleaching worldwide for October 2015-January 2016. Image Credit: NOAA
When the ocean gets even a smidge too toasty, zooxanthellae start producing harmful oxygen radicals. Polyps respond to the sudden toxicity of their tenants by expelling them. This, in turn, causes the coral to turn white and stop growing. Unless the zooxanthellae return, the coral will eventually die.
In 1998, a massive underwater heat wave killed 18% of corals around the world in the first global bleaching event. This year's coral bleaching is expected to impact 38% of the world's coral reefs, decimating over 4,630 square miles (12,000 square kilometers) of reefs by 2016.
Coral reefs support an estimated 25% of all marine species. From a biodiversity perspective, losing them would be the equivalent of losing our planet's tropical rainforests. As scientists concluded earlier this year, a sixth mass extinction event is definitely upon us. And right now, the warm, salty front lines are looking especially grim.
A close-up image of bleached staghorn coral in American Samoa in February 2015.
A scientist records a bleached fire coral in Bermuda.
A fire coral in Bermuda. The one on the left is a healthy fire coral, while the one on the right is completely bleached.
A bleached fire coral in Bermuda.
An image showing the bleaching in the famous Kaneohe Bay in Hawaii.
Alice Lawrence, a marine biologist, assesses the bleaching at Airport Reef in American Samoa.
A green turtle photographed in Hawaii on a bleached reef in late 2014.
A close up image of staghorn coral bleaching in American Samoa during the bleaching event in February 2015.
A long-nose file fish struggling to find coral polys to eat. File fish are iconic reef fish that are totally reliant on healthy corals for food.
A completely bleached coral photographed by the XL Catlin Seaview Survey in Hawaii during the main islands' first ever mass bleaching event in late 2014.
A completely bleached coral photographed in Hawaii during the main islands' first ever mass bleaching event in late 2014.
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Top: Airport Reef in American Samoa photographed in August 2015 (after the bleaching event). Approximately 70% of the corals here are dead.
Image Credits: XL Catlin Seaview Survey