Bleached coral in the Pacific ocean. Photo: Helmut Corneli / Alamy Stock Photo

Coral reefs are disappearing—is it too late to save them?

As the oceans get warmer, almost all the world’s coral could die without serious climate action
October 31, 2021

For most of the world’s corals, the COP26 negotiations must seem like mere talk. The great majority are doomed—even if the summit succeeds in its stated planet-saving aim of holding temperature rises to 1.5C above pre-industrial levels. 

It’s not just coral that will be lost. Coral reefs are one of nature’s most generous hosts: although they cover only 0.2 per cent of the ocean’s floor, they are home to around a quarter of sea species. But they’re also fascinating in themselves. “Coral is animal, vegetable and mineral,” explains Helen Fox, conservation science director at the Coral Reef Alliance. “The coral itself is an animal related to a jellyfish. Reef-building coral has tiny symbiotic algae living inside its tissues. The photosynthesis from the algae creates energy that feeds the coral, and the coral’s waste products—the nitrogen and the phosphorus—are fertiliser for the algae.”  

Sadly, coral is losing its colour. Its famously vivid pinks and reds come from those algae, called zooxanthellae. When corals are stressed by changes in their conditions, they are said to “bleach,” expelling the algae, and leaving the coral animal translucent. The underlying white calcium carbonate “skeleton”—its mineral part, which forms the underlying structure of the reef—then becomes visible through the living tissue. Worse, without zooxanthellae living in it, the coral cannot feed and will eventually die, leaving just the bleached skeleton.

Coral can be stressed by pollution, overfishing or another invasive species. But the biggest threat is climate change, which makes storms frequent, and deoxygenates and acidifies oceans. And of course, it warms the water. Corals already endure close to the hottest temperature they can tolerate. They have not faced this kind of heat before in their evolutionary history. 

The first mass global bleaching event occurred in 1998, destroying 8 per cent of all coral. This October, scientists reported that around 14 per cent of the world’s coral was lost between 2009 and 2018. The Great Barrier Reef has lost more than half of its corals since 1995, especially over the last five years. But all is not (quite) lost. The difference between a global temperature rise of 1.5C and 2C would be crucial to saving some of what is left. It is estimated that if global warming can be kept to 1.5C, coral reefs will decline by between 70 and 90 per cent, as opposed to more than 99 per cent destruction with a rise of 2C. 

“Coral is animal, vegetable and mineral”

The looming catastrophe is human as well as environmental. More than half a billion people worldwide depend on coral reefs for food, income or protection from threats such as tsunamis. “They’re really important for a lot of communities,” says Fox. “And for a lot of other people, even if they don’t see them, they enjoy knowing that they exist.” 

No wonder. With their colour, the creatures they host and even their strange hybridity they make our world more interesting. We’ve already doomed most of them. Here’s hoping we can summon the collective will to save the rest.