Oceans in Crisis: Coral Bleaching and Ocean Acidification

By Richard Brown


Divers at Australia’s Great Barrier Reef are emerging from the water smelling of rotting flesh. Almost a quarter of the coral of the world’s largest coral reef ecosystem is now dead.

As global warming and repeated El Niño years have caused consecutive years of drought in Guatemala, they also have raised ocean temperatures, causing corals to “bleach,” or release the algae that give them their brilliant color and their food. When corals release these algae, they starve and then decompose. (El Niño is a flood of warm water that spreads across the Pacific Ocean in irregular intervals, on average once every five years, and affects weather around the world.)

A recent study found that 93% of the Great Barrier Reef’s coral was bleached this year. When coral is bleached, it dies if ocean conditions do not allow it to recover. Terry Hughes, the lead researcher, tweeted in April, “I showed the results of aerial surveys of #bleaching on the #GreatBarrierReef to my students, and then we wept.”

The Great Barrier Reef, like many coral reef ecosystems, rivals a jungle’s biodiversity; it’s home to hundreds of coral species, over 1,600 species of fish, 133 kinds of sharks and rays, over 30 species of whales and dolphins, and over 100 species of jellyfish, along with turtles, crocodiles, and thousands of other creatures. It stretches for 2,300 kilometers. The Belize Reef, the world’s second-longest reef, stretches along the Central American coast for 290 kilometers.

All of them depend on corals. When corals bleach, the small fish that feed on coral or use it for protection, like the Finding Nemo clownfish, starve or migrate. Animals of all sizes, up to dolphins and the mighty tiger shark, that feed on the smaller fish also disappear. The many birds that eat coral reef fish can no longer find them, and plants on nearby islands that depend on bird droppings for nourishment whither.

The white tips on this coral are a reflection of "bleaching" and declining coral health. Photo by Oregon State University.

The white tips on this coral are a reflection of “bleaching” and declining coral health. Photo by Oregon State University.

The bleaching is not limited to the Great Barrier Reef. For just the third time we are witnessing a “global bleaching event” in which coral reefs from Australia to the Caribbean are dying. This year’s bleaching appears far worse than the last two cases in 1982 and 1998.

The source of the crisis is the unfathomable scale of human pollution. Land temperatures have risen across the globe because human activity has released so much carbon dioxide and other gases into the atmosphere that the atmosphere has become thicker, allowing less heat to escape into space. But the oceans disguise the true quantity of heat that human pollution has created, because they absorb 90% of the excess heat trapped by the atmosphere. That quantity of heat is so extreme that these oceans, which cover 70% of the planet, have become 1C hotter over just the past 35 years. That’s over 1.2 billion kilometers cubed of water that humans have managed to heat in just three and a half decades.

 Bleached Coral in Comparison to Healthy Coral. Photo by Smithsonian's National Zoo

Bleached Coral in Comparison to Healthy Coral. Photo by Smithsonian’s National Zoo

Human pollution has changed not only the temperature of the oceans, but also their level of acidity, or pH. The oceans have absorbed so much of the carbon dioxide that humans create by burning oil, gas, and coal that in many places, mollusks like oysters and clams, crustaceans like crab, corals, and plankton are unable to build their shells because of increased acidity. This phenomenon, known as ocean acidification, is increasingly wreaking ecological and economic havoc. Along the United States’ northwest coast, the important oyster industry is dying. Entire populations of Dungeness crab, a commercial staple in the western US and western Canada, are in jeopardy.

Ocean acidification simply adds to the burden on coral reefs, on which half a billion people depend worldwide both for fishing and other foods as well as for protection from flooding.