What’s The Link Between Coral Die-Off & Global Warming?

Dear EarthTalk: I’ve heard about the die-off of coral reefs due to global warming. I’ve also read that coral reefs themselves store carbon dioxide (CO2), one of the main global warming gases. So if coral reefs are dying out, isn’t that a double whammy that increases the CO2 in the atmosphere?

—Tom Ozzello, Maplewood, MN

According to marine scientists, the world’s coral reefs—those underwater repositories for biodiversity that play host to some 25 percent of all marine life—are in big trouble as a result of global warming. Data collected by the international environmental group WWF (formerly World Wildlife Fund) show that 20 percent of the world’s coral reefs have been effectively destroyed and show no immediate sign of recovery, while about 50 percent of remaining reefs are under imminent or long-term threat of collapse.

Most scientists now agree that global warming is not a natural phenomenon but a direct result of the continual release of excessive amounts of CO2 and other “greenhouse” gases into the atmosphere by human industrial and transportation activity. And the small but prolonged rises in ocean temperature that result cause coral colonies to expel the symbiotic food-producing algae that sustain them. This process is called “bleaching,” because it turns the reefs white as they die.

But researchers working with the Coral Reef Alliance have found that while coral reefs do store CO2 as part of photosynthesis, they tend to release most of it back into the ocean (so they are not what are known as “carbon sinks”). As such, the release of CO2 from dying coral reefs is not a major concern.

Of course, the ocean itself is a large carbon sink, storing about a quarter of what would otherwise end up in the atmosphere. Landmasses (and their plants) soak up another quarter of all the CO2 emanating from the Earth’s surface, while the rest rises up into the atmosphere where it can wreak havoc with our climate.

Recent findings indicate that the Antarctic Ocean is getting less efficient at storing CO2, and this raises serious questions about the ability of our oceans to handle everything we throw at them. The study’s authors fear that “such weakening of one of the Earth’s major carbon dioxide sinks will lead to higher levels of atmospheric carbon dioxide in the long-term.”

Not everyone is forecasting gloom and doom. Some Australian researchers believe that coral reefs around the world could expand in size by up to a third due to increased ocean warming. “Our finding stands in stark contrast to previous predictions that coral reef growth will suffer large, potentially catastrophic, decreases in the future,” says University of New South Wales oceanographer Ben McNeil, who led the controversial 2004 study that was published in the peer-reviewed scientific journal, Geophysical Research Letters. “Our analysis suggests that ocean warming will foster considerably faster future rates of coral reef growth that will eventually exceed pre-industrial rates by as much as 35 per cent by 2100,” he adds.

In spite of such theories, the majority of marine scientists remain pessimistic about the future of coral reefs in a warmer world. One can only hope that the optimists are right.

CONTACTS: WWF; Coral Reef Alliance; “Coral reefs may grow with global warming,” New Scientist