A Swim Through the Ocean's Future

Can a remote, geologically weird island in the South Pacific forecast the fate of coral reefs?

As ocean water becomes more acidic, corals and shellfish must spend more energy to make their calcium carbonate shells. (Photos courtesy of NOAA Pacific Islands Fisheries Science Center Coral Reef Ecosystem Division, Photo by Benjamin Richards)

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“Already, the rate at which corals grow in the Great Barrier Reef has fallen by 15 percent in less than 20 years,” says Ove Hoegh-Guldberg, director of the Center for Marine Studies at the University of Queensland, Australia. “At this rate, corals will start to decline probably in 25 years.” Calcification will plummet to minimal levels–in which coral growth is barely perceptible–by the middle of the century, Hoegh-Guldberg calculated in a paper published in Science in December 2007.

Many reefs are already stressed by temperature spikes that kill coral colonies in a process called bleaching and by the overharvesting of fish that keep coral-smothering algae in check. “It doesn't take much of a decrease in reef calcification for coral reefs to begin to crumble and erode away,” Hoegh-Guldberg explains. “That leaves all the fish and the species that support them in the coral reefs essentially homeless, so they’ll just disappear.”

Hoegh-Guldberg published evidence for this scenario in 1999. Today he is no longer a lonely Cassandra. Many researchers believe that by mid-century, when atmospheric CO2 will be double what it was in 1800, “all coral reefs will cease to grow and start to dissolve,” asserts Jacob Silverman of Hebrew University of Jerusalem in a paper published in March in Geophysical Research Letters.

“Predicting the effect of acidification on coral is relatively easy,” says Caldeira of Carnegie, who coined the term “ocean acidification.” “We know it will make it harder for a lot of other marine species to reproduce, we just don’t know to what extent.”

The problem is that while there have been several periods during which the ocean has been much more acidic than it’s expected to be in a century, the process took at least 5,000 years, which gave marine species much more time to adapt. Now the process is 1,000 times faster than ever before, says Hoegh-Gulberg, which is why he worries about mass extinctions.

Which brings us back to Maug.

The unique death zone, where the water is so acidic from the volcanic vents that no coral can survive, is only 30 feet across and 200 feet long. After I swim out of it into gin-clear water full of fish and coral, I turn around and head back south, with the current, past the vents. Both the death zone and the transition zone, where highly acidic water merges with normal sea water, should yield insights into how corals will react to a changing ocean.

“This is the only shallow reef we know of where certain spots look just like we expect a lot of reefs will look like in 50 or 100 years,” Brainard says in his office in Honolulu. “We need to study what goes on in the space between the dead zone and normal one to find out exactly how corals react to increasingly acidic water.”

Because the ocean absorbs CO2 so slowly and there is already so much in the atmosphere, acidification will be much harder to reverse than climate change. As I swim back to the boat, I wonder how much coral my (hypothetical) grandchildren will see. They will certainly see ample and captivating photographs and film footage of coral reefs. Unlike the unphotographed dodo, our reefs, even if they are reduced to sand, will live on in our imagination.


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