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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)

A Swim Through the Ocean's Future

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

smithsonian.com

I drop the dinghy’s anchor below the red-streaked cliffs of Maug. The uninhabited island group is among the most remote of the Mariana Islands, which are territories of the United States in the Western Pacific. Maug's three steep, parentheses-shaped islands are the top of an underwater volcano.

Maug, part of the Marianas Trench Marine National Monument, is one of a string of underwater volcanoes—some active and spewing mud, sulfur and carbon dioxide—that boasts some of the world’s most spectacular geology. A nearby seamount hosts the only known sulfur lake this side of Jupiter. And Maug is the only place in the world where underwater volcanic vents emit carbon dioxide in a tropical, shallow-water environment.

Wearing a mask and fins, I slide into the water and swim north for a short trip into the future of our oceans.

As I start paddling against the current along the inner shore of the eastern island, I see abundant small fishes and plenty of coral heads washed in pastel blue, pink and white. Then the water gets warmer and murkier and there is less and less coral. Another 100 feet and I start seeing little bubbles rising in bursts from among the small rocks on the bottom, which are covered in brown slime called cyanobacteria. The small fishes disappear, a sign the water quality is changing.

Where the bubbles come out, the pH of the water is 6.07, a level of acidity that would kill all life in the oceans. A few feet away, where I swim, the acidic emissions have been diluted to a pH of 7.8, which is what scientists predict surface ocean water will average in half a century.

This is the death zone—dark and foreboding, and not a pretty sight. But it’s utterly fascinating to Rusty Brainard, head of the National Oceanic and Atmospheric Administration’s Coral Reef Ecosystems Division, who was among the first to swim through it in 2003. That's because, he says, it may help us understand how reefs around the world will react to an acidifying ocean.

Since the Industrial Revolution, humans have emitted a mind-boggling 500 billion metric tons of carbon dioxide (CO2) into the atmosphere. This heat-trapping gas caused the planet—which was poised for 100,000 years of cooling driven by variations in the Earth’s orbit—to get warmer instead.

Ken Caldeira, a climate change scientist at the Carnegie Institution’s Department of Global Ecology at Stanford University, says we’re spewing out 30 billion tons of CO2 a year from burning coal and oil, plus another 7 billion tons from the indirect effects of deforestation and making cement. In comparison, he says, the natural CO2 production from volcanic vents, in the air and in the water, is about half a billion tons a year.

About a third of the CO2 that has been emitted since the year 1800 has been absorbed by the oceans, which means our climate isn’t as hot as it would be otherwise. But what’s good for polar bears is not good for corals: CO2 in the water turns into carbonic acid, which has increased the acidity in the top 300-foot layer of the ocean by 30 percent.

As ocean water becomes more acidic, corals and shellfish must spend more energy to make their calcium carbonate shells, a process known as calcification.

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