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Ocean acidification, global warming, and the Great Barrier Reef

The decline in the rate of reef-building along the Great Barrier Reef is severe, sudden, and unprecedented in at least 400 years. Ocean acidification is a leading suspect.

By / January 1, 2009

These Porites and other corals provide a critical nursery and long-term habitat for tens of thousands of marine species that rely on coral reefs

Image courtesy of Jurgen Freund of Freund Factory

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Perhaps it's time to begin talking about global warming and acidifying oceans in the same breath, rather than as related-but-separate issues.

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In mid-December, The Monitor ran a story on research showing that some areas of the world's oceans are acidifying faster than marine scientists had predicted even three years ago.  The culprit: the excess carbon dioxide that human industrial activity and deforestation are pumping into the atmosphere -- and that the oceans are absorbing. (That article came on the heels of another, more-general Monitor article on the topic a month earlier.)

Now comes word that corals along Australia's Great Barrier Reef have been growing at an increasingly slow pace since about 1990. The process coral colonies use to build their crusty superstructures is called calcification; it's fallen off by some 14 percent since '90, according to a study published in the journal Science on Jan. 2. (You need a subscription to get the full research paper.)

The decline is severe, sudden, and "is unprecedented in at least 400 years," according to Glenn De'ath, a scientist at the Australian Institute of Marine Science outside of Townsville, Australia, who led the research team.

Dr. De'ath's team says more work is needed to pin down the relative contributions among several possible causes, including pollution, warming ocean temperatures, and ocean acidification.  But the group says it sees warming temperatures and ocean acidification as leading candidates.

The reason? Declines in coral far offshore were comparable to those near shore, where reefs would be more strongly affected by nutrient or soil run-off from land.

Researchers are concerned about the issue worldwide because it undermines the ability of shell-forming marine creatures -- many of which are key links in the marine food chain -- to build their homes. And reefs are nurseries and havens for a range of creatures.

Rock candy and calcium carbonate

To paint a picture of the process, Dwight Gledhill, a reef scientist with the US National Oceanic and Atmospheric Administration turns to kids making rock candy as an analogy. During a phone chat, he recalls the recipe: 1) Add lots of sugar to boiling water -- more than you'd think the water could take and still stay dissolved. The water in effect becomes "supersaturated" with dissolved sugar.  2) Suspend a string in the water, which gives the dissolved sugar a "nucleus" on which to gather back into crystal form. 3) Wait for a few days. Viola: an array of sugar crystals -- the rock candy -- clings to the string.

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