The finding raises alarms for scientists who say that warming and acidifying ocean waters are the latest and perhaps greatest threat to reefs already stressed by pollution, overfishing and other problems.
Coral growth has fallen by more than 13 percent since 1990, researcher Glenn De'Ath and his colleagues at the Australian Institute of Marine Science found. And if the rate of global warming continues on its current course, De'ath says, the reef could stop growing altogether by 2050.
The researchers measured the growth of a bulbous type of coral called Porites, which is found throughout the Great Barrier Reef. Like tree rings, the layers inside Porites coral reveal the organism's age and its history.
De'Ath and his colleages studied 328 Porites colonies from 69 reefs that spanned the Great Barrier Reef's nearly 1300 miles. Porites, like other corals, grows by extracting dissolved calcium carbonate from ocean water and using it to form the continually growing skeleton of the reef.
Because Porites coral grows in annual layers, like tree rings, De'Ath and his colleagues could examine the size and composition of the layers to figure out how much the coral had grown each year -- in some cases stretching back to the 16th century.
They found that around 1990, the growth rate began to drop. The researchers believe that the drop is probably due to the rising amount of carbon dioxide in the earth's atmosphere -- the same carbon dioxide that causes global warming.
The ocean absorbs more than a quarter of this excess carbon dioxide, which makes the water more acidic and makes it difficult for coral to get the calcium carbonate it needs to grow.
Although the current research can't directly link the declining coral growth rate to rising carbon dioxide levels, the researchers say that no other cause makes sense to explain such a widespread decline.
"The study covered basically the whole Great Barrier Reef," De'Ath says. "And the results were consistent across the whole reef. That's pretty important because there are other factors driving coral decline, like terrestrial runoff [pollution]. But with those, we'd expect to see effects mostly close to the coast. And this decline was widespread."
Some previous laboratory studies had suggested that ocean acidification might slow coral growth, but the new study is the first to show evidence that it could already be having a significant effect in the ocean.
Kent Carpenter, a marine biologist at Old Dominion University, was the lead author of a study that earlier this year found that one-third of the world's coral reef species could face extinction due to overfishing, pollution and other threats. The study, however, didn't include the threat of ocean acidification.
"At the time the evidence that it was actually causing growth problems was too scant," Carpenter says.
Now, with evidence mounting, Carpenter says the situation is likely even worse than his paper predicted.
"It will probably be necessary to redo our extinction risk [assessment]," he says.
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