Jan. 5 (Bloomberg) -- Growth of coral along the 2,000- kilometer (1,200-mile) Great Barrier Reef is declining more than at any time in four centuries as the climate warms globally and ocean waters become more acidic, Australian scientists found.

Coral skeletal growth slowed by 14.2 percent since 1990, the steepest decline in at least 400 years, Glenn De’ath, who led a team of researchers at the Australian Institute of Marine Science in Townsville, Queensland, wrote in the journal Science.

“This severe and sudden decline in calcification is unprecedented,” De’ath said in a statement on the institute’s Web site about the Great Barrier Reef. “The combination of increasing temperature stress and ocean acidification may be diminishing the ability of GBR corals to deposit calcium carbonate.”

The loss of coral growth threatens food webs and may lead to “precipitous” changes in biodiversity, the authors said. Their research is the latest showing coral’s decline due to man-made emissions of greenhouse gases and follows last month’s Global Coral Reef Monitoring Network report that said a fifth of the organisms have already been lost and many remaining reefs may die by 2050.

As corals grow, they deposit a layer of aragonite, a form of calcium carbonate, onto the existing coral skeleton. De’ath’s team examined the thickness and density of annual layers of calcium carbonate to determine the rate of deposits for a type of coral called porites going back more than 400 years. They examined the growth of 189 coral colonies from the Great Barrier Reef from 1990 to 2005 and data from a further 139 colonies from before 1990.

‘Imminent’ Changes

After discounting potential factors inhibiting coral growth including salinity changes, runoff of minerals from the land, diseases and competition with other corals, the researchers concluded that the main drivers of the current decline are probably increased sea surface temperatures and growing acidity, both products of man-made emissions of gases blamed for global warming.

The calcification rate of the corals, measured by multiplying the depth in centimeters of an annual layer by the density in grams per centimeter, dropped to 1.51 in 2005 from 1.76 in 1990, the scientists found. While the current rate of growth is still higher than the level of 1.4 in the late 1500s, it’s the steepest decline in growth in the four-century period, according to the paper.

“Clearly, more studies are needed,” the team wrote Jan. 2 in Science. “If porites calcification is representative of that in other reef-building corals, then maintenance of the calcium carbonate structure that is the foundation of the Great Barrier Reef will be severely compromised.”

The fate of corals is crucial to the livelihoods of millions of coastal dwellers around the world. Reefs are worth about $30 billion a year to the global economy through tourism, fisheries and coastal protection, according to the Millennium Ecosystem Assessment, a United Nations-supervised study.

“These organisms are central to the formation and function of ecosystems and food webs,” De’ath’s team wrote. “Precipitous changes in the biodiversity and productivity of the world’s oceans may be imminent.”

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: January 5, 2009 08:01 EST