Growth rates among corals on the Great Barrier Reef have slumped to their slowest in at least four centuries and growth is expected to cease within 26 years.

The process of calcification, which gives the reefs their structure and strength, has slowed by 14.2 per cent in less than 20 years, researchers in Australia have discovered.

The slowdown is so abrupt that they fear the natural process of reef-building will stop by 2050 and perhaps as early as 2035, when the Great Barrier Reef will start to fall apart.

Other reefs around the world are feared to be similarly affected, with disastrous implications for fish and other creatures. Global reef cover is already shrinking by 1 per cent annually.

Stress from changes in surface temperatures and an increase in acidity caused by more carbon dioxide being absorbed by the water were cited as the most likely causes.

Scientists analysing data on 328 colonies of Porites corals collected since 1572 within the Great Barrier Reef system fear that the decline passed a “tipping point” within the past decade and may be irreversible.

Glenn De’ath, of the Australian Institute of Marine Science, said that once calcification stopped the reefs would crumble. “The reefs will slowly break down, be taken over by algae,” he said. “The loss of habitat for small fish will lead to reductions in their populations, which in turn cascade up the food chain to predators, and so on. The reef will still exist, but will be very different and far less diverse.”

Changes in the pace of growth have been most striking over the past century, with the research revealing that from 1990 to 2005 it fell from the highest to the lowest rate recorded on the reef. The decline was so dramatic, said the institute’s scientists, that there should be an immediate investigation.

“This study shows that the causes are probably large-scale in extent and that the observed changes are unprecedented within the last 400 years,” they reported in the journal 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 findings may have implications for other marine ecosystems because of the role that corals in parts of the world play in the food chain. In their paper, the researchers said: “These organisms are central to the formation and function of ecosystems and food webs, and precipitous changes in the bio-diversity and productivity of the world’s oceans may be imminent.”

Comparing corals from the period 1900-30 with others from 1970, it was found that the calcification rate had risen from 1.67 grams per square centimetre each year to 1.76g. Since 1990, however, calcification has declined from 1.76g per sq cm to 1.51g in 2005.

Equally startling to the researchers was the increase in the rate of decline.

In 1990 the rate of growth fell by 0.3 per cent, but by 2005 it was falling by 1.5 per cent in a year. The researchers discounted natural cycles as a likely cause because the increase in calcification up until 1970 was gradual, whereas the recent fall has been more rapid.

Samples from 69 reefs were assessed.Porites, which are huge, were chosen because they have annual density bands, similar to tree rings. They can grow for centuries.

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