Coral reefs in the Caribbean are producing less than half of the key ingredient that makes their calcium skeleton compared to pre-industrial times, scientists said on Tuesday, describing the findings as "extremely alarming."

The amount of new calcium carbonate being added by coral reefs is at least half, and in some places 70 percent lower, than it was thousands of years ago.

Biologists have long sounded the alarm for reef-building corals, on which nearly half a billion people depend for their livelihood from fishing and tourism.

Previous research has estimated that coral cover is declining by as much as two percent per year in parts of the Indian Ocean and the Pacific. In the Caribbean, cover has shrunk by around 80 percent on average since the mid-1970s.

According to a June 2012 update of the "Red List" compiled by the International Union for Conservation of Nature (IUCN), 33 percent of reef-building corals are at risk of extinction.

Habitat destruction, pollution and more recently global warming are the factors blamed in the decline.

But data which compares today's trends with the pre-industrial past is sketchy.

Calcium carbonate is secreted by polyps, the tiny animals that live symbiotically with coral. Patiently accumulated, it provides the structure that enables coral to grow vertically.

A multinational team led by Exeter University in southwestern England measured ancient corals at 19 sites in the Bahamas; Belize; Grand Cayman, which is part of the Cayman Islands; and Bonaire, a Dutch territory in the Leeward Antilles islands.

Their study, published in the journal Nature Communications, found that in shallow waters of around five metres (16.25 feet) in depth, reef growth rates today were between 60 and 70 percent lower compared to the regional averages of the distant past.

The fall was smaller -- around 25 percent -- in deeper waters of around 10 metres (32.5 feet).

Many reefs may have lost their ability to produce enough carbonate to grow vertically, according to the study. Some are already below the threshold by which enough carbonate is produced to maintain the skeletal reef structure, and thus are at risk of erosion.

The estimates "are extremely alarming," said Chris Petty, an Exeter University professor.

"Our findings clearly show that recent ecological declines are now suppressing the growth potential of reefs in the region, and that this will have major implications for their ability to respond positively to future sea-level rises."