Reefs in the Caribbean and Florida Keys have lost most of the colorful corals that feed a rich ecosystem and made the region a diving and snorkeling mecca, a major conservation group reported Friday. On average, reefs have live coral on just 8 percent of their surface area, down from more than 50 percent in the 1970s.

Impacts including warming seas and human sewage have contributed to a steady decline that shows "no signs of slowing," the International Union for Conservation of Nature (IUCN) said in releasing its report, which was based on new data compiled by 36 experts earlier this year.

The decline was not uniform, the IUCN noted, and those areas with less human impact fared better. "Corals declined precipitously on the Jamaican north coast in the 1980s ... but not at Curacao and Bonaire where coral has more gently declined to about 25-30% today," the IUCN said in the report.

In contrast, total coral cover in the Florida Keys, the U.S. Virgin Islands and Puerto Rico "has progressively declined from 25 to 35% in the 1970s to less than 15% today."

Many of those severely deteriorated reefs instead are covered with large algae, which make it harder for coral to get established, "and virtually no fish larger than" a few inches, the report stated.

The report cited a number of factors causing the decline: disease, pollution, overfishing, hurricanes and "coral bleaching" — a process triggered by stress such as warm seas or pollution whereby the coral expels the tiny single-celled algae inside it that provide its color.

The IUCN did not try to weigh the importance of each factor, but some experts voiced their belief that global warming is paramount.

John Bruno, a University of North Carolina marine biologist who contributed to the new data, told NBC News that a study published last July shows the key driver in the decline is a warming ocean.

"Our preliminary analysis suggests that the state of Caribbean reefs continues to worsen, primarily due to ocean warming," he said. "To reverse this dire trend, job one is to halt the increase of greenhouse gas emissions."

Related: Study ties coral crisis to climate change
Related: Slideshow on threats to coral

The IUCN released the report at its annual convention and urged nations to step up efforts to reduce fossil fuel reliance, thereby reducing greenhouse gases. It also called on nations with coral reefs in their waters to take several actions:

• Limit fishing through catch quotas;
• Create or extend marine protected areas, which provide havens for coral and fish populations to recover;
• Halt runoff from land of sewage and fertilizers, among other pollutants.

The impacts on coral must be "immediately and drastically" reduced, said Carl Gustaf Lundin, director of IUCN’s Global Marine and Polar Program, "if coral reefs and the vitally important fisheries that depend on them are to survive in the decades to come."

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