By Anna Mudeva

POZNAN, Poland, Dec 10 (Reuters) - The world has lost about a fifth of its corals and many of the remaining reefs could die in the next 20 to 40 years unless humans reduce greenhouse gas emissions, a report said on Wednesday.

Further coral loss will have alarming consequences for some 500 million people who depend on reefs for their livelihood, said the report by the Global Coral Reef Monitoring Network (GCRMN) presented at a Dec. 1-12 U.N. conference on global warming.

"Climate change must be limited to the absolute minimum to save corals," Julia Marton-Lefevre, head of the International Union for Conservation of Nature (IUCN) which is a member of the GCRMN, told a news conference.

"If nothing is done to substantially cut emissions, we could effectively lose coral reefs as we know them, with major coral extinction," she said.

The impact of the main climate threats, such as rising sea surface temperatures and seawater acidification, is being strengthened by other negative factors like overfishing, pollution and invasive species, said the report.

As emissions of the greenhouse gas carbon dioxide increase, oceans absorb more of them and become more acidic, and this is damaging a wide range of marine life from corals to plankton and from lobsters to seagrasses, it said.

Scientists cannot say how much of the coral loss in recent years has been caused by global warming but agree that climate change is the biggest threat to reefs.

The report said one encouraging sign was the ability of some corals to recover after major "bleaching" events -- when colourful algae living in corals die off -- and to adapt to climate change.

But the global trend of recent years of a worsening environment for corals has not been reversed, it said.

Coral reefs offer economic and environmental benefits to millions of people, including coastal protection from waves and storms and as sources of food, pharmaceuticals and jobs. (Reporting by Anna Mudeva, editing by Tim Pearce)

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