LONDON, England (CNN) -- The world has lost almost one-fifth of its coral reefs according a new report released by the International Union for Conservation of Nature (IUCN).



Coral reefs could be wiped out in 30-40 years according to a new report.



Compiled by the Global Coral Reef Monitoring Network, the report has brought together the work of researchers from 15 countries with data stretching back 20 years.



It's not just climate change -- which raises ocean temperatures and increases seawater acidification -- which is damaging reefs. In some parts of the world overfishing, pollution and invasive species are proving equally harmful.



Scientists are warning that reef destruction will have alarming consequences for around 500 million people who rely on coral reefs for their livelihood.



Left unchecked, remaining reefs could be completely wiped out by 2050, the report says.



Professor Olof Linden from the World Maritime University in Malmo, Sweden, told CNN: "We see a great and imminent threat of more reefs being lost."



Speaking from the U.N. Climate Conference in Poznan, Poland, Professor Linden said that the 19 percent figure is an average.



"For many developing countries like Sri Lanka and countries in East Africa the percentage of damage is much worse. Sometimes three times as high in some places," he said.



"In these areas we have local effects like dynamite fishing and other destructive fishing techniques combined the threat of coral mining, unmanaged tourism and all kinds of pollution from agriculture."



But overall the biggest threat to reef survival is climate change.



"The most destructive climate event to impact the coral reefs so far," said Linden, "was the 1998 El Nino which caused major coral bleaching and disrupted ecosystems all over the planet." iReport.com: What little things are you doing to save the planet?



Scientists say reefs have recovered somewhat from those bleaching events. But the Indian Ocean tsunami in 2004, coupled with coral disease and human effects, have slowed their recuperation.



Coral reefs not only provide an income and food for those who live near them, but are also effective natural barriers against storm surges.



Despite the report's pessimism, researchers see some encouraging signs. Forty-five percent of the world's reefs are currently in good health and the hope remains that damaged reefs can recover and adjust to the changing conditions.



"We must focus on helping corals to adapt to climate change and on diverting people away from destructive practices such as overfishing," Linden said.
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