Global warming is causing trillions of dollars of damage to coral reefs, aggravating risks to tropical small island states threatened by rising sea levels, a UN report said.

The rise in sea levels off some islands in the Western Pacific was four times the global average, with gains of 1.2 cms (0.5 inch) a year from 1993 to 2012, due to shifts in winds and currents, said the United Nations' Environment Programme (Unep).

The study, released to mark the UN's World Environment Day on June 5, said a warming of waters from the Indian Ocean to the Caribbean was damaging reefs by killing the tiny animals that form corals with their stony skeletons.

"These 52 nations, home to over 62 million people, emit less than one per cent of global greenhouse gases, yet they suffer disproportionately from the climate change that global emissions cause," said Achim Steiner, head of Unep.

"Some islands could become uninhabitable and others are faced with the potential loss of their entire territories," the study said.

The loss of corals is wiping trillions of dollars a year off services provided by nature, usually counted as free. Corals are nurseries for many types of fish, they help to protect coasts from storms and tsunamis and also attract tourists.

"Our fishermen are reporting less and less catches in areas where there was once a thriving trade," Grenada's Environment Minister Roland Bhola said on the sidelines of UN talks on climate change in the western German city of Bonn.

"We have been able to associate that with the issues of climate change ... the destruction of our coral reefs and other ecosystems like mangroves," he said.

ECOSYSTEMS

A study last month estimated that each hectare (2.5 acres) of the world's coral reefs provided services worth $350,000 a year. That means that a loss of 34 million hectares of corals since the late 1990s is worth $11.9 trillion a year.

"Corals .. are probably the most threatened ecosystems on the planet," Robert Costanza, of the Australian National University and lead author of the study, told Reuters.

Some people in small island developing states are considering moving inland due to rises in sea level that are causing erosion and bringing more salt onto farmland, said Jacqueline McGlade, chief scientist of Unep.

"But many of them don't have places to retreat towards."

The UN panel of climate scientists said in March there were warning signs that warm water corals were already experiencing "irreversible" shifts. It also says it is at least 95 percent probable that human emissions of greenhouse gases are the main cause of a rise in average world temperatures.

"Addressing climate change ... is absolutely vital to the survival of small island states," Christiana Figueres, head of the UN Climate Change Secretariat, told a news conference.

The report said that small islands could shift to abundant solar and wind power to help cut fuel import bills, which are often between five and 20 percent of gross domestic product.

"We are doing what we can," said Marshall Islands Environment Minister Tony de Brum, pointing to plans to invest in solar energy. His nation also has the world's largest shark sanctuary as part of efforts to protect nature, he added. - Reuters