POZNAN, Poland — The world has lost nearly one-fifth of its coral reefs and much of the rest could be destroyed by increasingly acidic seas if climate change continues unchecked, an environmental group warned Wednesday.

Global warming and the rising temperature of the oceans are the latest and most serious threats to coral, already damaged by destructive fishing methods and pollution, the International Union for Conservation of Nature said.

"The world has lost about 19 per cent of its coral reefs during the last 20 years," said IUCN's director general, Julia Marton-Lefevre, on the sidelines of the 190-country UN talks on a new climate change treaty.

"If current trends in carbon dioxide emission continue, many of the remaining reefs will be lost in the next 20 to 40 years," she told reporters.

"Climate change must be limited to the absolute minimum if we want to save coral reefs. We need to move forward and substantially cut emissions," she said.

Increasing carbon dioxide levels in the atmosphere, which fuels global warming, is raising the level as well as the temperature of the oceans, said Olof Linden of the World Maritime University in Malmo, Sweden.

That makes the water more acidic, adversely affecting reef-building coral that rely on calcification to build their shells.

A report by the Global Coral Reef Monitoring Network, of which IUCN is a member, said all the world's coral reefs could be considered threatened if current forecasts from the UN's Intergovernmental Panel on Climate Change and coral reef experts prove accurate.