The world's wetlands, threatened by development, dehydration and climate change, could release a planet-warming "carbon bomb" if they are destroyed, ecological scientists said on Sunday. Wetlands contain 771 billion tons of greenhouse gases, one-fifth of all the carbon on Earth and about the same amount of carbon as is now in the atmosphere, the scientists said before an international conference linking wetlands and global warming.

If all the wetlands on the planet released the carbon they hold, it would contribute powerfully to the climate-warming greenhouse effect, said Paulo Teixeira, coordinator of the Pantanal Regional Environment Programme in Brazil."We could call it the carbon bomb," Mr Teixeira said by telephone from Cuiaba, Brazil, site of the conference. "It's a very tricky situation."

Some 700 scientists from 28 nations are meeting this week at the INTECOL International Wetlands Conference at the edge of Brazil's vast Pantanal wetland to look for ways to protect these endangered areas. Wetlands are not just swamps. They also include marshes, peat bogs, river deltas, mangroves, tundra, lagoons and river flood plains.

Together they account for six per cent of earth's land surface and store 20 per cent of its carbon. They also produce 25 per cent of the world's food, purify water, recharge aquifers and act as buffers against violent coastal storms.

Historically, wetlands have been regarded as an impediment to civilisation. About 60 per cent of wetlands worldwide have been destroyed in the past century, mostly due to draining for agriculture. Pollution, dams, canals, groundwater pumping, urban development and peat extraction add to the destruction.

"Too often in the past, people have unwittingly considered wetlands to be problems in need of a solution, yet wetlands are essential to the planet's health," said Konrad Osterwalder, UN Under-Secretary-General and rector of United Nations University, one of the hosts of the meeting.