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, 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 Program in Brazil.

Wetlands are not just swamps: they also include marshes, peat bogs, river deltas, mangroves, tundra, lagoons and river flood plains.

Together they account for 6 percent of Earth's land surface and store 20 percent of its carbon. They also produce 25 percent of the world's food, purify water, recharge aquifers and act as buffers against violent coastal storms.

As the globe warms, water from wetlands is likely to evaporate; rising sea levels could change wetlands' salinity or completely inundate them.

In addition, northern wetlands, where permanently frozen soil locks up billions of tons of carbon, are most at risk from climate change because warming is forecast to be more extreme at high latitudes, said Eugene Turner of Louisiana State University, a participant in the conference.