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 Program in Brazil.
"We could call it the carbon bomb," 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 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.
Historically, wetlands have been regarded as an impediment to civilization. About 60 percent 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.
So far, the impacts of climate change are minor compared to human depredations, the scientists said in a statement. As is the case with other environmental problems, it is far easier and cheaper to maintain wetlands than try to rebuild them later.
As the globe warms, water from wetlands is likely to evaporate, rising sea levels could change wetlands' salinity or completely inundate them.
Even so, wetland rehabilitation is a viable alternative to artificial flood control for coping with the larger, more frequent floods and severe storms forecast for a warmer world.
Northern wetlands, where permanently frozen soil locks up billions of tons of carbon, are 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.
The melting of wetland permafrost in the Arctic and the resulting release of carbon into the atmosphere may be "unstoppable" in the next 20 years, but wetlands closer to the equator, like those in Louisiana, can be restored, he said.
Teixeira admitted wetlands have an image problem with the public, which is generally well-disposed to saving the rainforest but not the swamp.
"People don't have a good impression about wetlands, because they don't know about the environmental service that wetlands provide to us," he said.
Editing by Alan Elsner
© 2008 Reuters