A new study says the world's tropical coastal forests store more planet-warming carbon dioxide than almost any other ecosystem.
But rapid loss of these forests - known as mangroves - is releasing substantial and previously unrecognized quantities of greenhouse gases into the atmosphere.
Mangroves provide rich breeding grounds for fish, and they help buffer coastal areas from storm surges. But their role in trapping climate-warming carbon dioxide has not been studied much.
A new study in the journal "Nature Geoscience" provides a first look.
More carbon per hectare than tropical rainforests
Daniel Donato with the Forest Service of the U.S. Department of Agriculture and his colleagues surveyed tree mass, dead wood, and soil carbon in 25 mangrove ecosystems around the tropical Indian and Pacific Oceans.
A mangrove landscape on the Ganges River Delta, Bangladesh. Mangroves are being rapidly deforested globally.
Donato says study shows, "They are right up there with the most carbon-rich forests in the world."
Donato's results showed mangroves store about three to four times more carbon per hectare as temperate forests or even tropical rainforests. But Donato says most of it is hidden.
"Carbon in mangroves is really about the below-ground story," he says. "Most of the carbon stored in that ecosystem is in the soil and the tree roots."
Mangroves stored about five times as much carbon in deep soils compared to temperate or tropical forests.
But the researchers note that a third to a half of the world's mangroves have been cut down in the last half-century for timber, coastal development and aquaculture.
The researchers estimate that the amount of greenhouse gases released each year from mangrove deforestation may be equivalent to as much as 10 percent of the global toll from deforestation.That's despite the fact that mangroves make up less than 1 percent of the tropical forest area.
Donato says the research should get the attention of policymakers."It shows mangroves are probably pretty good candidates for things like carbon-market trading to encourage sustainable forest management."
Carbon markets, where industries buy the right to emit carbon dioxide in exchange for protecting forests or other offsets, could be a powerful force for conservation, environmentalists say.
Many developing countries, especially in Asia, are clearing mangroves for fish and shrimp aquaculture because it can be profitable. The value of the mangrove often is not immediately obvious to policymakers, says Emily Pidgeon, director the marine climate change program at the environmental group Conservation International.
"By suddenly having an actual mechanism to value these systems in dollars, it gives you a potentially very large new mechanism for countering the other financial and economic arguments," she says.
Other groups are approaching the economics of mangrove conservation from the buyer's side. They are encouraging Western supermarkets and other large consumers to only buy farmed seafood that meets certain standards, including limited impact on mangrove loss.
Pidgeon says it works well in some places. But other developing countries are not equipped to meet or enforce the standards.
"The government structures are not very strong," she says, "and the expertise and capacity to apply some of these approaches is often very challenging."
Mosaic of services
One of the most important ways to help local communities value the mangroves, Pidgeon says, is to see them as part of a mosaic that provides a whole range of services, including nurturing the fisheries that many developing-world coastal fishing communities depend on for their livelihoods.
"Although you may catch your fish in one particular piece of the mosaic, the fish are only there to catch because of the time they spend in all the other pieces," she adds.
For many coastal communities, carbon storage is just the latest service they may see mangroves providing.