For the past few years, people have planted trees in the effort to stop global warming.

But now some scientists think they've found another solution: restoring wetlands.

These marshy areas absorb carbon dioxide, the principal heat-trapping gas blamed for warming the planet. Their storage capacity may be greater per acre than forests, scientists said.

That little-known function of wetlands could lead to a major role in federal efforts to combat climate change and an environmental boon for the Texas coast.

Climate change legislation that has passed the U.S. House and is similar to measures under consideration in the Senate would allow industries to use offsets worth up to 2 billion tons of carbon dioxide a year.

Offsets are credits that enable polluters to compensate for their greenhouse gas emissions by investing in projects that remove greenhouse gas from the atmosphere.

They are part of legislation creating a broader system called cap-and-trade, which would set a limit on carbon emissions and create a market for polluters to buy and sell emissions permits.

Already, some electric power generators are paying private foresters to plant seedlings and allow trees to grow larger before logging, increasing the amount of carbon stored in the woods.

Other companies are paying farmers to cultivate their fields without plowing, so the soil retains the carbon that would otherwise seep into the air.

Lots of unknowns

As for restoring wetlands to absorb carbon dioxide, the idea is still in its infancy.

The value of wetlands is considerable: they filter and cleanse water, provide habitats for many wildlife species and serve as buffers against hurricanes and storm surges. But their worth as a carbon commodity isn't clear.

“The science isn't really there yet,” said Bob Stokes, president of the Galveston Bay Foundation, which works to restore wetlands in the region. “It's more advanced with hardwood forests, but there is a good supposition that coastal wetlands would work well.”

Scientists know that marsh plants absorb carbon dioxide through photosynthesis. As the plants die and decay, they build up the wetlands' soil, where the carbon is stored for the long term.

Preliminary data suggest that wetlands can store six times more carbon dioxide per acre than forests, said Ron Sass, a professor of natural sciences at Rice University.

“Wetlands are about the richest ecosystem we have,” Sass said.

Despite the enthusiasm, some scientists are concerned that freshwater wetlands emit methane, a climate-altering gas more potent than carbon dioxide. Salt marshes release a negligible amount, making them more suitable for carbon sequestration.

There is also concern about how climate change would affect wetlands. Higher sea levels could drown marshes, while drought could make wetlands too salty.

That raises the possibility that a carbon market would look at a wetlands restoration project on a shorter horizon — maybe 10 years versus as much as 30 years for a forest.

Scientists, policymakers and conservation groups are still trying to frame rules to quantify and verify carbon capture for the credibility of the offset.

Critics argue that offsets are largely unverifiable and delay genuine industrial changes that could thwart global warming. Whether wetlands along the Texas coast can be part of a trading scheme to combat climate change depends on the trustworthiness of the offset.

Timing in idea's favor

The timing, it seems, is right.

Over the past century, much Texas marshland has disappeared because of coastal development, subsidence, storms and rising sea levels. The state is losing part of its natural storm defense, as well as habitats for wildlife, with each vanishing acre.

That makes restoration efforts appealing.

To replace marshes along Galveston Bay, for example, federal and state agencies and environmental groups use dredged materials to build mounds in open water that can support vegetation and become feeding grounds for the next 40 years or so.

“If you could fund more restoration through offsets, it's a win-win,” Stokes said. “It seems like a good deal for everyone.”

Reforestation advantages

Wetlands restoration as a carbon offset has piqued the interest of some companies, but its business potential, like the science, is still developing.

Bill Baker, who manages ecological programs for power generating company RRI Energy, said he sees marshes as a potential offset but will stay with reforestation for now. That's because there are more opportunities for forest projects and the process is more clearly defined.

“There is a need to restore wetlands,” Baker said, but it's difficult to find one large enough for RRI Energy, which would need at least 500 acres to make the offsets worthwhile.

Last year, the company — known as Reliant Energy before NRG purchased Reliant's retail electricity business — gave $300,000 for the rights to offsets that will be created by 1,100 acres of forest known as the Columbia Bottomlands.

Houston attorney Jim Blackburn, meanwhile, paid $1,500 to the Galveston Bay Foundation for the rights to the carbon sequestration of 50 acres of wetlands for three years. It's more of a personal statement by Blackburn, who is seeking to neutralize the carbon emissions of his law office, than a formal offset, Stokes said.

“This could be a great model, and it could be something that we move toward in the future,” Stokes said, “but I want to make sure the science on it is specific before we formally offer something like that.”