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As efforts to curb global warming continue in Washington and at this week's Copenhagen summit, there may be a partial solution in the fields of Marin County, California. It's called carbon ranching. The idea is to coax carbon dioxide out of the air and to capture it in the grass and soil of pastureland.

NPR's Christopher Joyce explains.

CHRISTOPHER JOYCE: Marin County isn't just hot tubs and wineries. Try going up to where the cows and deer live.

Dr. WHENDEE SILVER (Co-Founder/Lead Scientist, Marin Carbon Project): So in another month or so, this road becomes really challenging.

JOYCE: It's not challenging now?

(Soundbite of laughter)

Dr. SILVER: No. This is pretty good.

JOYCE: Whendee Silver is steering a Jeep up a hill as steep as a ski slope. Silver is a soil scientist at the University of California, Berkeley. If soil is the Earth's skin, then Silver might as well be its dermatologist.

Dr. SILVER: What we're interested in doing out here is trying to figure out how much carbon is added to the soil and how much carbon is lost.

JOYCE: Soil and plants that grow in it depend on carbon. Plants mostly absorb it from carbon dioxide - essentially it's a plant food. Silver wants plants to eat more.

Dr. SILVER: So this is a compost plot.

HEADLEE: Silver has spread compost that were plots of pastureland; a mix of plant clippings, food waste, the kind of stuff you put on your garden.

Dr. SILVER: And what that does is it increases plant growth. It actually also lowers the temperature a little bit so the soils don't get quite as hot, it doesn't stimulate as much microbial activity.

JOYCE: You can see the grass is taller here, that means more carbon in the plants and more food for cows. Ranchers like that part.

But those microbes, Silver mentions, are tricky. Soil is full of them and when they eat plants and animals and bugs, they emit carbon dioxide into the air. So Silver's composting has to work a delicate balance between supercharging plants but not microbes.

Dr. SILVER: What we're really trying to do here is understand what makes the microbes go and what makes the plants go.

JOYCE: So far, the grass in the composted plots grows so well that it captures 50 percent more carbon from the air than grass in the untouched plots. And the soil is taking up almost all the carbon in the compost � carbon that most likely would have gone up into the atmosphere if it hadn't been added to the pastureland. Silver is now measuring exactly how much that is. To do that, her team pounds plastic domes into the ground that capture the greenhouse gases seeping up from the soil.

(Soundbite of pounding)

JOYCE: But Silver says just the extra grass from composting could make a big dent in greenhouse gas emissions, especially in this part of California.

Dr. SILVER: Grasslands, because they're in these dry regions, have really, really high root biomass, so it tends to go pretty deep. Those plants are looking for water and that's what builds that dark, organic rich soil and that carbon-rich soil.

JOYCE: Silver thinks composting could work for about 30 years before the soil is saturated with carbon. And during that time, ranchers could get paid for the practice.

Dr. SILVER: Hopefully, they'll be able to participate in a carbon market, where we can quantify how much carbon is being stored on the land, and they can sell that as a carbon offset.

JOYCE: That idea intrigues John Wick. His back deck looks out over his grazing land where Silver is conducting her experiments, which are part of the Marin Carbon Project.

Mr. JOHN WICK (Project Director, Marin Carbon Project): Now, I think about carbon in everything I'm doing, and it's completely changed my life. This whole ecosystem down there is alive. I mean, up until this point, it was just dirt to me and I - something I pushed around with my bulldozer.

Mr. BOB GIACOMINI (Dairy Farmer): You know, all these things sound good, but you got to look at the cost of them all and see what the payback is.

Dr. SILVER: Absolutely. Yeah.

JOYCE: Bob Giacomini is a little more skeptical. Giacomini stands in a trailer above a milking room at his a dairy farm. It's noon and he's already been up eight hours. Whendee Silver is trying to persuade him to try composting to see if his pastures will grow better, as well as store more carbon.

Mr. GIACOMINI: I'm certainly interested in finding that out. I mean, how much time and/or money am I willing to put forward to find that answer, you know?

JOYCE: That's where the carbon market would come in. If a climate law is passed, industries will be looking for ways to reduce their carbon footprint. Paying farmers to soak up carbon in their pastures could be one way to do that.

As for the time and know-how, there's a new office in the U.S. Department of Agriculture in Washington, D.C., designed to create a market for exchanges just like that. Sally Collins runs it.

Dr. SALLY COLLINS (Director, Office of Ecosystem Services and Markets): The potential for landowners is huge, so we started looking at that as a way to keep people on the land.

JOYCE: Because land that stores carbon would generate extra income. Actually, measuring carbon in soil and plants isn't easy, though. Collins acknowledges that a market won't work unless the buyers and sellers know exactly what they're getting.

Dr. COLLINS: We have got to figure out how to have one set of scientifically based, credible standards.

JOYCE: Standards to measure exactly how much carbon stays in the soil and the grass, and even what happens to it after the microbes or the cows eat it.

It sounds complicated, and it is. But as negotiators at the Copenhagen climate meeting struggle with ways to reduce CO2 in the atmosphere, storing carbon in soil and plants may start to look pretty attractive.

Christopher Joyce, NPR News.

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