Sequestering carbon in soil and trees

This post is by Martha Roberts, an economist at Environmental Defense.

This is the first of a two-part series on carbon sequestration - storing carbon or carbon dioxide (CO₂) in soils, trees or geological formations.

Global warming is occurring because -- day after day, hour after hour -- human activities pump large amounts of greenhouse gases into the atmosphere. One way to decrease emissions is to store carbon or CO₂ someplace other than the atmosphere.

There are two vastly different ways of sequestering carbon: biological and geological. The topic of this post is biological sequestration, which could offer quick, substantial cuts in emissions.

Our lands and forests have huge potential for storing carbon -- they are nature's "carbon sinks."

Green plants take CO₂ out of the atmosphere and convert it into organic carbon as they grow -- a process called photosynthesis. Organic carbon is converted back to CO₂ when it is eaten or decomposed -- a process called respiration.

Farmers and foresters can do many things to increase photosynthesis and/or decrease respiration. For example, they can replant forests or delay timber harvests to increase photosynthesis. Tilling increases the respiration of microbes in the soil by improving the conditions for decomposition, so no-till farming reduces respiration.

Are trees the only plants that can store carbon? No, but trees store the most carbon because they're large and long-lived.

As long as the wood doesn't decompose or burn, it stores carbon away from the atmosphere. Still, restoring tilled fields to grasslands can help. Since the grass isn't harvested and the land isn't tilled, more of its organic carbon remains in the soil.

How do we get farmers and foresters to use better land management practices?

The answer is a carbon market in which farmers and foresters can participate. In an inclusive carbon market, those who emit carbon pollution pay for credits, whereas farmers and foresters sell them through agricultural offsets.

Agricultural offsets require a robust monitoring and verification process -- a reliable way to measure the carbon stored in soils and wood. This is challenging but doable, as Bill Chameides and his colleagues demonstrated in their technical manual on the topic. (Duke University, which published the book, offers excerpts online [PDF].)

Globally, soil carbon sequestration alone could offset as much as 15 percent of fossil fuel emissions. In addition, thoughtful offset projects can have side benefits such as improved soil quality, increased crop yields, and better wildlife habitat.

And all that's keeping us from these benefits is a lack of economic incentives -- a carbon market where farmers can sell their carbon credits.

In 2004, the USDA published an economic analysis of biological carbon sequestration in the U.S. agricultural sector. The report evaluated how landowners would respond if they could be paid to sequester carbon.

Not surprisingly, the results showed that the higher the price for carbon, the more farmers would do to enhance carbon sinks. At the highest price considered in the analysis, U.S. farmers and landowners would implement practices that could sequester as much as 160 million additional tons of carbon in forests and agricultural soils every year.

That's equivalent to nearly 10 percent of America's CO₂ emissions in 2005! Even at lower carbon prices, agriculture can play a very important role in our fight against global warming.

When people talk about reducing emissions, they often focus on high-tech solutions. But let's not forget the low-tech strategies that can bring immediate results, like improved agricultural practices and increased energy efficiency.