Reversing the Trend

Researchers seek creative ways to slow the buildup of atmospheric carbon dioxide.

In March 2007 a Reuters reporter telephoned Gregg Marland with this comment and question: "China is growing rapidly. When will China's carbon dioxide emissions catch up to U.S. emissions, which currently are number one in the world?"

Marland, a climate researcher at Oak Ridge National Laboratory, expressed interest in the question and promised to get back to the reporter. He had read reports that China was building an average of one new coal-fired power plant every week.

"I did a rough extrapolation and discovered, much to my astonishment, that CO₂ emissions in China will likely surpass U.S. emissions in 2007," Marland says. In March Reuters reported the finding, which was carried by most of the international media. The following month the International Energy Agency, which uses a carbon emission estimation methodology Marland helped develop, reached the same conclusion, reshaping the climate debate.

Sources and sinks

CO₂ emissions do not observe political boundaries. Recognizing this fact, one strategy that could have a global impact on the CO₂ buildup in the atmosphere is mitigation, broadly defined as efforts to reduce the sources and enhance the sinks of greenhouse gases.

Improving the energy efficiency of buildings and producing electricity using nuclear, wind and solar energy instead of fossil fuels will reduce CO₂ emissions. ORNL Corporate Fellow David Greene, a lead author for the "Transport" chapter of the mitigation report of the Intergovernmental Panel on Climate Change's fourth assessment, says that reducing CO₂ emissions from transportation "will almost certainly require a transition to more electric drive vehicles, whether powered by hydrogen fuel cells or electricity from the grid." Although increased use of more-fuel-efficient vehicles, cleaner diesel vehicles, hybrid cars and biofuels would slow the growth of CO₂ emissions from the developed world's highway vehicles, the accelerating motorization of the developing world—especially China, India and Southeast Asia—will likely produce a net increase in the world's CO₂ emissions from motor vehicles through 2030.

ORNL's Rod Judkins leads a project designed to assess the ability of inorganic membranes and activated carbons to capture CO₂ before the gas is emitted from coal-fueled power plants. Dave Cole is participating with the Bureau of Economic Geology of the University of Texas at Austin in the Department of Energy's Frio Brine Experiment. The researchers study the ability of an underground brine-saturated, sandstone formation in Texas to sequester injected CO₂ in a well 5700 feet (1737 meters) deep.

The first ORNL report on the process of storing carbon in the earth was a 1980 technical memorandum entitled The Collection, Disposal and Storage of Carbon Dioxide. "We proposed injecting CO₂ into the ocean, the soil and depleted oil and gas fields," recalls Marland.

In 1977, when Marland collaborated with Freeman Dyson at the Institute for Energy Analysis in Oak Ridge, Alvin Weinberg, IEA director and former ORNL director, asked Dyson "to think great thoughts." Dyson proposed planting lots of trees to remove CO₂ from the air. "Freeman was one of the pioneers of carbon capture and storage," recalls Marland, who later advocated tree planting. "I have twice testified before Congress on the importance of trees as a way to capture and store carbon, " says Marland, author of one of the first chapters on land use change and forestry for the International Panel on Climate Change and a contributor to chapters for the panel's first three assessments.

Marland recently commented that dark forests absorb more light than grassy fields and snowy surfaces. The observation suggests that mature trees planted close together—which absorb less carbon dioxide than growing trees—might warm, rather than cool, the planet in some locations.

Carbon in soil

Cultivation and harvesting of crops and deforestation lead to the loss of about half of the carbon stored in these soils to the atmosphere. Oak Ridge scientists are examining ways to increase soil carbon storage to slow the buildup of atmospheric CO₂.

In March and July 2007, researchers collected soil cores containing grass roots a yard deep from a 40-acre switchgrass field in Milan, about 300 miles west of Oak Ridge. Switchgrass is a potential feedstock for producing cellulosic ethanol as a renewable, carbon-neutral, transportation fuel.

The experimental plot, administered by the University of Tennessee, is being used for a study of soil carbon storage by researchers from Oak Ridge, Argonne and Pacific Northwest national laboratories and several universities. ORNL's Robin Graham leads the project, called Carbon Sequestration in Terrestrial Ecosystems, which was initiated in 1999 by DOE's Office of Science.

"We had a multidisciplinary crew of about 20 microbiologists, plant ecologists, modelers and an economist collecting soil cores containing roots of different cultivars, or varieties, of switchgrass," Graham says. "The work in March was our first field event to get baseline information on a bioenergy crop.

"One of our tasks is to determine the carbon content of each soil core after weighing it and the separated roots. We analyze the roots and remaining soil samples for carbon and nitrogen content using an elemental analyzer that combusts the carbon and nitrogen in the samples to form gases we then measure."

The researchers seek to answer a variety of questions. Do different switch-grass cultivars vary in terms of root structure and chemistry, aboveground biomass and the influence of root morphology on soil and soil carbon? Do microbial communities change with switchgrass root structure and depth and affect soil carbon content? Does the root mass vary at different depths?

To learn how to optimize long-term storage of carbon in soil, researchers are planning to study the effect of amendments to the soil on soil carbon content in the switchgrass field. One carbon-containing fertilizer of particular interest is black char, powdery charcoal that can enhance soil's retention of carbon.

"In the Amazon some of the soil is dark, rich and productive," Graham says. "That productivity may be due to human manipulations a thousand years ago. People there may have started fires for cooking, tilled the resultant char into the ground and learned this practice increases soil productivity. We intend to amend some soil with black char and study the resultant effects on soil processes that control carbon sequestration."

Wilfred Post, Tony King and others use high-performance computing to model the roles of microbes in decomposing organic matter and forming mineral-humus structures involved in physically protecting soil carbon from decomposition. Soil contains three times as much carbon as vegetation. Soil carbon turnover time ranges from a few years to thousands of years, depending upon the soil's physical and chemical properties, temperature and moisture.

Humans and the carbon cycle

Tristram West has compiled and analyzed extensive data from hundreds of field experiments, primarily in the United States, Canada and Europe, which compare carbon releases and soil carbon storage associated with conventional plow tillage versus no-till agriculture. Using a no-till approach, farmers disrupt the soil less, typically reduce decomposition rates and increase soil carbon.

"Scientists speculated that these effects would occur but had no proof until they conducted experiments and analyzed data," West says. "The result indicated that soil carbon increases with no till, quantifying and confirming a near-term greenhouse gas mitigation strategy that works. In addition to the benefits of lower carbon emissions and less erosion, farmers who use no-till agriculture also experience reduced fuel costs."

One concern is that no-till farmers must increase use of herbicides to destroy the weeds normally killed by plowing. But, West says, monitoring of chemical accumulations in groundwater and waterways and on land has thus far revealed no adverse environmental effects of increased herbicide usage.

How humans influence the carbon cycle continues to intrigue Gregg Marland. He and his colleagues have calculated the CO₂ produced by tractors and trucks, by the manufacture of fertilizers and pesticides and by all the elements of managing farms and forests. "Humans affect the global carbon cycle not only through the carbon in plants and soils, but also through all of the activities involved in managing the production and harvesting of crops," he says.

Noting that the world's population has tripled within his lifetime, Marland makes this telling comment: "Humans have been intimately involved in perturbing the carbon cycle. We are now confronted with trying to minimize the impact."—Carolyn Krause

Contact: Gregg Marland

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