Forests and soils could be a short-term Band-Aid for global warming by sequestering manmade carbon.

“No one is saying that they provide the answer,” says Dr. G. Philip Robertson, professor of crop and soil sciences at Michigan State University. “However, they do buy us some time.”

Carbon sequestration involves long-term storage of carbon dioxide, or CO2. CO2 is one of six “greenhouse gases” that help make the earth inhabitable by trapping heat from the sun. Many climate scientists now predict, however, that rising levels of greenhouse gases may be creating an enhanced greenhouse effect, or global warming.

In January 2001, the Intergovernmental Panel on Climate Change reported that the 1990s was the warmest decade in the Northern Hemisphere in the past 1,000 years. The
National Oceanic and Atmospheric Administration estimates that CO2 concentrations are now higher than they have been in more than 400,000 years. While many scientists believe that there is a connection between rising global temperatures and greenhouse gas levels, the extent of this connection is still uncertain.

Carbon dioxide is produced naturally through biological respiration and decomposition of organic matter. However, the majority of atmospheric CO2 is now produced through human activities, such as combustion of fossil fuels and landscape alterations, like deforestation. Scientists estimate that humans add about six billion tons of CO2 to the atmosphere every year, with about one-fourth of that produced by the United States.

Reducing CO2 concentrations in the atmosphere may be one way to reduce the threat of global warming, either by reducing emissions or by capturing and storing atmospheric CO2. Reduced emissions can come about through increasing energy efficiencies or by increasing reliance on renewable energy sources, such as solar and wind power.

Sequestering carbon can be accomplished naturally, as with the planting of forests that metabolize CO2 during photosynthesis. It can also be accomplished through more direct means. Many look at sequestration as a complement to reducing CO2 emissions, not a stand-alone carbon management strategy.

“It essentially buys time until economical carbon conservation technologies can be developed,” Robertson says.

Carbon is stored both terrestrially and non-terrestrially. Terrestrial sinks include soils and vegetation, while the oceans and fresh water bodies provide non-terrestrial sinks. In addition to planting new forests and conserving existing forests, carbon can be sequestered by improving agricultural tillage practices and taking marginal cropland out of production. One idea for more direct sequestration involves capturing CO2 from stationary sources like factories and power plants, and storing it permanently.

Power plants account for about one-third of all carbon dioxide emitted to the atmosphere worldwide. Researchers are now looking at the possibility of using very thin membranes that capture CO2 but let other exhaust gases pass through. Once captured, the gas could be injected into underground geological formations like unmined coal beds, rock caverns, salt domes and depleted oil and gas fields, as is now being done in the North Sea.

Another possibility being studied involves the solidification of CO2 into a permanent hydrate. By exposing CO2 gas to a mild acid and minerals, such as olivine and serpentine, and then applying pressure for a short period of time, the resulting solid stores the CO2 permanently.

While carbon sequestration techniques like these could provide a complementary CO2 strategy, many policy makers say the emissions side of the equation will eventually need to be addressed. To date, all CO2 emissions programs in the U.S. are voluntary.
The U.S. Department of Energy (DOE) has a voluntary emissions reduction program called Climate Challenge. Electric utilities are encouraged to reduce greenhouse gas emissions and report them to the DOE for potential credits against future mandatory requirements. In April, the Senate introduced an amendment to the energy bill that would create a voluntary greenhouse gas registry that also provides credit for past and future emissions reductions. In five years, the proposed program would become mandatory if 60 percent of the total national greenhouse gases being emitted were not being reported voluntarily.

Another potential CO2 management strategy involves the development of a market-based approach, creating carbon credits that can be traded nationally or internationally among emitters. In this scenario, generators of CO2, such as utilities and industries, would be issued emission allowances. If they could not meet these allowances by reducing their emissions, they could purchase surplus allowances from more efficient emitters or buy offset credits from landowners that plant forests or adopt certain farm conservation techniques. Robertson said the global carbon market is currently under development, with carbon exchanges being tested right now in Chicago and New York.

Robertson foresees the day when carbon will be sold as a commodity, just like corn and soybeans, which farmers could depend on as another source of income. They could be paid to take marginal land out of production or adopt conservation tillage practices, both of which reduce CO2 emissions. Although some farmers are now being paid in Iowa to implement conservation measures as part of the test market, Robertson said it is not enough to encourage new farmers to do the same. He said until the Kyoto Protocol is ratified, financial incentives will not be there for producers.

“Once the markets are created, we will find out quickly what it will take for farmers to change their practices,” he says.

Still, all of these efforts may not be sufficient. In November 2000, researchers from the United Kingdom predicted in Nature that forests and soils will become net producers of carbon by 2050 because of increased decay from global warming.

A dirt cheap solution

Erosion control practices reap benefits for environment, agriculture industry

Soil provides a huge potential for sequestering carbon. Dr. G. Philip Robertson, professor of crop and soil sciences at MSU, says sequestering carbon in soils has certain advantages over forests. First, there is less risk of catastrophic loss of stored carbon, like what could occur in a forest fire. There is also less risk from a change in landscape management, or from the often inevitable need to harvest a forest resource.

Robertson says other advantages include increased soil fertility, improved nutrient cycling and availability, reduced reliance on synthetic fertilizers and reduced water pollution. Some agricultural soils that have been plowed for 30 or 40 years have lost up to half of their original carbon content, making them ripe for carbon sequestration. However, research indicates they will never be able to retain as much carbon as they did originally.

Soil carbon is both organic and inorganic, although most carbon near the soil surface is in an organic form. Carbon can be stored in soils as both living biomass and biological residue. Humus is one type of soil residue that provides particularly effective carbon storage because it is very resistant to biological decay. It can persist for decades.

Like forests, soils can be carbon sources as well as sinks. Because most organic carbon is concentrated in the top layer of soil, it is particularly vulnerable to the effects of erosion. Organic material is more easily oxidized when it is exposed, and oxidation increases emissions of CO2. Reducing wind and water erosion on cropland allows biomass and residue to be retained in the soil longer, thus reducing CO2 emissions. In a 1999 report to the Greening Earth Society, Dr. David Wojick estimates that about 100 billion tons of carbon originally contained in soil has been lost globally—primarily from agriculture.

Robertson says in the U.S., addressing the soil erosion problem no longer requires additional research but implementation of existing solutions. Many have been developed, including creating buffer strips, maintaining stream riparian areas and managing crop cover and residue.

Conservation tillage, which limits plowing used to control weeds and mix soil, is a common and effective erosion control practice. Robertson says that nearly 60 percent of farmers in the U.S. now use some sort of conservation tillage, which does not cause a “productivity penalty.” Aside from carbon trading test markets like one in Iowa, farmers are generally not paid to use conservation tillage methods. The rewards for these farmers are still economic, however, in terms of conserved soil and moisture.

Without formal economic incentives for adopting carbon-friendly farming techniques, expecting farmers to participate voluntarily may be unrealistic. Farmers are still rewarded for “taking as much product off the land as possible,” Robertson says. A system of carbon credits should be put into place that would pay farmers for taking CO2 out of the environment, he continues. This could be done through tillage management, or by taking marginal cropland out of production and converting it into land cover like forests, wetlands or grasslands.

In the United States, some existing farm programs aimed at conservation have an added benefit of reducing CO2 emissions. The Conservation Reserve program pays farmers to remove marginal croplands, like those on highly-erodable soils, from production. Similarly, the Wetlands Reserve Program pays farmers to restore farmland that was once wetlands to its former state. Robertson says these programs could serve as models for new programs aimed at paying farmers for sequestering carbon. He adds, however, that there is a finite amount of farmland that should be taken out of production.

“The most efficient thing to do in terms of global warming might be to turn all of this land into forest, but we can’t eat forests,” he says. “It’s not a practical solution.”