Carbon sequestration is the long-term storage of carbon in forests, the earth, or the oceans in order to stop it from being emitted as carbon dioxide in the atmosphere. All organisms that undergo photosynthesis, from trees to phytoplankton, absorb carbon dioxide from the air and store it in the form of glucose molecules. This process of carbon sequestration is a natural process that is part of the carbon cycle. Enhancing it could help reduce levels of carbon dioxide in the atmosphere, which have been increasing because of emissions from fossil fuels.
Carbon sequestration can be increased by planting trees, improving agricultural practices, and, possibly, from seeding the oceans or trapping fossil fuel emissions in geologic formations or the oceans. However, carbon sequestration rates vary with the technology used. The process may also be reversed, sometimes rapidly. Carbon sequestration, therefore, is only part of the answer to controlling greenhouse-gas emissions.
Historical Background and Scientific Foundations
Carbon dioxide levels in the atmosphere have risen from 280 parts per million (ppm) in the pre-industrial era to 375 ppm today. A major contributor to this change is the burning of fossil fuels, which lies at the heart of modern economies. Carbon dioxide is a greenhouse gas, which has the effect of warming the planet, leading to climate change with potentially catastrophic consequences. Therefore, there is increasing interest in ways of reducing atmospheric carbon dioxide levels. One way forward could be carbon sequestration, which is the absorption and long-term storage of carbon by terrestrial and oceanic biomass.
Carbon sequestration is a natural process, and it is best understood in the context of the carbon cycle. Carbon dioxide in the atmosphere is taken up by photosynthetic organisms, which could be trees in a forest or phytoplankton in an ocean’s euphotic zone (the upper area of a water body that receives sunlight sufficient for photosynthesis). Photosynthesis, a series of biochemical, enzyme-catalyzed reactions driven by the energy of sunlight, then incorporates the carbon into molecules of glucose within the organism. Some of this glucose is consumed by animals feeding on the photosynthetic organisms. Both plants and animals use glucose as a source of energy in the process of respiration, in which the glucose is broken down under the influences of oxygen and releases carbon dioxide back into the atmosphere.
Some of the carbon fixed by photosynthesis ends up in fossil fuels, which form from the long-buried remains of dead carbon-based plants and animals. When this is burned, the carbon dioxide again ends up in the atmosphere. Therefore, an individual carbon atom may be released back into the atmosphere either very rapidly, by respiration, or it may take millions of years if it has been locked up in coal or petroleum. Carbon sequestration is the storage, either short-term or long-term, of carbon in biomass as part of the carbon cycle.
There are two main types of natural carbon sequestration. Terrestrial carbon sequestration refers to absorption of atmospheric carbon dioxide by trees, plants, and crops and its storage in leaves, roots, branches, tree trunks, and soil, which are known collectively as biomass. Aquatic carbon sequestration is a similar process, only occurring in the phytoplankton, which are algae or bacteria with photosynthetic pigments, in the oceans or other waterways.
Human activities can either increase or decrease carbon sequestration. For example, tree planting or forest preservation increases it; deforestation decreases it. The latter is estimated to contribute as much as 20% to carbon dioxide emissions. The level of sequestration achieved by tree planting varies with tree species, soil type, climate, and forestry management practices. In the United States, carbon sequestration rates for most tree species are known. For example, pine plantations in the southeastern United States will accumulate almost one metric ton of carbon per acre per year. In the oceans, there are plans to stimulate phytoplankton production by seeding the water with iron. The idea is to thereby sequester carbon at the bottom of the ocean in sediments made up of dead phytoplankton remains.
There is also a growing interest in sequestering carbon in underground geologic formations and at the bottom of the oceans. The idea is to develop technologies that trap carbon dioxide emissions from power plants and industries by, for instance, precipitating it as calcium carbonate, before it reaches the atmosphere. The carbonate would then be buried. These options may prove more expensive than those based on biomass and their long-term impacts are not yet known.
Impacts and Issues
The forestry and agricultural practices that encourage carbon sequestration have other positive effects. They generally enhance the quality of soil, water, air, and wildlife habitats. Preservation of tropical rain forest, in particular, not only stores carbon, but also protects biodiversity. Soil erosion is generally reduced by tree planting, which can thereby reduce agricultural runoff and water pollution. However, the creation of single species plantations may sequester carbon, but reduces biodiversity. Moreover, the effect of carbon sequestration on emissions of two other major greenhouse gases, methane and nitrous oxide, has to be considered. Applying nitrogen-based fertilizer to increase biomass will increase nitrous oxide emissions. However, rotational grazing will reduce emissions of all three greenhouse gases.
Carbon sequestration is best viewed in a broad context. An overall positive impact on carbon emissions is only possible when biomass in croplands, forests, and farmland is acting as a carbon sink, where carbon sequestration is greater than carbon releases over a period of time. Agriculture and forestry may release carbon to the atmosphere as well as sequestering it. In the United States, biomass acts as a carbon sink to the extent of offsetting 15% of total carbon emissions from transport and industry.
Rates of carbon sequestration in the United States have been declining, however, and will continue to do so because of maturing forests, declining harvests, and changes in land use. Therefore, more can perhaps be done, but carbon sequestration does have its limits, because biomass can reach a saturation point where no more carbon can be accumulated. This happens when
WORDS TO KNOW
BIOFUEL: A fuel derived directly by human effort from living things, such as plants or bacteria. A biofuel can be burned or oxidized in a fuel cell to release useful energy.
BIOMASS: The sum total of living and once-living matter contained within a given geographic area; or, organic matter that can be converted to fuel and is regarded as a potential energy source.
CARBON CYCLE: The circulation of carbon atoms through natural processes such as photosynthesis.
CARBON SINK: A location like a forest where there is net storage of carbon as sequestration exceeds release.
FOSSIL FUEL: Hydrocarbon fuel that has been obtained from the death and decay of living matter millions of years ago.
GREENHOUSE GASES: Gases whose accumulation in the atmosphere increase heat retention.
PHOTOSYNTHESIS: The process by which plants fix carbon dioxide from the atmosphere using the energy of sunlight.
trees reach full maturity or when organic matter concentrations in soil build up to a certain level. At this point, the area still needs to be carefully maintained to keep the carbon in storage in the long-term. Carbon sequestration is subject to rapid or gradual reversal due to harvesting, abandonment of a project, or other disturbance.
However, carbon sequestration has a significant role to play in reducing climate change. The Intergovernmental Panel on Climate Change (IPCC) says that 100 billion metric tons of carbon could be sequestered through forest preservation, tree planting, and improving agricultural practices. To put this into context, this would offset 10-20% of the world’s projected fossil fuel emissions.
Nevertheless, carbon sequestration is not the only answer to global warming. Replacing fossil fuels with alternatives, such as wind power, which generate less or no carbon dioxide, is important too, as is cutting down on energy consumption. Offsetting carbon emissions by sequestration should not be used as a justification for maintaining or even increasing emissions elsewhere.
Primary Source Connection
The following news article explores a question that environmentalists and energy companies are asking: Can countries bury CO2 (shown as CO2 in the article) and other greenhouse gases? Such issues as the huge amount
of greenhouse gases that would be pumped underground every day, to issues that could result in a change in groundwater, are making scientists and others think twice. However, many scientists are positive about injecting CO2 into sandstone caverns, and are testing the areas where CO2 has been underground for years to determine the safety and efficiency of geologic storage.
EARTH TOO WARM? BURY THE CO2
Under a blazing west Texas sun, with a whiptail lizard and cattle looking on, Rebecca Smyth works with an assistant to lower a measuring line, then a hose, and finally a slender plastic capsule down an old water well 200 feet deep.
She’s hoping the water samples she collects will yield clues to what is, arguably, one of mankind’s most pressing environmental questions: Can nations bury their greenhouse gases?
If they can, then governments will have bought themselves a decades-long respite as they search for less carbon-intensive energy sources. If they can’t, then a significant rise in global temperatures by 2100 looks inevitable, if fossil-fuel consumption continues at its current pace.
And the answer may well lie here, atop an old west Texas oil field known simply as SACROC, where more CO2 has been pumped underground over a longer period of years than anywhere else on Earth. Her efforts—and those of the rest of a small army of scientists funded by the US Department of Energy—are being closely watched. Energy companies want to know their options as Congress mulls over legislative options to global warming. Environmentalists are eager to find ways to slow the rise of greenhouse gases.
“If we don’t sequester carbon from coal, we won’t be able to stabilize the concentration of CO2 in the atmosphere,” says John Thompson, director of the coal transition project of the Clean Air Task Force, a Boston-based environmental group. “It’s the linchpin.”
Admittedly, pumping huge amounts of carbon dioxide into underground caverns sounds audacious. If the US captured just 60 percent of the CO2 emitted by its coal-burning power plants and reduced it to a liquid for injection underground, the daily volume would roughly equal what the US consumes in oil each day—about 20 million barrels, according to a report by the Massachusetts Institute of Technology in Cambridge. And the risks are substantial.
Inject too much CO2 and the resulting pressure could cause earth tremors or push deep super-salty groundwater up into freshwater aquifers. Once pumped in, the CO2 may not even stay put in the sandstone formations, below layers of shale and other rock.
Nevertheless, researchers sound confident. “I grew up near Love Canal, so I know the problems of putting stuff underground,” says Sue Hovorka, a research scientist at the University of Texas at Austin. “But we’re cautiously optimistic this is going to work.”
She is one of the scientists tracking the movement of carbon dioxide underground in the nation’s first deep-sequestration experiment.
Under a torrid midday sun in the old Liberty oil field south of Houston, she is tracking the progress of about 2,000 tons of food-grade CO2 that she had injected into a well in 2004 and again last fall. Unlike SACROC (Scurry Area Canyon Reef Operators Committee), no CO2 had ever been injected here before, so it should be straightforward to track. But at the moment, Dr. Hovorka is not happy.
Nearly a mile below, her sensitive instruments are trapped in a five-inch steel pipe, and the roughnecks on the rig have spent hours trying to pull them out. A colleague opts to use small explosives to dislodge them. An hour later, the instruments are on their way to the surface and water samples are being analyzed from an adjacent well.
So far, the results are positive.
“Right now the CO2 is stored as small bubbles in the pore spaces of the sandstone,” Hovorka says. “We believe it’s immobilized and will sit there on a 10,000-year time frame and that when we open this well later nothing will happen. We don’t expect any geysers of escaping CO2 or any of the things that people worry so much about.”
The amount of potential storage is vast. Three of the five US geologic storage possibilities under review—salt basins a mile or more deep, mature oil and natural-gas reservoirs, and deep unminable coal seams—could permanently hold at least two centuries’ worth of US CO2 emissions—about 6 billion metric tons a year, researchers estimate.
But many steps lie ahead. These geologic formations must be tested for environmental safety and their ability to retain CO2. New power-generation technologies that can economically capture CO2 emissions must be developed. Finally, pipelines and infrastructure must be built to collect CO2 from emitters to move it to geologic storage.
Perhaps America’s best hopes for geologic sequestration lie with the sandstone formations holding super-salty groundwater here on the Texas coast—as well as the dwindling oil fields across its vast breadth, says Ian Duncan, associate director of the Bureau of Economic Geology at the University of Texas at Austin. Together, these two geological assets could hold all of America’s CO2 emissions for at least the next 40 years, he estimates, enough time to help bridge the gap until solar power or other emissions-free sources of energy become common.
“The question will end up being: How much capacity can we find for injecting large amounts of CO2 over
IN CONTEXT: “HUMAN AND NATURAL DRIVERS OF CLIMATE CHANGE”
“Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land use change, while those of methane and nitrous oxide are primarily due to agriculture.
SOURCE: Solomon, S., et al, eds. “IPCC, 2007: Summary for Policymakers.” In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
decades?” says Ernest Moniz, an MIT professor and coauthor of the March report that criticized the government for not expediting large-scale sequestration research. “Will we, for instance, be able to inject the CO2 output of 50 big power plants in the ground and have it stay there?”
High-volume CO2 injections of 1 million tons or more are expected to begin in Cranfield, Miss., later this year to push out hard-to-reach oil and to test further the feasibility of geologic storage.
Back in Snyder, Smyth keeps a lookout for rattlesnakes from under her broad-brimmed hat as she collects water samples. She’ll compare them with other samples from nearby areas where CO2 is not a factor. Slight chemical differences could yield clues about whether the CO2 is staying put or expanding upward.
“We’re not sure we’re going to see any significant impact from CO2 here,” Smyth says. “But if the impacts are going to show up anywhere in the world, they should show up here where CO2 has been injected so long.”
CLAYTON, MARK. “EARTH TOO WARM? BURY THE CO2.” CHRISTIAN SCIENCE MONITOR (JULY 31, 2007).
Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
U.S. Department of Energy. “Carbon Sequestration.” http://cdiac2.esd.ornl.gov/ (accessed April 14, 2008).
U.S. Environmental Protection Agency. “Carbon Sequestration in Agriculture and Forestry.” http://www.epa.gov/sequestration/faq.html (accessed April 14, 2008).