Carbon Dioxide Sequestration to Mitigate Climate Change:-
What Is Carbon Sequestration?
The term "carbon sequestration" is used to describe both natural and deliberate processes by which CO2 is either removed from the atmosphere or diverted from emission sources and stored in the ocean, terrestrial environments (vegetation, soils, and sediments), and geologic formations. Before human-caused CO2 emissions began, the natural processes that make up the global "carbon cycle" (Figure 1) maintained a near balance between the uptake of CO2 and its release back to the atmosphere. However, existing CO2 uptake mechanisms (sometimes called CO2 or carbon "sinks") are insufficient to offset the accelerating pace of emissions related to human activities.
How Big is the Problem?
Annual carbon emissions from burning fossil fuels in the United States are about 1.6 gigatons (billion metric tons), whereas annual uptake amounts are only about 0.5 gigatons, resulting in a net release of about 1.1 gigatons per year (Figure 2). Controlling atmospheric CO2 will require deliberate mitigation with an approach that combines reducing emissions and increasing storage.
Scientists at the U.S. Geological Survey (USGS) and elsewhere are working to assess both the potential capacities and the potential limitations of the various forms of carbon sequestration and to evaluate their geologic, hydrologic, and ecological consequences. The USGS is providing information needed by decision makers and resource managers to maximize carbon storage while minimizing undesirable impacts on humans and their physical and biological environment.
Oceanic Carbon Sequestration-
The world's oceans are the primary long-term sink for human-caused CO2 emissions, currently accounting for a global net uptake of about 2 gigatons of carbon annually (Figure 1). This uptake is not a result of deliberate sequestration, but occurs naturally through chemical reactions between seawater and CO2 in the atmosphere.
While absorbing atmospheric CO2, these reactions cause the oceans to become more acidic. Many marine organisms and ecosystems depend on the formation of carbonate skeletons and sediments that are vulnerable to dissolution in acidic waters.
Laboratory and field measurements indicate that CO2-induced acidification may eventually cause the rate of dissolution of carbonate to exceed its rate of formation in these ecosystems. The impacts of ocean acidification and deliberate ocean fertilization on coastal and marine food webs and other resources are poorly understood. Scientists are studying the effects of oceanic carbon sequestration on these important environments.
Terrestrial Carbon Sequestration
Terrestrial sequestration (sometimes termed "biological sequestration") is typically accomplished through forest and soil conservation practices that enhance the storage of carbon (such as restoring and establishing new forests, wetlands, and grasslands) or reduce CO2 emissions (such as reducing agricultural tillage and suppressing wildfires).
In the United States, these practices are implemented to meet a variety of land-management objectives. Although the net terrestrial uptake fluxes shown in Figure 2 offset about 30 percent of U.S. fossil-fuel CO2 emissions, only a small fraction of this uptake results from activities undertaken specifically to sequester carbon. The largest net uptake is due primarily to ongoing natural regrowth of forests that were harvested during the 19th and early 20th centuries.
Existing terrestrial carbon storage is susceptible to disturbances such as fire, disease, and changes in climate and land use. Boreal forests and northern peatlands, which store nearly half the total terrestrial carbon in North America, are already experiencing substantial warming, resulting in large-scale thawing of permafrost and dramatic changes in aquatic and forest ecosystems. USGS scientists have estimated that at least 10 gigatons of soil carbon in Alaska is stored in organic soils that are extremely vulnerable to fire and decomposition under warming conditions.
The capacity of terrestrial ecosystems to sequester additional carbon is uncertain. An upper estimate of potential terrestrial sequestration in the U.S. might be the amount of carbon that would be accumulated if U.S. forests and soils were restored to their historic levels before they were depleted by logging and cultivation. These amounts (about 32 and 7 gigatons for forests and soils, respectively) are probably not attainable by deliberate sequestration because restoration on this scale would displace a large percentage of U.S. agriculture and disrupt many other present-day activities.
Decisions about terrestrial carbon sequestration require careful consideration of priorities and tradeoffs among multiple resources. For example, converting farmlands to forests or wetlands may increase carbon sequestration, enhance wildlife habitat and water quality, and increase flood storage and recreational potential - but the loss of farmlands will decrease crop production. Converting existing conservation lands to intensive cultivation, while perhaps producing valuable crops (for example, for biofuels), may diminish wildlife habitat, reduce water quality and supply, and increase CO2 emissions. Scientists are working to determine the effects of climate and land-use change on potential carbon sequestration and ecosystem benefits, and to provide information about these effects for use in resource planning.
Geologic Carbon Sequestration--
Geologic sequestration begins with capturing CO2 from the exhaust of fossilfuel power plants and other major sources. The captured CO2 is piped 1 to 4 kilometers below the land surface and injected into porous rock formations. Compared to the rates of terrestrial carbon uptake shown in figures 1 and 2, geologic sequestration is currently used to store only small amounts of carbon per year. Much larger rates of sequestration are envisioned to take advantage of the potential permanence and capacity of geologic storage.
The permanence of geologic sequestration depends on the effectiveness of several CO2 trapping mechanisms. After CO2 is injected underground, it will rise buoyantly until it is trapped beneath an impermeable barrier, or seal. In principle, this physical trapping mechanism, which is identical to the natural geologic trapping of oil and gas, can retain CO2 for thousands to millions of years.
Some of the injected CO2 will eventually dissolve in ground water, and some may be trapped in the form of carbonate minerals formed by chemical reactions with the surrounding rock. All of these processes are susceptible to change over time following CO2 injection. Scientists are studying the permanence of these trapping mechanisms and developing methods to determine the potential for geologically sequestered CO2 to leak back to the atmosphere.The capacity for geologic carbon sequestration is constrained by the volume and distribution of potential storage sites. According to the U.S. Department of Energy, the total storage capacity of physical traps associated with depleted oil and gas reservoirs in the United States is limited to about 38 gigatons of carbon, and is geographically distributed in locations that are distant from most U.S. fossil-fuel power plants.
The potential U.S. storage capacity of deep porous rock formations that contain saline ground water is much larger (estimated by the U.S. Department of Energy to be about 900 to 3,400 gigatons of carbon) and more widely distributed, but less is known about the effectiveness of trapping mechanisms at these sites. Unmineable coal beds have also been proposed for potential CO2 storage, but more information is needed about the storage characteristics and impacts of CO2 injection in these formations. Scientists are developing methods to refine estimates of the national capacity for geologic carbon sequestration.
To fully assess the potential for geologic carbon sequestration, economic costs and environmental risks must be taken into account. Infrastructure costs will depend on the locations of suitable storage sites. Environmental risks may include seismic disturbances, deformation of the land surface, contamination of potable water supplies, and adverse effects on ecosystems and human health. Scientists are pioneering the use of new geophysical and geochemical methods that can be used to anticipate the potential costs and environmental effects of geologic carbon sequestration.
Why Action Is Needed Now?
Cumulative historical CO2 emissions from fossil fuels in the United States are equivalent to more than the total amount of carbon stored in U.S. forests. If current trends continue, cumulative U.S. emissions are projected to double by 2050 and increase by a factor of three to four by 2100. According to the Intergovernmental Panel on Climate Change Fourth Assessment Report of 2007, sequestration and reduction of emissions over the next two to three decades will potentially have a substantial impact on longterm opportunities to stabilize levels of atmospheric CO2 and mitigate impacts of climate change.
Can Sequestration Control Atmospheric CO2?
Computer models of future CO2 emissions and controls on atmospheric CO2 have been developed and summarized by the U.S. Climate Change Science Program (CCSP). These models indicate that projected annual global emissions during the next century would need to be reduced by more than 75 percent in order to stabilize atmospheric CO2 at about 550 ppm. This concentration would be about twice the level of CO2 in the pre-industrial atmosphere and about 45 percent higher than the atmospheric CO2 concentration in 2007. According to the CCSP, stabilizing atmospheric CO2 would "require a transformation of the global energy system, including reductions in the demand for energy.and changes in the mix of energy technology and fuels."
The CCSP models have been used to evaluate scenarios for aggressive implementation of geologic carbon sequestration. As shown in Figure 3, the estimated amount of geologic sequestration in the U.S. over the next century is projected in one model to be substantially smaller than the cumulative emission reductions anticipated from changes by all other methods. In this model, the needed amount of geologic sequestration would exceed U.S. capacity in depleted oil and gas reservoirs, implying the need to implement carbon storage in the Nation's relatively unknown deep formations that contain saline water. In other models, predicted geologic sequestration needs are smaller as a result of different assumptions about global and national economic and technological trends.
The CCSP model results have a large amount of uncertainty. The results shown in Figure 3 do not take into account many of the uncertainties in costs and environmental risks of geologic carbon sequestration. Additional uncertainties prevent comparison of future oceanic and deliberate terrestrial sequestration. Future disturbances of vegetation and soils may add to future CO2 emissions and increase the amount of mitigation required to stabilize atmospheric CO2. For example, if a substantial portion of the carbon stored in Alaskan organic soils were converted to atmospheric CO2 as a result of climate change, the resulting emissions could offset or even exceed the likely magnitude of any deliberate U.S. terrestrial sequestration measures.
The CCSP models illustrate the widely held view that sequestration is necessary but insufficient to control atmospheric CO2. Stabilizing atmospheric CO2 is likely to require substantial changes in energy sources and use as well as carbon management. Many of these changes will likely have significant, long-lasting impacts on land, water, and ecosystem resources.
