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10 Could Carbon Sequestration Solve the Problem of Global Warming? Stephen W. Pacala, Princeton University During the 21st century, it is anticipated that a trillion tons of carbon in the form of carbon dioxide from anthropogenic sources will be emitted into the atmo- sphere. ~ While it is uncertain what the long-term effects of this added carbon will be, ecological models indicate that a significant amount of damage to ecosystems could result. In addition to increased use of renewable carbon-neutral energy, an addi- tional "backstop" technology to decrease the amount of carbon emitted into the atmosphere is to sequester it. The key question for carbon sequestration tech- nology is: can the science and technology be developed to effectively remove carbon from the atmosphere and keep it sequestered? To answer this question, global biogeochemical constraints must be addressed. Research has been done on so-called natural biological sinks to determine the amount of carbon sequestration possible by this method. This work has indi- cated that natural biological sinks such as a pine forest do initially achieve elevated carbon dioxide fixing from the atmosphere but that this effect goes away after approximately 3 to 4 years. This effect, termed down regulation, can be the result of a long-term decrease of nitrogen or other needed nutrients in the forest over time. A global extension to this question is whether the world is as a whole down regulated or if carbon dioxide fertilization is actually occurring. U.S. Forest Ser- vice data for the past 70 years was analyzed to find if forest growth is currently iNational Research Council, 2001, Carbon Management: Implications for R&D in the Chemical Sciences and Technology, National Academy Press, Washington, D.C., pp. 8-14. 62
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COULD CARBON SEQUESTRATION SOLVE THE PROBLEM OF GLOBAL WARMING? 63 faster than in the past, indicating that carbon dioxide fertilization is occurring. The results indicated that growth was exactly the same presently as it was when the records were first kept. Extrapolating these data globally, the conclusion is that the world has indeed down regulated. However, a careful inventory of U.S. data indicates that, although the country is taking up a half billion tons of carbon dioxide annually, this is almost entirely the result of recovery from past land use. The problem with the land-use sink for carbon is that eventually the sink goes away. In addition, the extent of increasing anthropogenic carbon in the atmosphere over the 21st century is so extensive that the contribution that land use carbon sequestration could make is probably quite limited anyway. As stated previously, it is anticipated that a trillion tons of carbon in the form of carbon dioxide from anthropogenic sources will be added to the atmosphere during the 21st century. Assuming that all of this added atmospheric carbon must be removed, conversion of all agricultural lands and grasslands globally into old-growth forests would remove only 475 billion tons. A second possibility for a carbon sink is the world's oceans. At present, there is already some 37 trillion metric tons of carbon, mostly in the form of bicarbon- ate, dissolved in the oceans of the world. Of the carbon not taken up by terrestrial ecosystems, the oceans will be the eventual repository for about 85 percent of the rest of the carbon emitted to the atmosphere by human activities.2 However, this uptake occurs quite slowly. For example, the oceans are currently taking up only 40 percent (with an uncertainty of +16 percent) of the annual anthropogenic carbon emissions not removed by terrestrial processes. Because of the slow rate of mixing of the world's oceans, it would take many centuries for them to realize most of their uptake capacity, even if anthropogenic emissions were to stop today. The oceans capacity is such that all anthropogenic carbon dioxide can be absorbed. However, the problem faced with the carbon cycle is that this anthro- pogenic carbon dioxide is put into the atmosphere faster than the oceans can ab- sorb it. Extensive modeling on the absorption of carbon dioxide by the oceans has been confirmed through carbon-14 penetration in the oceans from atmospheric testing of nuclear weapons. Using these models, it is possible to predict methods to artificially remove carbon dioxide from the atmosphere, such as injecting it directly into the oceans as a gas or as supercritical carbon dioxide. The results indicate that significant problems result from this approach. First, injecting carbon dioxide into ocean shallows results in escape back into the atmosphere. To over- come this obstacle, carbon dioxide must be injected deep into the ocean abyss, 2P.,Tans, I.~. Sarmiento, and W.H. Schlesinger, 1998, "Changes in Carbon Sources and Sinks: The Outlook for Climate Change and Managing Carbon in the Future," USGCRP Seminar 8, December, Washington, D.C., http://www.usgcrp.gov/usgcrp/seminars/981201DD.html.
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64 ENERGY AND TRANSPORTATION which drives up costs astronomically. In addition, localized injections of massive amounts of carbon dioxide into oceans are likely to cause significant ecological damage, through localized changes in pH and inhibition in the growth and repro- duction of deep-sea animals. Therefore, a number of smaller injections at different points in the oceans must be done, which also dramatically drives up costs. Finally, the long-term ecological effects of such a scenario, such as formation of clathrates that would smother sea bottom organisms, are either unknown or detrimental. Other methods proposed for ocean sequestration may lower costs but present significant obstacles. One scenario envisions fertilization of the oceans with iron. However, studies of this method have indicated that only about 10 percent of the carbon fixed remains in the ocean. Also, the unintended ecological consequences of this method, such as abyssal nitrogen anoxia and risks to fisheries, may far exceed the benefit derived from carbon sequestration. Geological sequestration poses another possibility to fix atmospheric carbon. Presently, the oil and gas industries collectively move hundreds of millions of tons of gases, including carbon dioxide, and re-inject them into fossil-fuel-bear- ing geological formations either as waste gas or to enhance oil recovery as part of their normal operations. The key problem with this method is that these geological reservoirs potentially leak, either through natural fractures or by puncturing from hundreds of thousand of old wells that are sealed with concrete caps that have the potential to leak. Backstopping technologies, such as the formation and burial of carbonate rocks from carbon dioxide, have been proposed to overcome this limitation. The impediment to this method is the cost effectiveness of moving and burying these large rocks. Although leakage of carbon dioxide from these geological repositories is a concern, it is not necessary to completely seal all of these leaks to effectively avoid potential climate changes due to added carbon dioxide in the atmosphere. Mathematical modeling can be performed to balance anthropogenic carbon dioxide generation, geological carbon sequestration, and leakage of carbon dioxide from these reservoirs back into the atmosphere. Results indicate that geo- logical sequestration has the capacity to solve the problem of excess anthropo- genic carbon dioxide in the atmosphere, provided that the leakage rate from these repositories is kept beneath 1 percent per year. There are key challenges for the chemical sciences if sequestration is adopted as a method to reduce atmospheric carbon dioxide. Carbon dioxide capture after generation is generally estimated to represent three-fourths of the total cost of a carbon capture, storage, transport, and sequestration system.3 To make carbon 3http://www.fe.doe.gov/coal_power/sequestration/sequestration_capture.shtml
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COULD CARBON SEQUESTRATION SOLVE THE PROBLEM OF GLOBAL WARMING? 65 sequestration practical, research in the chemical sciences in the following areas will be required: absorption (chemical and physical), adsorption (chemical and physical), low-temperature distillation, and gas separation membranes. In addition, more information is needed about the chemistry of carbon dioxide in brine with mineral surfaces. Chemical tracking of carbon dioxide is required to determine how to plug leaks without eliminating the storage capacity of a reser- voir. Also, understanding how to keep gases (such as hydrogen sulfide from coal) in solution that would cosequester with carbon dioxide (which would also leak from reservoirs but unlike carbon dioxide would cause a substantial localized problem) is also a fundamental problem to be solved by the chemical sciences.4 4It is important to note that any sequestering process requires energy. For sequestering to be a useful process, this energy cannot produce carbon dioxide, or at least should produce much less than is being sequestered.
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