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28
Geoengineering

In this chapter a number of "geoengineering" options are considered. These are options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry. Most of these options have to do with the possibility of compensating for a rise in global temperature, caused by an increase in greenhouse gases, by reflecting or scattering back a fraction of the incoming sunlight. Other geoengineering possibilities include reforesting the United States to increase the storage of carbon in vegetation, stimulating an increase in oceanic biomass as a means of increasing the storage and natural sequestering of carbon in the ocean, decreasing CO2 by direct absorption, and decreasing atmospheric halocarbons by direct destruction. It is important to recognize that we are at present involved in a large project of inadvertent "geoengineering" by altering atmospheric chemistry, and it does not seem inappropriate to inquire if there are countermeasures that might be implemented to address the adverse impacts.

Our current inadvertent project in "geoengineering" involves great uncertainty and great risk. Engineered countermeasures need to be evaluated but should not be implemented without broad understanding of the direct effects and the potential side effects, the ethical issues, and the risks. Some do have the merit of being within the range of current short-term experience, and others could be "turned off" if unintended effects occur.

Most of these ideas have been proposed before, and the relevant references are cited in the text. The panel here provides sketches of possible systems and rough estimates of the costs of implementing them.

The analyses in this chapter should be thought of as explorations of plausibility in the sense of providing preliminary answers to two questions and encouraging scrutiny of a third:



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Page 433 28 Geoengineering In this chapter a number of "geoengineering" options are considered. These are options that would involve large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry. Most of these options have to do with the possibility of compensating for a rise in global temperature, caused by an increase in greenhouse gases, by reflecting or scattering back a fraction of the incoming sunlight. Other geoengineering possibilities include reforesting the United States to increase the storage of carbon in vegetation, stimulating an increase in oceanic biomass as a means of increasing the storage and natural sequestering of carbon in the ocean, decreasing CO2 by direct absorption, and decreasing atmospheric halocarbons by direct destruction. It is important to recognize that we are at present involved in a large project of inadvertent "geoengineering" by altering atmospheric chemistry, and it does not seem inappropriate to inquire if there are countermeasures that might be implemented to address the adverse impacts. Our current inadvertent project in "geoengineering" involves great uncertainty and great risk. Engineered countermeasures need to be evaluated but should not be implemented without broad understanding of the direct effects and the potential side effects, the ethical issues, and the risks. Some do have the merit of being within the range of current short-term experience, and others could be "turned off" if unintended effects occur. Most of these ideas have been proposed before, and the relevant references are cited in the text. The panel here provides sketches of possible systems and rough estimates of the costs of implementing them. The analyses in this chapter should be thought of as explorations of plausibility in the sense of providing preliminary answers to two questions and encouraging scrutiny of a third:

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Page 434 1.  Does it appear feasible that engineered systems could actually mitigate the effects of greenhouse gases? 2.  Does it appear that the proposed systems might be carried out by feasible technical means at reasonable costs? 3.  Do the proposed systems have effects, besides the sought-after effects, that might be adverse, and can these be accepted or dealt with? An exhaustive literature search and analysis has not been completed, but it has been possible to find useful material in the literature and to make first-order estimates that suggest positive answers to these first two questions. This being the case, it seems appropriate to continue consideration of the range of geoengineering possibilities and to pursue answers to question 3 above. In virtually all cases, there are significant missing pieces of scientific understanding. Carrying the examination further would first require more detailed understanding, theoretical modeling, and simulation analyses of the physics, chemistry, and biology in the light of what is known about the geophysical, geochemical, climate, and ecological systems. If these further analyses suggest that the answers to the questions continue to be positive, experiments could then be carried out. These would not be full-scale climate mitigation experiments, but rather experiments intended to answer questions that might still remain after theoretical analysis, e.g., questions concerning optical effects and properties of various kinds of dust or aerosols, lifetimes and cloud stimulation properties of tropospheric sulfate aerosol, and so on. There is also a need for more detailed design, development, and cost analysis of the proposed deployment systems, perhaps including experimentation with specific hardware for deployment. Such work would give much more information with which to decide whether such systems could be deployed at a reasonable cost, and whether they would be likely to work as suggested by the preliminary evaluations included below. If the theoretical analyses, experiments, and development work show that these mitigation ideas continue to have promise, the possibility of actual deployment would raise additional issues. The global climate and geophysical, geochemical, and biological systems under examination are all highly nonlinear systems involving the interaction of many complicated feedback systems. Such systems are likely to exhibit various forms of instability, including dynamic chaos, as well as various unintended side effects. These possibilities must be seriously considered before deployment of any mitigation system, and the risks involved weighed against alternatives to the proposed system. Would attempts to mitigate greenhouse warming using one of these geoengineering systems result in putting a global system into some unintended and undesired state? Effects that have been suggested as possible

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Page 435 results of greenhouse warming itself, and which might result from attempts to mitigate it, include a shift to a glacial state and major shifts in ocean currents. Our current models and understanding of geophysical systems do not allow us to predict such effects. Our understanding and modeling have so far not even permitted us to make a map of the possible states of the system. We might require a different modeling approach even to be able to do so. It can be argued that, in the face of such uncertainty, we should not consider "tinkering" with the only earth we have. However, we are not entirely without understanding of this matter. The principal characteristic of chaos instability, for example, is that the behavior of states with only slightly different initial conditions may be totally different. This is frequently expressed by the statement that "the alighting of a butterfly may change the future of the earth." However, in the sense that we know something of the effects of various kinds of events on parts of the geophysical system, we do know a good deal about this. For example, we know something of the effect of the dust and aerosols resulting from volcanic eruptions on the climate system and on atmospheric chemistry, and we know something of the effect of industrial sulfur emissions on the climate system. It seems reasonable to assume that mitigation systems that put dust or aerosols into the atmosphere at altitudes and in quantities that are within the bounds of the natural experiments or of previous experiments would not produce instabilities or effects that had not been produced before. This expectation could provide one criterion for use of a geoengineering option: the activity must be within the natural variability of the geophysical system. We could use natural variability, or what are effectively previous experiments, as tests of the stability of the geophysical system and as opportunities to search for possible side effects. However, we must also consider that the chemistry of the atmosphere is changing, particularly from the injection of chlorofluorocarbons (CFCs) and from the increased injection of other greenhouse gases, so past chemistry will be an incomplete guide to the future. We can use the past and our understanding of the nature of the physics and chemistry to guide us in looking for new effects as natural events occur: the next significant volcanic eruption, for example, can be used as an opportunity to extend our understanding of the effects of dust, sulfuric acid aerosol, and chemicals produced by volcanic eruptions on stratospheric chemistry and the climate system. The possibility would have to be taken into account that a natural event occurring during a mitigation activity could push the system beyond its normal bounds. For example, a large volcanic eruption occurring while artificial volcanic dust was in place might result in a dust loading beyond that previously experienced. Given some knowledge of the statistics and occurrence of eruptions (but noting their current unpredictability in detail)

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Page 436 and of the lifetime aloft of the dust or aerosol in question, it should be possible to make a reasonably prudent statistical design for such a mitigation system and to compare its risks with other alternatives. In many simple nonlinear systems the phenomenon of hysteresis is observed. In these cases, as some physical variable is changed, the system changes its state in a particular way, but if the same physical variable is then returned to its initial value, the system does not retrace the path; it changes state along a different path. Thus attempting mitigation by decreasing the quantity of greenhouse gases in the atmosphere could, in principle, lead the system into a region of instability even though increasing them had not done so. The problem we face is that, given that the climate system is nonlinear and that we do not understand its state space, all actions can potentially lead to instability, and even a small-scale action is not necessarily less likely to do so than a large-scale action. Because of the possible sensitivity of geophysical systems to chaotic instability, we must proceed with caution in any geoengineering effort. We have to compare the nature and size of proposed actions with what we know about what has already been observed to occur in the system as a result of similar stimuli to it. This gives us a way of testing proposed actions. We can also try to learn the structure of the state space of the geophysical system by theoretical, modeling, and simulation analysis combined with observation of the system and its history, perhaps using small stimulus experiments that we believe to be safe to add to our understanding. While geological history provides evidence of what appear to be major changes in state, there is a great deal of observed variation in the system and in stimuli to the system that do not appear to result in changes of state. Improving our understanding of these matters in this way may enable us to make rational decisions on what risks to take if we desire to use geoengineering or other means of mitigation to counter any greenhouse warming produced by greenhouse gas increases. Particular caution must be exercised because although changing atmospheric chemistry and changing global reflectivity may both have an impact on global mean temperature, the relevant physics for each is very different. The geographic distribution of effects may also be very important. The kinds of steps that may be taken include the following: • Theoretical modeling and simulation analyses of the physics, chemistry, and biology of the relevant geophysical, geochemical, climate, and ecological systems. • Study of the potential for induced instability and chaos. • Small-scale mitigation experiments to determine physical, chemical, and biological properties where these are unknown. • Detailed design, development, and cost analysis of proposed deployment systems.

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Page 437 • Study of related natural events to understand their relevant properties, including the statistics of their occurrence. • Study of possible ecological, geophysical, geochemical, and atmospheric side effects, including considerations of reversibility. Reforestation Reforestation is one possible method of slowing the buildup of CO2 in the atmosphere. While some countries are a source of emissions because of deforestation (as discussed in Chapter 27), most temperate countries (such as the United States) are a net sink of emissions in that new growth at least compensates for trees harvested. This analysis focuses on the opportunities for rural reforestation in the United States. Urban reforestation is discussed in Chapter 21. It should be noted that uncertainties regarding land availability and cost make extrapolation from the few available figures difficult. Reforestation efforts are relevant to the mitigation of greenhouse gas emissions because during photosynthesis green plants take in CO2 and release oxygen. The carbon ''fixed" during photosynthesis in excess of that released during respiration is stored in plant tissue. For perennial species such as trees, the amount of stored carbon can accumulate for decades. Therefore reforestation could potentially take in (or sequester) some of the CO2 the United States generates from energy sources. Recent Trends Forests cover about one-third of the earth's land surface, stretching from evergreen forests in the moist tropics to vast boreal forests in the subarctic. Terrestrial biota and soils are an important part of the carbon cycle. They store 2,280,000 Mt (2,280 petagrams) of carbon compared to the 750,000 Mt (750 petagrams) of carbon in the atmosphere (World Resources Institute, 1990).1 Although it should be noted that the amount of carbon stored in the ocean and lithosphere is much larger than that in either the atmosphere or the land, the time scales over which they equilibrate with the atmosphere are very large. The land area of the United States has lost close to 25 percent of its forest cover since settlement of the North American continent began, and forest cover continues to decline. This is notwithstanding the planting of more and more trees over time (Figure 28.1). The 1989 decrease in tree planting is due to a decline in planting under the Conservation Reserve Program (CRP). In 1989, tree planting increased on National Forest and other federal lands, but decreased on private, state, and nonfederal public lands. A breakdown of total planting and seeding by ownership category in 1989 is given in Table 28.1; private sources planted 85 percent of the trees

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Page 438 image FIGURE 28.1 Historical summary of U.S. forest planting. SOURCE: U.S. Forest Service (1990). TABLE 28.1 Total Planting and Seeding by Ownership Category in FY 1989     Acres Percent of All Planting Federal government       National forests 307,138 10.2   Department of the Interior 52,006 1.7   Other federal agencies 9,257 0.3   TOTAL 368,401 12.2 Nonfederal public       State forests 57,133 1.9   Other state agencies 6,013 0.2   Other public agencies 13,515 0.4   TOTAL 76,661 2.5 Private       Forest industries 1,248,565 41.3   Other industry 22,225 0.8   Nonindustrial owners 1,306,096 43.2   TOTAL 2,576,886 85.3 GRAND TOTAL 3,021,948 100.0

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Page 439 image FIGURE 28.2 Total planting and seeding by region in FY 1989. SOURCE: U.S. Forest Service (1990). in the United States in that year. Figure 28.2 shows that the great majority of planting (76.3 percent) is in the southern United States (U.S. Forest Service, 1990). Storing Carbon in Trees Forests take up carbon fastest during their early years of rapid growth (which may be up to 80 years for some species). As trees age, their growth rates decline and the rate at which they sequester carbon also declines. As a result, stands of young trees, on a net basis, actively increase stores of carbon (per unit area) more rapidly than mature forests, where photosynthesis is more closely balanced by respiration and death. On the other hand, a mature forest generally contains more stored carbon overall than a younger forest does. The ability of a particular type of tree to store carbon depends on a number of factors, including its intrinsic growth rate as well as site and stand attributes. Carbon is stored in stemwood, branches, and roots, and in the soil around the tree. Carbon incorporated in leaves is recycled rapidly, often on an annual basis, and thus is less important from a carbon storage standpoint. Researchers have documented drastic improvements in the present net productivity (carbon uptake) of several species of trees. Heilman and Stettler (1985) managed short rotations of hybrid cottonwoods on fertilized, irrigated plots in western Washington and achieved nearly 14 t of carbon uptake per hectare (ha) per year of total production. A study by Steinbeck and Brown (1976) of intensely managed American sycamores on a 4-year rotation

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Page 440 in Georgia yielded a carbon uptake of 6.5 t C/ha/yr. On the average, however, Marland (1988) estimates that U.S. commercial forests have an uptake of roughly 0.82 t C/ha/yr. If they were fully stocked, the average forest's productivity could increase to 1.35 t C/ha/yr. In sum, both the species of tree and the management practices are important considerations in reforestation policy, as are the kinds of land on which the trees are planted and the climatic zones in which planting occurs. Therefore a reforestation strategy for sequestering carbon might theoretically involve the use of fast-growing species with advanced silviculture techniques on optimal sites. However, there is a trade-off between maximum carbon storage and maximum rate of carbon uptake. For example, in the sycamore experiment mentioned, trees were harvested every 4 years to maintain the high growth rates of young vigorous plants. These data on short-rotation forestry demonstrate that rates of carbon uptake can be dramatically increased by forest management strategies. For net U.S. carbon emissions to be reduced, trees must be either protected from oxidizing to CO2 or used to replace fossil fuel burning. In general, more intensive management requires more energy inputs, and these must be compensated to determine net carbon benefit. The most comprehensive analysis of the potential for sequestering carbon in trees in the United States is that undertaken by Moulton and Richards (1990) of the U.S. Forest Service. This is a detailed analysis of the land available in the United States that could support trees, the carbon uptake that might be expected, and actual costs for each type of land to be managed. According to Moulton and Richards, it is possible to sequester up to 720 Mt C on economically marginal and environmentally sensitive pasture and croplands and nonfederal forestlands. After analyzing the potential carbon uptake and cost per ton in 70 region and land-type classes, Moulton and Richards arrange these in order by cost per ton and assemble a supply curve for carbon sequestering. The analysis concludes that up to 56.4 percent of U.S. CO2 emissions could be sequestered in domestic trees at costs ranging from $5.80 to $47.75/t C. Recognizing that the Moulton and Richards analysis suggests that 56.4 percent of U.S. CO2 emissions could perhaps be offset with a massive commitment to a reforestation program, the Mitigation Panel takes a very conservative approach in estimating the carbon offset that might be envisioned. As discussed in Appendix P, the Mitigation Panel's analysis accepts that the 10 percent objective described by Moulton and Richards is a reasonable initial target and that reforestation of economically marginal or environmentally sensitive pasturelands and croplands and nonfederal forestlands to a total of 28.7 Mha could take place at costs as described in their analysis. Several factors in the Moulton and Richards analysis, however, heavily influence the numeric results and are likely to elicit some discussion as to the magnitude and cost of reforestation. Their analysis has a 40-year time

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Page 441 horizon, so it does not confront the consequences of declining growth rates as trees approach maturity or of the long-term possibilities for tree maintenance or harvest. In addition, land rental rates and the ratio between carbon uptake in marketable timber and total ecosystem carbon uptake are somewhat uncertain. Taking these factors into account, the Mitigation Panel's analysis suggests that 240 Mt CO2 equivalent per year could be sequestered at costs between $3 and $10 per ton of CO2 (average cost is $7.20/t CO2). Demonstration projects could verify the lower costs and higher targets for total sequestration suggested by Moulton and Richards. Obstacles to Implementation There are several constraints on implementing a reforestation policy. First, there are land use commitments. For reforestation to be pursued on a large scale, planting would have to take place on marginal agricultural lands. This represents a long-term commitment to nonagricultural uses. Second, there may be resource constraints (e.g., water). Great care and understanding would be required to select tree species, species mixes, and management strategies to maximize the potential of sites with widely different available resources. Policy Options Public policy decisions to increase carbon storage through reforestation involve such silviculture issues as replanting, selection of species to be planted, and land management practices such as fertilization. To implement these reforestation options, however, someone must pay for the reforestation itself and for the cost of maintaining land in forest cover. Landowners face a variety of alternative opportunities and liabilities. Policies to increase and maintain forests to store carbon must therefore address questions of economics as well as silviculture. The cost of sequestration, considered apart from the value to be recaptured by sales of timber, ranges upward from zero. The economic cost—which can, of course, be negative if the investment in sequestration is less than the return from the sale of harvested products and other benefits—is determined both by the value of forest products and by the value of alternative land uses. Because forests have a biological time scale on the order of decades, significant uncertainty is the norm in economic analyses. Public policies can affect management choices by changing, for example, the taxes levied on timber harvest, the regulations that govern forest practices, or real estate taxes. To ensure long-term sequestering of wood on private lands, governments may need to purchase title or limited property rights.

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Page 442 Other Benefits and Costs Reforestation can have many other positive benefits, including enhanced biodiversity, wildlife, air and water quality, aesthetics, forest products, and recreational opportunities. Reforestation can also raise environmental concerns, and there is some apprehension about the implications if planting were to occur as broad expanses of monocultural plantations. Research and Development A recent National Research Council (1990) report entitled Forestry Research: A Mandate for Change provides a number of research recommendations relative to societal concerns regarding the relationship of forests and climate, biological diversity loss, forest product demand, "pristine" forest area demand, sustainable wood production in conjunction with environmental protection, and maintenance of forest health. The NRC Forestry Research Committee (1990) recommends • improving understanding of the basic biology and ecology of forests, • developing information to sustain productivity of forests as well as to protect their inherent biological diversity, and • understanding the economic and policymaking processes that affect the fate of forests. Conclusions Reforestation has the potential to offset a large amount of CO2 emissions but at a cost that increases as the amount of offset increases. This analysis recognizes the large land resource required and adopts a conservative approach with respect to the U.S. Forest Service analysis of the amount of carbon that might be sequestered. It also recognizes that forests will mature and that reforestation is thus an interim approach to the long-term concerns of greenhouse warming. In addition, if a forest is harvested, the only true CO2 offset is the amount of carbon stored in soil, roots, and as lumber or other long-lived products. Furthermore, there is some apprehension about the implications for biodiversity if planting were to occur as broad expanses of monocultural plantations. Overall, however, reforestation seems to provide a method of storing carbon with little adverse societal impact and a number of benefits. Increasing Ocean Absorption of Carbon Dioxide The Approach The oceans already play an enormous role in establishing planetary climate, both through the transport of heat and supply of water vapor and

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Page 443 through the absorption of a large fraction of fossil fuel CO2. Estimates of the net ocean sink for CO2 range from the traditionally accepted value of some 40 percent of fossil fuel CO2 emissions (through reaction of CO2 gas with carbonate ion over the entire ocean surface, and based upon models derived from Oeschger et al. (1975)), which today gives close to 3 Gt C/yr, to the much lower value of 0.6 Gt C/yr recently reported by Tans et al. (1990). No realistic model of earth's climate can escape simulation of the oceans in some form. While the oceanic role in moderating the present-day fossil fuel increase depends almost totally on the rate of mixing and the alkalinity, the potential future role of ocean biota cannot be neglected. The potential amount of total carbon that could be utilized by oceanic photosynthesis has been estimated to be 35 Gt/yr. However, this figure represents the gross fixation of carbon in the ocean; the net effect on the atmosphere will depend on the return flow from decomposition and will eventually reach steady state. Ice core records (Neftel et al., 1982; Barnola et al., 1987) show that in the past, the atmospheric CO2 level has fluctuated independently of the activities of man, with ice age CO2 concentrations some 30 percent lower than the most recent preindustrial value. A key question is thus, Can this state be achieved today? In 1984, three independent research groups published hypotheses on this phenomenon (Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984). Each reached the conclusion that the key lay in the surface nutrient concentrations in polar ocean regions. In areas such as the far North Pacific and the antarctic circumpolar ocean, high concentrations of nitrate and phosphate (the key ingredients for plant growth) are unused. The problem did not seem to be insufficient light, or bitter cold, but some other variables not yet recognized. The 1984 models showed that if these nutrients were assimilated, the conversion of CO2 to organic carbon could readily account for the ice age signal. These nutrients can be regarded as an important unused chemical capacity of the ocean, one of a scale to significantly affect the global carbon balance. A radical solution to this ice age CO2 puzzle has been proposed by Martin and co-workers (Martin and Fitzwater, 1988; Martin and Gordon, 1988; Martin, 1990; Martin et al., 1990). These scientists achieved the first reliable measurements of "dissolved" iron, at the nanomolar level, in ocean waters through stringent avoidance of the all-pervasive contamination problem. They further showed that addition of trace amounts of iron to natural populations of phytoplankton stimulated photosynthesis, and they hypothesized that iron limited the phytoplankton growth in these areas. Thus trace inputs from atmospheric dust events could trigger blooms of the plankton and ultimately lower atmospheric CO2. Finally, the ice core record shows that glacial times, with dry and dusty continents, are characterized by strong dust input to oceans. The route to contemporary utilization of this unused oceanic potential is

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Page 454 (U.S. Bureau of the Census, 1988). This gives a cost of slightly more than $1 per ton-mile for freight. If a dust distribution mission requires the equivalent of a 500-mile flight (about 1.5 hours), the delivery cost for dust is $500/t, and ignoring the difference between English and metric tons, a cost of $0.50/kg of dust. If 1010 kg must be delivered each 83 days, (provided dust falls out at the same rate as soot), 5 times more than the 1987 total ton-miles will be required. The question of whether dedicated aircraft could fly longer distances at the same effective rate should be investigated. However, if the requirement is to mitigate the 1989 U.S. emissions of CO2, 500 times less dust is needed, the cost is about $10 million per year, and implementation would require about 1 percent of the ton-miles flown in 1987. If 10 percent of the ton-miles flown in 1987 were used, the system could mitigate 80 Gt CO2. These costs should probably be increased by the cost of delivered dust (say, $0.50/kg) and of delivery systems in the aircraft, but better-than-average freight rates could probably be arranged. Thus the costs appear to be about $0.0025/t CO2. Clearly, the amount of dust required could be greater by a factor of 10, and the cost would be $0.025/t CO2. This provides a cost estimate in the range of $0.003 to $0.03/t CO2. Multiple Balloon Screen A screen can be created by putting a vast number of aluminized, hydrogen-filled balloons at a high enough altitude that they do not interfere with air traffic. They would provide a reflection screen. The properties of such a system are examined in Appendix Q. The multiple balloon parasol system requiring billions of 1- to 6-m-diameter balloons would appear to cost about 20 times as much as distributing dust in the stratosphere. The large number of balloons, and the trash problem posed by their fall, make the system somewhat unattractive. Changing Cloud Abundance A more detailed discussion of the possibility of changing cloud abundance appears as Appendix Q. The Approach Independent studies estimated that an approximately 4 percent increase in the coverage of marine stratocumulus clouds would be sufficient to offset CO2 doubling (Reck, 1978; Randall et al., 1984). Albrecht (1989) suggests that the average low-cloud reflectivity could be increased if the abundance of cloud condensation nuclei (CCN) increased due to emissions of SO2. It

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Page 455 is proposed that CCN emissions should be released over the oceans, that the release should produce an increase in the stratocumulus cloud albedo only, and that the clouds should remain at the same latitudes over the ocean where the surface albedo is relatively constant and small. Albrecht (1989) estimates that a roughly 30 percent increase in CCN would be necessary to increase the fractional cloudiness or albedo of marine stratocumulus clouds by 4 percent. Albrecht's idealized stratocumulus cloud, which he argues is typical, has a thickness of 375 m, a drizzle rate of 1 mm per day, and a mean droplet radius of 100 mm, and he assumes that each droplet is formed by the coalescence of 1000 smaller droplets. The rate at which the CCN are depleted by his model is 1000/cm3 per day. Consequently, about 300/cm3 per day (30 percent of 1000) of additional CCN would have to be discharged per day at the base of the cloud to maintain a 4 percent increase in cloudiness. This assumes that the perturbed atmosphere would also remain sufficiently close to saturation in the vicinity of the CCN that additional cloud cover would be formed every time the number of CCN increased. Mass Estimates of Cloud Condensation Nuclei With Albrecht's assumption in mind that cloudiness in a typical ocean region is limited by the small number of CCN, we now extrapolate to the entire globe. On the average, 31.2 percent of the globe is covered by marine stratiform clouds (Charlson et al., 1987). If no high-level clouds are present, the number n of CCN that need to be added per day is 1.8 × 1025 CCN/day. The mass of a CCN is equal to 4/3pr3 × density, and it is assumed that the mean radius r is equal to 0.07 × 10-4 cm (Charlson et al., 1987). Because the density of sulfuric acid (H2SO4) is 1.841 g/cm3, the CCN mass is 2.7 × 10-15 g. The total weight of H2SO4 to be added per day is 31 × 103 t per day SO2 if all SO2 is converted to H2SO4 CCN. To put this number in perspective, a medium-sized coal-fired U.S. power plant emits about this much SO2 in a year. Consequently, the equivalent emissions of 365 U.S. coal-burning power plants, distributed homogeneously, would be needed to produce sufficient CCN. To estimate the value of the sulfur directly, the total weight of SO2 to be added per day would equal 32 × 103 t, or about 16 × 103 t of sulfur (S) per day, which is equivalent to about 6 × 106 t S/yr. If the average market price of sulfur delivered at the mine or plant is taken as $96.60/t for the years 1983 to 1987, the cost would be about $580 million per year. Equating this yearly cost to the 300 parts per million by volume (ppmv) of CO2 necessary for full compensation gives $580 × 106/yr/(3890 × 106 t C/ppmv CO2 × 300 ppmv CO2), or about a fraction of 1 cent/t CO2. To obtain an equivalence to conserved carbon, known emissions of carbon in 1978, 1979, and 1980

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Page 456 have been compared with the total measured increase of CO2 to obtain the equivalence: 3890 × 106 t C @ 1 ppmv CO2. A 4 percent increase in cloudiness was then equated to a 300-ppmv CO2 decrease, which translates into a reduction of 1200 Gt C or 4400 Gt CO2. Cost Estimates The primary cost of this process involves the mechanism for distributing SO2 in the atmosphere at the correct location. Assume a fleet of ships, each carrying sulfur and a suitable incinerator. The ships are dedicated to roaming the subtropical Pacific and Atlantic oceans far upwind of land while they burn sulphur. They are vectored on paths to cloud-covered areas by a control center that uses weather satellite data to plan the campaign. In addition to choosing areas that contain clouds, it would be important to distribute the ships and their burning pattern so as not to create major regional changes, or the kind of change with a time or space pattern likely to force unwanted wave patterns. These restrictions (which perhaps cannot now be defined) could present a difficult problem for such a system to solve. From the above, 16 × 103 t per day or 6 × 106 t/yr would be needed. If we allocate 102 t per ship per day, and a ship stays out 300 days each year, roughly 200 ships of 10,000-t capacity (one reprovisioning stop every 100 days) are required. At a cost of $100 million per ship (surely generous), the capital cost of the fleet is $20 billion. Amortized over 20 years, the annual capital cost is $1 billion. Sulfur will cost another $0.6 billion per year, and $2 million per ship per year for operating costs (this is $10,000 per operating day), giving a total cost of $2 billion per year. Over 40 years (until 2030), this gives a cost of $80 billion, or approximately $100 billion. This continuously mitigates 103 Gt, for a cost of $0.10/t C/yr, or $0.025/t CO2/yr. This provides a cost estimate in the range of $0.03 to $1/t CO2. Of course, this continues to be a yearly cost of $2 billion per year. The SO2 could also be emitted from power plants. These plants could be built out in the ocean near the equator (the Pacific gives more room than the Atlantic) and could furnish power for nearby locations (e.g., South America). Transmission or use of the power in the form of refined materials, or possibly by the use of superconducting power transmission systems, could be considered. It would likely require eight large power plants using "spiked" coal (with 4 times the normal amount of sulfur), at a cost of $2 to $2.5 billion per plant. Most of the cost might be borne by those buying the power; so imagining a cost of, at most, 10 percent per year (the interest on the investment), total cost would be $2 billion per year (with the above conversion, $2 × 109/3890 × 106 × 300 image $0.0005/t CO2).

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Page 457 Possible Acid Deposition One must consider whether the injection of this much additional SO2 into the atmosphere would cause an acid deposition problem. It must be kept in mind that the principal component of naturally occurring CCN is sulfate from marine algae. Schwartz (1988) quotes estimates of 16 to 40 × 1012 g/yr or perhaps about 25 × 109 kg/yr emitted from this source. The addition of about 6 × 109 kg/yr, one-quarter of the total natural amount, is being considered, although locally much more would be added to the amount naturally present. The oceans have an enormous buffering capacity (Stumm and Morgan, 1970), so that the additional rainout of sulfate (especially after dilution through cloud dispersal and droplet coalescence) seems unlikely to have any effect, even locally, although there is clear disagreement on this point. The principal concern would be to avoid additional sulfate deposition over land. With a 30 percent rainout per day, this could be ensured to a 90 percent level by operating about a week upwind of land. Such a constraint would have to be added to the others stated above. Another possible way of dealing with the problem of acid rain would be to introduce sulfate in the form of ammonium sulfate or bisulfate, both of which are neutral salts. This would avoid the acid question from the start. These salts are frequently used as fertilizers and, in the dilutions to be seen here, would have a mild fertilizing effect locally. These salts can be made by reacting ammonia with sulfuric acid. The price of ammonia is about $100/t, so the cost of the CCN might double, and there would be an additional cost for equipment to run the reaction at sea. These additional costs might increase the total by as much as 50 percent, to $0.15/t C mitigated per year or $0.04/t CO2. It may also be sensible to consider using ships that pump a seawater aerosol into the air above the ocean, thus increasing the density of sea salt aerosol crystals, which can act as CCN (Latham and Smith, 1990). Atmospheric Chlorofluorocarbon Removal Another option for mitigating greenhouse warming could be to remove chlorofluorocarbons (CFCs) from their principal reservoir, the lower atmosphere or troposhere. The expected tropospheric residence time for CFCs exceeds 65 years (cf. Table 19.2); evidently these highly inert gases disappear only by very slow loss to the stratosphere, where ultraviolet rays from the sun cause molecular decomposition. A reasonable query is whether this natural process of CFC depletion can be significantly enhanced by large-scale technical means. It has been suggested that extremely powerful lasers might be used to break up tropospheric CFCs (Stix, 1989). Vast arrays of pulsed lasers at

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Page 458 mountain altitudes would launch intense infrared beams into the atmosphere. The laser beams would then selectively destroy chlorofluorocarbon molecules in the atmosphere through the process of multiphoton dissociation. Due to the low atmospheric concentration of the CFCs (less than one part per billion by volume), any process to remove them must be highly selective. That is, the process cannot afford to waste energy in reactions involving any of the far more abundant non-CFC molecules in the atmosphere. The suggested laser scheme then depends first upon finding bands of strong laser-light absorption by CFC molecules. Second, within these bands, one must find ''spectral windows" where absorption of the laser light by non-CFC molecules in the atmosphere is virtually absent. Computer calculations making use of an extensive atmospheric-gas infrared cross-section data base suggest that 90 percent transmission over 50-km paths would be possible through dry atmospheres. Nevertheless, a large number of questions remain unexplored, among them laser and optical technology, electro-optical conversion efficiency, anomalous or unexpected laser-light absorption channels including excited-state processes and stimulated rotational Raman scattering, infrared bandpass mirrors, adequate laser selectivity, pulse shaping benefits, wind velocity and atmospheric humidity patterns, site availabilities, and safety and ecology. Even making very optimistic assumptions about the resolution of these and other questions, the expense associated with the installation and operation of the elaborate and extensive laser facilities would be prohibitive: to remove 10 percent of the atmospheric CFCs per year, the electric power bill alone is estimated to exceed $10 billion. Nevertheless, if technological breakthroughs were to introduce a factor of 10 to 20 improvement in overall efficiency, the cost of such processing of the atmosphere, although very large indeed, might be worth evaluating. In conclusion, the panel does not believe that the use of lasers to remove CFCs from the atmosphere is currently feasible. Conclusions Several of the geoengineering possibilities discussed in this chapter, including atmospheric CFC removal, space mirrors, and the multiple balloon stratospheric screen, appear, with current technology or that expected to be available soon, to be either impractical, too cumbersome to manage, or too expensive. These ideas might merit some further study to be certain of this conclusion but do not now seem worth great effort. They should be kept in mind, however, because technological changes may make them more attractive. Reforestation is a low-cost, ecologically attractive option that could be adopted rapidly as an expanded program. It is, however, limited in its low-cost

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Page 459 form by the easy availability of appropriate land. Therefore the panel hesitates to look beyond its initial potential mitigation of 240 Mt CO2/yr. In addition, a number of years would be required to build reforestation to its full mitigation potential. Stimulation of ocean biomass with iron may be feasible and would be a relatively low-cost option. Its application appears to be limited at most to the mitigation of about 7 Gt CO2 equivalent per year (about 1.5 times U.S. annual CO2 emissions). The biological, ecological, and ocean chemical and physical dynamics of this possibility are not well understood and should be investigated further, both theoretically and experimentally. There continue to be questions as to whether iron is the limiting nutrient. Furthermore, the circulation dynamics of the antarctic ocean might severely limit the effect. If feasible, the mitigation potential of the possibility—storage of CO2 in a standing crop and as dissolved CO2 with slow sequestering of carbon to the ocean bottom—could probably be established over several years. If applications of iron were stopped, the standing crop would be expected to die within days or weeks, thus ending the mitigation effect. Cloud stimulation by provision of cloud condensation nuclei appears to be a feasible and low-cost option capable of being used to mitigate any quantity of CO2 equivalent per year. Details of the cloud physics, verification of the amount of CCN to be added for a particular degree of mitigation, and the possible acid rain or other effects of adding CCN over the oceans need to be investigated before such system is put to use. Once a decision has been made, the system could be mobilized and begin to operate in a year or so, and mitigation effects would be immediate. If the system were stopped, the mitigation effect would presumably cease very rapidly, within days or weeks, as extra CCN were removed by rain and drizzle. Several schemes depend on the effect of additional dust (or possibly soot) in the stratosphere or very low stratosphere screening out sunlight. Such dust might be delivered to the stratosphere by various means, including being fired with large rifles or rockets or being lifted by hydrogen or hot-air balloons. These possibilities appear feasible, economical, and capable of mitigating the effect of as much CO2 equivalent per year as we care to pay for. (Lifting dust, or soot, to the tropopause or the low stratosphere with aircraft may be limited, at low cost, to the mitigation of 8 to 80 Gt CO2 equivalent per year.) Such systems could probably be put into full effect within a year or two of a decision to do so, and mitigation effects would begin immediately. Because dust falls out naturally, if the delivery of dust were stopped, mitigation effects would cease within about 6 months for dust (or soot) delivered to the tropopause and within a couple of years for dust delivered to the midstratosphere. Such dust would have a visible effect, particularly on sunsets and sunrises, and would heat the stratosphere at the altitude of the dust. The

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Page 460 heating would have an effect on the chemistry of the stratospheric ozone layer, and this possibility must be considered before major use of such a mitigation system. The amount of dust to be added is within the range of that added from time to time by volcanic eruption, so the effects on climate would not be expected to go beyond those experienced naturally. However, either the natural or the artificial effects on the chemistry might be very serious under conditions of increased CFC chlorine in the stratosphere, and the result of having these effects continuously must be considered, so the option might not be usable. Better specification of dust characteristics and size for best effect and better data on the fallout rate of dust from various altitudes as well as on chlorine chemistry are needed. It wil be important to observe the effects on stratospheric chemistry of any volcanic eruptions that occur, with special attention to separating the effects of dust, aerosol, and hydrochloric acid. Of these systems to alter the planetary albedo, the increase of low-level marine clouds by increasing CCN and the delivery of dust to the stratosphere by using large rifles seem the most promising. The rifle system appears to be inexpensive, to be relatively easily managed, and to require few launch sites. However, the possible effect of the additional stratospheric dust on ozone chemistry may be a serious problem, and the noise of the rifles would have to be managed. Balloons also appear to be a good possibility, but the return of the balloons to ground level would require management. Sunlight screening systems would not have to be put into practice until shortly before they were needed for mitigation, although research to understand their effects, as well as design and engineering work, should be done now so that it will be known whether these technologies are available if wanted. Perhaps one of the surprises of this analysis is the relatively low costs at which some of the geoengineering options might be implemented. If, however, further analyses support the preliminary conclusions, it will bear further inquiry to decide if they can produce the targeted responses without unacceptable additional effects. The level at which we are currently able to evaluate the cost-effectiveness of engineering the global mean radiation balance leaves great uncertainty in both technical feasibility and environmental consequences. This analysis does suggest that further inquiry is appropriate. Notes 1. Throughout this report, tons (t) are metric; 1 Mt = 1 megaton = 1 million tons; 1 Gt = 1 gigaton = 1 billion tons. 2. The ships can distribute material across the lane by towing hoses spread away from the ship with paravanes, a well-known minesweeping technology.

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Page 461 3. 4p(6.6)2 × 106 km2 ×(103 m/km)2 = 547 × 1012 m2 @ 5 × 1014 m2. The screening only requires covering the illuminated disk, or pr2, but in many of the cases treated it will not be possible to maneuver the screening material so as to remain only above the sunlit side of the disk, therefore 4pr 2 is used. 4.The correct parasol coverage area may be 1.4 percent because the Ramanathan computation is for 1 percent increase in the 30 percent albedo, but this change will have slight effect on the estimates. See also Penner et al. (1984), who estimate a dust requirement of 1.168 × 1010 kg. 5.The current space transportation system costs about $5200/kg; $130 million per launch with a capacity of 55,000 pounds to 160 nautical miles at 28.5 degrees. A Delta rocket costs $45 million per launch for 11,000 pounds to 100 nautical miles, or $9000/kg. Even at $100/kg, the cost of the material would be only 2 percent of the cost of putting the material into orbit. 6. If the correct equivalence is 1 percent to 1200 Gt, for example, all material quantities, etc., are smaller by a factor of 10/12; if the equivalence is 800 Gt, the numbers are larger by a factor of 10/8, etc. For costs per ton of CO2, all costs per ton of carbon should be divided by 44/12 @ 4. 7. In this connection, see also Early (1989). Estimates range between $1/t CO2/yr for the lower launch cost and long-lived mirrors and $10/t CO2/yr for the higher cost and annual replenishment. 8. From Toon and Pollack (1976), one can make a crude estimate of the mass of lower stratospheric dust by noting that they give the density at 20 km as about 1 mg/m3. The "all sizes" curve in their Figure 9 suggests a reasonably constant concentration in the 8 km from 12 to 20 km. So the mass can be taken to be roughly 5 × 1014 m2 × 8 × 103 m × 10-6 g/m3 = 40 × 1011 g = 4 × 1012 g = 4 × 109 kg, or roughly half the amount to be injected to form the screen. 9. It is interesting that this is the same mass as that computed above for the space mirror, given an assumed density of 2 g/cm3 for dust instead of the 1 g/cm3 used previously (clay has a density of 1.8 to 2.6, alumina of 4, basaltic lava of 2.8 to 3.0). 10. Another suggestion is to shape the dust into highly conductive needles about 0.1 m in radius by 0.5 m long, the scattering at an optical wavelength of 1 m would be dipole scattering with an effective scattering cross section 100 times greater than for spheres, thus requiring 100 times less material. Mack and Reiffen (1964) computed these effects in connection with the West Ford project. The maximum cross section expected for a perfectly conducting half-wave resonant dipole is 0.86l. In the case of a dipole such as that specified above, which has an area of 1/2 × 1/5 l a scattering cross section enhancement of 0.86 × 10 = 8.6 is obtained. This enhancement would be decreased by averaging over all angles of the dipole to the incoming radiation. Mack and Reiffen compute this effect to be about 0.1 for backscattering for several polarizations of the incoming light. In addition, highly conducting dipoles would have Q values too large to cover the necessary optical bandwidth effectively. It appears that assuming Mie scattering of dust with a size spectrum optimized to the scattering of the visible part of the solar spectrum, roughly comparable to the estimate of Ramaswamy and Kiehl (1985), is fairly efficient. 11. Material from staff at Naval Surface Weapons Center, Dahlgren, Virginia, was furnished to John I. Connally, Jr., vice president of Scientific Applications

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