Engineers must appropriately scale solutions to problems. Since changes in climate are global in scale, the problems associated with these changes must be addressed at very large scales. Engineers also must select from approaches to a problem, ranging from prevention to remedies. To date, proposals regarding climate change have been preventive, that is, to reduce emissions of greenhouse gases. Remedies may also be employed. These large-scale remedies are the domain of geoengineering.
Global-scale interventions have yet to be taken to address climate change. However, intentional large-scale interventions are being considered. These are known as geoengineering. Currently, removal of greenhouse gases and management of solar radiation are the two prominent geoengineering solutions being considered (see table). Most climate-change models indicate that the buildup of carbon dioxide (CO2) is the principal driver for warming. Removal of CO2 includes methods for extracting the gas from the atmosphere and storing or sequestering it. Solar radiation management does not attempt to address the underlying causes of climate change; rather, it attempts to counteract increases in greenhouse gases by blocking solar radiation or increasing the reflectivity of clouds or the Earth's surface.
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Solar radiation management |
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Spraying SO2 into the stratosphere to enhance cloud albedo |
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Spraying engineering nanoparticles into the stratosphere to enhance cloud albedo |
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Placing reflective mirrors, discs, or particles in Earth orbit |
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Painting roofs and other structures with reflective material |
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Placing solar reflectors in the desert |
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Carbon dioxide removal |
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Using physical and chemical processes to remove and store CO2 |
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Enhancing CO2 removal and storage by terrestrial plants |
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Burial of biomass |
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Reforestation and afforestation (creating new forests) |
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Fertilizing the oceans to stimulate CO2removal by plankton |
Source: D. B. Resnik and D. A. Vallero, Geoengineering: An idea whose time has come?, J. Earth. Sci. Climate Change, 2011, S1.
Sulfate particles
Scientists make hypotheses based on their observations of natural phenomena. For example, the 1991 Mount Pinatubo eruption emitted 10 million metric tons of sulfur dioxide (SO2) into the stratosphere, which was converted to sulfate particles. Sulfate particles in the atmosphere increase cloud albedo, which is the reflectivity of solar radiation into space. Noting this, Nobel Prize–winning geochemist Paul Crutzen hypothesized that spraying sulfur dioxide into the stratosphere would increase cloud albedo and could result in mean cooling of the Earth by 0.5°C. According to Crutzen, sulfate particles will last one or two years longer if SO2 is sprayed into the stratosphere rather than the troposphere, and have a greater impact on cloud albedo. To generate this amount of cooling, about 2 million metric tons of SO2 would need to be sprayed annually. This represents 3.6% of the 55 million metric tons of SO2 emitted into the atmosphere each year from the burning of fossil fuels.
From a thermodynamic and climatic perspective, this type of geoengineering appears to be achievable. However, the atmosphere is quite complex. Changing one variable, SO2 concentration, will affect other variables. For example, spraying SO2 into the stratosphere may adversely affect human health and the environment if it interferes with the ozone layer or other chemical balances. In the troposphere, SO2 is converted into sulfuric acid (H2SO4), producing acidic precipitation that damages ecosystems and threatens sensitive plant and animal species.
Also, inhalation of SO2 contributes to respiratory problems, such as airway constriction and asthma exacerbation. As such, most nations regulate the emissions of SO2. Short-term exposure is associated with increased visits to emergency departments and hospitalization for respiratory problems, especially among young children and asthmatics. So, it may be difficult to justify increasing the global mean concentration of SO2 while decreasing local concentrations.
Other problems can result from inaction. That is, solving only the cooling problem does nothing about the increasing concentrations of atmospheric CO2, which can be associated with other problems. For example, when CO2 dissolves in seawater, it changes the ionic strength and other chemical properties of the water, which increases the net acidity of oceans and other surface waters. The oceans have increased in acidity by 30% since 1750, due to increases in anthropogenic carbon dioxide in the atmosphere. This trend may decrease the availability of calcium carbonate, threatening species that form shells from this compound, such as mollusks, corals, and some types of plankton. A reduction in these species could have wide-ranging effects on other marine species and ecosystems, because many organisms feed on mollusks or plankton or depend on coral reefs for shelter.
Increasing cloud albedo may also lead to environmental problems. Even though the global temperatures may be stabilized, the reflectivity increase could affect precipitation patterns, tropical storm activity, temperature distribution, and wind. Also, much of the radiation that is reflected by sulfate particles strikes the Earth as diffuse light. This increases the whiteness of the daytime sky and may reduce the efficiency of plant photosynthesis and solar power.
Further, the complexity of the atmosphere means that the expected results will be highly uncertain, whatever actions are taken. Thus, spraying too much SO2 into the stratosphere could lead to excessive cooling, droughts, floods, or other meteorological events.
Nanoparticles
Photophoretic forces occur when there is a temperature differential between an aerosol and the surrounding gas (see illustration). This process can enhance albedo. One recently proposed approach is to spray disc-shaped engineered nanoparticles composed of layers of aluminum oxide (Al2O3), metallic aluminum, and barium titanate (BaTiO3) into the stratosphere. The aluminum layer provides high solar-band reflectivity with high transparency to outgoing thermal infrared radiation. This produces large mass-specific cooling. The Al2O3 layer reduces the rate of oxidation of the aluminum surface. The BaTiO3 layers thickness is determined by the electrostatic torque from the atmospheric electric field so as to orient the disc to optimize levitation, overcoming gravity. That is, the disc is analogous to a colloidal suspension within a mixture of stratospheric gases. Nanoparticles' low mass and large relative diameters could take advantage of photophoretic and electromagnetic forces to levitate above the stratosphere.
The relative low reactivity and resistance to oxidation substantially increase the time that such nanoparticles may remain suspended in the atmosphere, compared to the sulfate particles generated from spraying SO2. Their specific reflective properties could also be controlled through engineering and design, so that they would produce correct amount of diffuse light. Also, because the nanoparticles would be above the stratosphere, they would be less likely to interfere with ozone chemistry and, unlike sulfate, would not produce acid rain.
The efficacy of nanoparticles in reducing atmospheric temperatures is much less certain than that of SO2. The eruption of Mt. Pinatubo indicates the cooling potential of SO2, but no analogous process for nanoparticles has been observed in the troposphere.
Stabilizing global temperatures at higher CO2 levels would not address the problem of ocean acidification, and it might affect precipitation patterns, temperature distribution, tropical storms, and winds. The risks may be even more uncertain than those for SO2.
Little is known about the direct and indirect risks from nanoparticles. Indeed, there could be significant environmental and public health risks of spraying nanoparticles into the stratosphere, which are not well understood at this point.
Weighing benefits and risks
Ideally, any decision to implement a geoengineering proposal should be based on a thorough understanding of the benefits and risks. Proposals should only be initiated when there is sufficient evidence that the benefits outweigh the risks, and serious harms can be prevented or avoided. However, we currently lack a thorough understanding of the benefits and risks of most geoengineering approaches, because of their scale and complexity. Such large-scale engineering efforts have no direct precedents and may require a high degree of international cooperation. Large uncertainties about their potential success and potential effects on human health and the environment may keep such large projects from occurring, because people may not want to take risks that are difficult to predict or manage.
Because the benefits and risks of most geoengineering proposals are uncertain at this point, a precautionary approach is warranted. Smaller-scale, lower-risk projects should be implemented before larger and riskier ones.
Arguably, there is less controversy associated with greenhouse gas removal than with enhancing albedo, due to a better understanding of the benefits and risks of greenhouse gas removal. Indeed, carbon sequestration projects are underway, although these are also controversial as their scale increases, due to questions about the efficiency of storing CO2 in geological formations and potential escape of gases from deep ocean storage, as well as concerns about costs.
Traditional cost–benefit analyses have not been conducted at the global scale. Even for relatively small-scale projects, these analyses often cannot quantify social scientific factors, such as political and economic variables. Thus, even though reducing greenhouse gas emissions is preferable to either removing them or addressing the climatic changes brought about by their increased concentrations in the troposphere, the benefits of prevention may well be underestimated. Notably, three of the world's largest greenhouse gas emitters (China, India, and the United States) did not ratify the Kyoto Protocol on climate change, principally because of concerns about its effect on their economies. Although negotiations continue on a new climate-change treaty, it is not known whether the international community will be able to reach an agreement to reduce greenhouse gas emissions. Barring a significant reduction in worldwide greenhouse gas emissions, geoengineering proposals will likely continue to receive serious consideration.
[Acknowledgment: This research is the work product of an employee or group of employees of the National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH), however, the statements, opinions or conclusions contained therein do not necessarily represent the statements, opinions or conclusions of NIEHS, NIH or the United States government.]
See also: Acid rain; Aerosol; Albedo; Atmosphere; Atmospheric chemistry; Global climate change; Greenhouse effect; Nanoparticles; Stratosphere
