Stratospheric Aerosol Injection

The most frequently discussed geoengineering option for mitigating climate change is the injection of aerosols (solid or liquid particles) into the stratosphere, to mimic the cooling effects of volcanic eruptions. The majority of effort by those interested in geoengineering, to date, has been applied to understanding whether purposeful injection of sulfate aerosol would be a viable strategy to control Earth's surface temperature. This idea was originally introduced by Budyko [32] and was recently revitalized in an editorial by Nobel Laureate Paul Crutzen. Crutzen [16] noted that tropospheric particles will decrease in concentration in response to public health measures, a fact which has been supported by recent trends in sulfur emissions [33]. As anthropogenic particulate pollution has an overall cooling effect [1] , the lowering of particle concentrations while greenhouse gas concentrations continue to increase would shift the energy balance towards accelerated warming. In order to counteract both this effect and that of poor progress regarding greenhouse gas emission reductions, Crutzen [16] advocates that stratospheric sulfate aerosol injection be given serious consideration and further research.

Sulfate aerosol has been the most well-studied and popular choice for stratospheric aerosol injection proposals, due to its ease of production (in fact, the coal and oil power industry invests in flue gas desulfurization to be rid of it) and the ability to observe natural volcanic events which demonstrate the results of large-scale sulfate aerosol injection. Other aerosol types have also been proposed, including dust [22], soot [34], and the development of novel aerosols optimized to scatter solar radiation [17]. As the sulfate option has been more thoroughly analyzed, in comparison to other aerosol types, the following discussion will be confined to sulfate injection.

Mitigation Strategy: Inject sulfate aerosol into the stratosphere, with continued injections on an annual or biennial basis to sustain climate forcing.

Feasibility: Using past volcanic eruption events as a guide, it has been estimated that annual injections of 5.3 Tg sulfur would meet the goal of 1.8% reduction in incident radiation and compensate for the temperature effects of doubling CO2 levels [16]. Rasch et al. [35] calculated that less sulfur may be needed annually (1.5-3 Tg) if the particle size were optimized. Using the cost metrics developed by [22] and extending the implementation out to 50 years, the deployment of this option was estimated to range $2-7 trillion (1992 USD). It was estimated that the amount of sulfur needed is likely attainable, with 5.3 Tg sulfur equal to less than 5% of inadvertent global sulfur emissions in the year 2000 [33]. Several mechanisms for lofting sulfate aerosols into the stratosphere have been proposed, including dispersal by aircraft, ground-based artillery, or balloons. The optimal generation of sulfate aerosol and mechanisms of dispersal are areas of needed research and development.

Co-benefits and undesirable consequences: The only known co-benefit of aerosol addition to the stratosphere is aesthetic - sunrises and sunsets would likely be intensely colorful, as often observed after volcanic eruptions. A major area of concern is the fate of the stratospheric ozone layer, which blocks biologically harmful ultraviolet radiation. After Mt. Pinatubo injected tens of teragrams of sulfate aerosol in the stratosphere, the ozone hole over Antarctica grew 25% greater in geographical extent [36]. A recent modeling study by Tilmes et al. [37] also demonstrated that sulfate aerosol addition to the stratosphere may lead to a loss of ozone, predicting that this geoengineering scheme could delay the ozone hole recovery by 30-70 years. An inverse relationship with particle size was also found, with the higher surface area associated with smaller particles enhancing ozone loss through heterogeneous chemical reactions. Although smaller aerosols may provide a cost savings [35], it appears they also increase the risk of stratospheric ozone destruction.

The human health, ecosystems, and climate effects of air pollution-related and stratospheric sulfate aerosols have been extensively studied by the atmospheric sciences community, as discussed for tropospheric cloud enhancement strategy. Known consequences include reduced PAR, altered hydrological cycles leading to shifts in regional precipitation, and potential human health effects. The knowledge that now exists on these effects can assist us in assessing the environmental impacts from boosting sulfate aerosol loadings in the atmosphere. For manufactured aerosols, however, we are in uncharted territory. Proposals to spread novel aerosols [17] in the atmosphere at a global scale need careful evaluation prior to any level of implementation. As we learned the hard way with manufactured chlorofluorocarbon compounds (CFCs), what is an inert compound within the troposphere can be reactive in the stratosphere and cause long-term global impacts.

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