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Climate Engineering with
Stratospheric Aerosols and
Associated Engineering Parameters

BEN KRAVITZ
Pacific Northwest National Laboratory

Climate engineering with stratospheric aerosols, an idea inspired by large volcanic eruptions, could cool Earth’s surface and thus alleviate some of the predicted dangerous impacts of anthropogenic climate change. However, the effectiveness of climate engineering to achieve a particular climate goal, and any associated side effects, depend on certain aerosol parameters and how the aerosols are deployed in the stratosphere. Through the examples of sulfate and black carbon aerosols, this paper examines “engineering” parameters—aerosol composition, aerosol size, and spatial and temporal variations in deployment—for stratospheric climate engineering. The effects of climate engineering are sensitive to these parameters, suggesting that a particle could be found or designed to achieve specific desired climate outcomes. This prospect opens the possibility for discussion of societal goals for climate engineering.

BACKGROUND

Large volcanic eruptions cause cooling of Earth’s surface by creating a layer of stratospheric sulfate aerosols that scatter incoming solar radiation. The 1991 eruption of Mount Pinatubo, which injected approximately 20 teragrams (Tg) of sulfur dioxide (SO2) into the stratosphere, caused global cooling by 0.5°C for the next 12 months (Soden et al. 2002). This and other eruptions have inspired study of a method of climate engineering: the deliberate creation of a layer of stratospheric aerosols to cool the planet (Budyko 1974).

In addition to surface cooling, large tropical eruptions such as that of Mount Pinatubo have other important effects. They induce patterns of winter warming over continents in the Northern Hemisphere, a dynamical response of the atmo-



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Climate Engineering with Stratospheric Aerosols and Associated Engineering Parameters Ben Kravitz Pacific Northwest National Laboratory Climate engineering with stratospheric aerosols, an idea inspired by large vol- canic eruptions, could cool Earth’s surface and thus alleviate some of the predicted dangerous impacts of anthropogenic climate change. However, the effectiveness of climate engineering to achieve a particular climate goal, and any associated side effects, depend on certain aerosol parameters and how the aerosols are deployed in the stratosphere. Through the examples of sulfate and black carbon aerosols, this paper examines “engineering” parameters—aerosol composition, aerosol size, and spatial and temporal variations in deployment—for stratospheric climate engineering. The effects of climate engineering are sensitive to these parameters, suggesting that a particle could be found or designed to achieve specific desired climate outcomes. This prospect opens the possibility for discussion of societal goals for climate engineering. BACKGROUND Large volcanic eruptions cause cooling of Earth’s surface by creating a layer of stratospheric sulfate aerosols that scatter incoming solar radiation. The 1991 eruption of Mount Pinatubo, which injected approximately 20 teragrams (Tg) of sulfur dioxide (SO2) into the stratosphere, caused global cooling by 0.5°C for the next 12 months (Soden et al. 2002). This and other eruptions have inspired study of a method of climate engineering: the deliberate creation of a layer of strato- spheric aerosols to cool the planet (Budyko 1974). In addition to surface cooling, large tropical eruptions such as that of Mount Pinatubo have other important effects. They induce patterns of winter warming over continents in the Northern Hemisphere, a dynamical response of the atmo- 29

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30 FRONTIERS OF ENGINEERING spheric circulation to stratospheric heating by the aerosols (Shindell et al. 2001; Stenchikov et al. 1998). At the same time, the summer monsoons in India and East Asia are weakened by a smaller temperature gradient between the Indian Ocean and the Asian continent and reduced evaporative flux from the Indian Ocean (Boos and Kuang 2010; Manabe and Terpstra 1974; Oman et al. 2006). Furthermore, an increase in available photochemical surfaces provided by the aerosols catalyzes ozone loss (Kinnison et al. 1994). High-latitude eruptions, such as that of Katmai in 1912, have somewhat different climate effects. Winter warming does not occur, and weakening of the Indian summer monsoon is more prominent (Oman et al. 2006). The aerosols have a shorter atmospheric lifetime (~8 months) than in tropical eruptions (~12 months) since both the aerosols’ travel from the tropics to the poles and midlatitude storm tracks (where they are removed) account for much of the lifetime of stratospheric aerosols injected into the tropics. The time of year of the eruption also plays a critical role in determining climate impacts. Aerosols injected in the winter at high latitudes will have reduced radiative effects because of less sunlight and will also be removed from the stratosphere more quickly, in part due to large-scale deposition (Kravitz and Robock 2011). SIMULATIONS Climate engineering with stratospheric sulfate aerosols has been studied repeatedly with climate models. Simulations in which globally averaged tempera- ture is returned to a reference state show that the tropics are slightly overcooled and that high latitudes, particularly the Arctic, are warmer than in the reference case (Govindasamy and Caldeira 2000; Kravitz et al. 2013). As distinct from the impacts of large tropical volcanic eruptions, Northern Hemisphere continents do not show winter warming patterns for climate engineering with stratospheric sulfate aerosols (Robock et al. 2008), a method that cools the surface more than the rest of the troposphere, stabilizing the lower atmosphere and weakening the hydrologic cycle (Bala et al. 2008). Studies have not yet revealed whether summer monsoon weakening is a robust feature of climate model response to this method of climate engineering. Simulated climate effects depend on the method of climate engineering, namely stratospheric sulfate aerosols that are similar to the aerosols from the Mount Pinatubo eruption. Such aerosols have particular compositions (approxi- mately 75% sulfuric acid and 25% water) and sizes (effective radius of ~0.5 µm) (Rasch et al. 2008). They are also assumed to be injected above the equator and distributed through an altitude of 16–25 km. If any of these parameters changes, the radiative and climate effects will likely change as well.

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CLIMATE ENGINEERING WITH STRATOSPHERIC AEROSOLS 31 This paper presents some options for “engineering” aerosol parameters, spe- cifically composition and size, as well as the latitude and time of year of aerosol injection. The discussion focuses largely on examples involving sulfate and black carbon aerosols, with less attention to particles designed to optimize particular radiative and climatic outcomes. The concluding section addresses societal impli- cations of various potential choices. ENGINEERING PARAMETERS Composition: Sulfate vs. Black Carbon Aerosols Sulfate aerosols scatter nearly 100% of visible and ultraviolet light, whereas black carbon aerosols are excellent absorbers. Although both types of aerosols will prevent some amount of solar radiation from reaching the surface if placed in the stratosphere, black carbon will cause significant stratospheric heating; for example, 1 Tg of black carbon aerosols (0.08 µm radius) in the lower stratosphere has been simulated to cause more than 20°C of stratospheric heating (Kravitz et al. 2012). Conversely, the eruption of Mount Pinatubo created 29 times the aerosol loading and produced 2–3°C of stratospheric heating (Stenchikov et al. 2002), which increased Arctic zonal winds, forcing a positive mode of the Arctic oscillation. The reactions governing catalytic ozone loss are temperature dependent, so stratospheric heating would also cause stratospheric ozone loss (Groves et al. 1978), particularly in the Arctic, but in the Antarctic evaporation of polar s ­ tratospheric clouds would slow ozone loss in the region. The addition of photo­ chemical surfaces to the stratosphere would promote ozone loss from both sulfate and black carbon aerosols; stratospheric climate engineering with 2 Tg S yr −1 would delay recovery of the Antarctic ozone hole by 30–70 years (Tilmes et al. 2008). Black carbon aerosols (typical radius 0.08 µm) cause more cooling per unit mass than volcanic sulfate aerosols. Stratospheric injection of 1 Tg yr−1 black carbon aerosols has been simulated to cause 0.4°C of surface cooling (Kravitz et al. 2012). In contrast, an injection rate of as much as 5 Tg SO2 per year would increase the cooling to 0.6°C (Robock et al. 2008). Stratospheric aerosols will fall into the troposphere within a few years. The amount of additional rain acidity resulting from climate engineering with 5 Tg SO2 per year would likely be insufficient to cause damage to most ecosystems (Kravitz et al. 2009), but black carbon is toxic and causes respiratory impairment (Baan et al. 2006). Moreover, if black carbon lands on snow or bright surfaces, it lowers the albedo of those surfaces and the planet retains more solar radiation, exacerbating global warming (Vogelmann et al. 1988).

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32 FRONTIERS OF ENGINEERING Size Depending on aerosol composition, particle size may be somewhat predeter- mined. Sulfate aerosols tend to coagulate, and SO2 can condense onto existing particles. Both factors tend to increase particle size. Sulfate aerosols are most efficient at scattering when the particles are small (~0.1 µm radius). As they grow larger, so does their infrared effect; above ~2 µm in radius, infrared effects overwhelm the scattering effects and become net absorbing particles. In addition, larger particles have a greater fall speed and thus a lower atmospheric lifetime. Simulations have shown that increasing the size of black carbon particles by 50% reduced surface cooling by more than a factor of 2 (Kravitz et al. 2012). The inclusion of aerosol microphysics in simulations increases the amount of SO2 needed from 5 Tg per year (Robock et al. 2008) to more than 50 Tg (English et al. 2011; Heckendorn et al. 2009; Pierce et al. 2010). One proposal to overcome microphysical limitations is direct condensation of sulfuric acid vapor to produce a monodisperse distribution of small sulfate aerosols, but this idea is untested (Pierce et al. 2010). Black carbon aerosols tend not to coagulate in ways that alter their radiative properties and are generally smaller. Moreover, in the stratosphere, they could be heated by the sun and self-loft, increasing the fall distance and thus atmospheric lifetime of the particles (Pueschel et al. 2000; Rohatscheck 1996). Spatial/Temporal Distribution Longitude of stratospheric aerosol injection is largely irrelevant, as the general circulation of the atmosphere will evenly distribute the aerosols across all longitudes within a matter of weeks. Conversely, the radiative and climatic impacts of climate engineering are quite sensitive to the latitude and altitude of the particles. Surface cooling from stratospheric aerosol climate engineering tends to increase with the stratospheric altitude of the aerosols, in part due to longer atmospheric lifetime (Ban-Weiss et al. 2012; Kravitz et al. 2012). Variation in solar radiation reductions by latitude and season results in modest improvements (<6%) in global temperature and precipitation residuals (climate engineering minus reference case) compared to a uniform solar reduction, but tar- geting regions with the highest residuals results in improvements in these regions by up to 30% (MacMartin et al. 2012). Aerosols injected extratropically tend to remain in the hemisphere of injection, and stratospheric sulfate aerosol climate engineering in only one hemisphere can shift the Intertropical Convergence Zone, a band of equatorial precipitation, potentially causing Sahelian greening or drying (J.M. Haywood and A. Jones, UK Met Office, personal communication, March 31, 2012).

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CLIMATE ENGINEERING WITH STRATOSPHERIC AEROSOLS 33 DESIGNED PARTICLES Changing the “engineering” parameters for sulfate and black carbon can “fine-tune” the climate effects of stratospheric aerosol climate engineering to some extent, but some side effects are unavoidable. For example, although they are excellent scatterers, sulfate aerosols mostly scatter light forward, whereas cool- ing is achieved by scattering sunlight back to space. Black carbon absorbs solar radiation, keeping the energy in the atmosphere. And the optimal sulfate aerosol size may not be achievable because of coagulation. These concerns suggest that it may be necessary to design particles in order to achieve desired aerosol parameters such as, for example, a perfectly scatter- ing particle that photophoretically levitates at 50 km in altitude (Keith 2010). Although the climate effects of such a particle are unknown, as are its side effects on stratospheric chemistry and atmospheric circulation, it may be possible to create particles that take advantage of certain properties and alleviate some side effects or shortcomings. CONCLUSIONS The goal of mapping “engineering” parameters for stratospheric aerosol par- ticles and their climate effects is to, eventually, be able to address the question of what society might want climate engineering to do. For example, if societal goals are primarily to preserve Arctic sea ice, climate engineering could be done by injecting sulfate aerosols into the Arctic troposphere during spring. If the primary goal is to cool the planet while avoiding any increase in rain acidity, perhaps black carbon would be preferable to sulfate. If the primary and side effects of climate engineering can be chosen, the foundation is laid for discussions to determine climate goals. Such a discussion will not have clear answers, though, as the goals of climate engineering do not depend solely on climatology. There are multiple stakeholders with myriad values that encompass scientific, social, political, legal, ethical, and personal dimensions, and there is no clear method of synthesizing and address- ing these issues on a global scale. Moreover, assuming that a method of climate engineering could be designed and chosen, society will need to decide how much climate engineering will be done. The choice of “engineering” parameters has not been fully explored, and there are many uncertainties in the predicted impacts of climate engineering. Dedicated research is needed to develop relevant engineering tools and enhance understand- ing in these areas. The purpose here is to illustrate areas of a potential research agenda that could be useful in choosing methods and goals of climate engineering.

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