PDF of this white paper available here:
Introduction
One method of solar radiation management is stratospheric aerosol injection, which entails spraying particles into the stratosphere to reflect light before it reaches the plant. By doing this, the amount of energy being trapped by the plant via greenhouse effect is reduced, and thus cools the planet. Figure 1 is a simplified representation of this method.
As with other forms of solar radiation management and geoengineering techniques, any potential pitfalls of this method should be discussed. There are known knowledge gaps on the widespread effect of application of this method. Below, we will review some of the most pressing atmospheric, environmental, ethical, and deployment issues as pointed out by the wider scientific community.
Atmospheric Issues
Implementation of stratospheric aerosol injections can cause atmospheric problems including Ozone depletion, changed weathering patterns, and hampered Earth-based astronomy [1]. Scientific studies have expressed concerns and uncertainties that increased high surface area density of sulfate aerosols would hinder the recovery of the polar ozone or cause ozone depletion from multi aspects. First, cold, liquid sulfate aerosols can potentially lead to activation of chlorine which further attacks or destroys ozone molecules [2]. Second, the temperature gradient caused by increased amounts of stratospheric aerosols in the low and high latitudes can lead to strengthened polar vortex and hence more frequent polar winds, resulting in great loss of ozone [2]. Further, increased stratospheric aerosols levels can disorder ozone photochemistry in the mid latitudes, leading to even more ozone depletion. Therefore, it is suggested that thorough chemistry-climate models are essential to evaluate the effects and consequential effects of increased stratospheric aerosols has on atmospheric ozone before the implementation.
Another issue facing stratospheric aerosols injection is the changed weather pattern. Stratospheric aerosols injection, both tropical and arctic, may disturb the summer monsoons in Asia and Africa, leading to reduced precipitation and reduced food production for billions of people [1]. Other studies on aerosols injection’s impact hold an uncertain attitude. For example, models on Africa’s climate response to stratospheric aerosol suggested uncertain precipitation disturbance of such injections due to varied local precipitation in space and time [3].What’s more, the injected aerosols can also ruin terrestrial astronomy. The pollution caused by injected aerosol will be sitting above moutain-top observatories, dampening the observation activities of such observatories that cost billions to build [1].
Environmental Risks
Much of the uncertainty surrounding stratospheric aerosol injection lies in the potential environmental effects and risks [4]. Aerosol injection comes with a list of potential environmental pitfalls, some including: changing weather patterns, regional drought, ocean acidification, ozone depletion, less solar power that could impact crops, and rapid warming if aerosol injection is suddenly stopped [4].
Some environmental concerns have been addressed by further research. While previously of concern, the reduction of solar radiation due to aerosol injection, which causes a more diffuse spectrum of light to reach Earth’s surface, has now been shown to actually be a benefit for photosynthesis [4]. Similarly, excess sulfate acid deposition has been shown to be a small enough amount that it would not significantly disrupt Earth's ecosystems [4].
However, while these two environmental issues are no longer of major concern when it comes to aerosol injection, there are still significant environmental risks that can’t be overlooked. Climate models and observations suggest that aerosol injection would cause “substantial ozone depletion,” and would cause reduced rainfall in Asia and Africa that likely lead to drought that would impact crop productivity [4].
While we can speculate as to potential side effects, the complexity and interconnectedness of possible risks resulting from aerosol injection, or any geoengineering method, are difficult to understand as a whole in the complex Earth system [4]. Similar to the fact that anthropogenic climate change due to greenhouse gasses has environmental consequences unique to it, aerosol injection would likely also have unique side effects [7]. These could include the sulfates
aerosols depositing on Earth, changes in local, regional, and worldwide weather patterns, and “substantial” changes to Earth’s hydrological cycle [7]. While aerosol inject has been shown in models to have a significant effect in counteracting global warming, it’s also worth noting that aerosol injection would not counteract ocean acidification [7]
Perhaps one of the most significant environmental concerns that persists about stratospheric aerosol injection is that this type of stratospheric geoengineering would harm the ozone. Climate models show that aerosol injection would cause substantial depletion of the ozone, lengthen the existence of the Antarctic ozone hole for several more decades, and would create seasonal ozone holes in the spring about the Antarctic, increasing the amount of UV radiation on the surface [4][7].
Ethical Issues
With the concerns discussed here, it is not unreasonable to see a growing group of experts asking if we should even consider using aerosol injection as a potential force to mitigate climate change. This is not only due to the previously mentioned environmental and atmospheric impacts, but also from an ethical view as well. One of the biggest concerns is how relatively cheap it is. With a price tag between 7- 72 billion dollars/year [5] this method could feasibly be completed by a single country, and even some larger companies such as Apple or Microsoft could afford it. This accessibility leads to concern as it could only take the decision of a few people to start this global geoengineering strategy. Due to the nature of aerosol injection, once one party deploys it, the entire world is subject to its effects. This creates a major ethical concern for the lack of consent for many people on earth. Since everyone will be facing the consequences of aerosol injection no matter who deploys it, it will be almost impossible for there to be a scenario where all people’s concerns are heard under this current government system [6]. This also means that there is no current governing body to properly regulate and make decisions on things like intensity, duration, or deployment. This lack of control can lead to situations that put the most vulnerable people on this planet at risk.
Aside from the difficulties with governing the deployment and implementation of aerosol injection, there is also the moral hazard associated with this kind of climate change management. With aerosol injection, this moral hazard is two fold. The first issue being that it is taking up resources and time away from strategies that are more proven and have fewer negative effects. The second issue being that aerosol injection risks giving a false idea that we can continue to pollute and not face consequences for the actions. While one may hope for the implementation of aerosol injection as a potential stop gap until the CO2 levels fall, this would leave the door open for companies to continue to pollute at the same rates as they no longer have to worry about the warming effects. This would mean that this temporary stop gap could be indefinitely extended, increasing the potential risks while also keeping companies from having motivation to mitigate their environmental impact.
Logistical/Deployment [8]
Stratospheric aerosol injection requires ~100,000 to ~1,000,000 tons of material taken to the lower stratosphere, ~20 km. Current aircrafts (commercial and military) cannot reach this altitude, even with the modifications. For instance, modified business jets are unable to reach altitudes above ~16 km. Hypothesized high altitude aerostats have not been tested successfully and cannot operate in some extreme weather conditions. Military fighters (e.g, F-15) can reach altitudes of ~18 km, but they cannot have regular operations at such high altitudes.
For the technologies capable of achieving the goal, the costs are expensive. For instance, NASA’ s existing high altitude aircraft with ~ 1t payloads is able to reach altitudes of ~20 km but it is quite costly. Balloons and large naval-style guns can be alternatives, but their costs are expensive (~$40,000 per ton) as well.
There are some proposed novel aircrafts, capable of achieving this mission with lower costs. For instance, Smith and Wagner (2018) proposed an aircraft, SAI lofter. The cost is ~ $ 1500 per ton. Over the first 15 years, the average cost is ~ $2.25 billion per year. This method has a much lower cost compared to other methods. However, it needs to have thousands of flights every year, which makes it easily detectable.
Categories | Issues |
Atmospheric | Ozone Depletion, Disturbed Weather Pattern |
Environmental | Ozone Depletion, Regional Drought, No More Earth Astronomy |
Ethical | Lack of Consent, Lack of Governing Body, Moral Hazard |
Logistical | No existing plausible aircrafts (with reasonable cost) |
Figure 2: This table shows some of the main issues with stratospheric aerosol injection indifferent categories.
Works Cited
[1]. Robock, A., Oman, L., & Stenchikov, G. L. (2008). Regional climate responses to geoengineering with tropical and Arctic SO2 injections. Journal of Geophysical Research: Atmospheres, 113(D16).[2]. Tilmes, S., Müller, R., & Salawitch, R. (2008). The sensitivity of polar ozone depletion to proposed geoengineering schemes. Science, 320(5880), 1201-1204.
[3]. Pinto, I., Jack, C., Lennard, C., Tilmes, S., & Odoulami, R. C. (2020). Africa's climate response to solar radiation management with stratospheric aerosol. Geophysical Research Letters, 47(2), e2019GL086047.[4] Robock, A., Marquardt, A., Kravitz, B., and Stenchikov, G. (2009). Benefits, Risks, and Costs of Stratospheric Geoengineering. Geophysical Research Letters 36, 19. https://doi.org/10.1029/2009gl039209.
[5] Wake Smith 2020 Environ. Res. Lett. 15 114004[6] F. Biermann et al., "Solar geoengineering: The case for an international non‐use agreement," WIREs Climate Change, 2022-01-17 2022, doi: 10.1002/wcc.754.[7] Zarnetske, P.L., Gurevitch, J., Franklin, J., Groffman, P.M., Harrison, C.S., Hellmann, J.J., Hoffman, F.M., Kothari, S., Robock, A., Tilmes, S., Visioni, D., Wu, J., Xia, L., and Yang, C.-E., 2021, Potential ecological impacts of climate intervention by reflecting sunlight to cool earth: Proceedings of the National Academy of Sciences, v. 118, no. 15, doi: 10.1073/pnas.1921854118.[8] Wake Smith and Gernot Wagner 2018 Environ. Res. Lett. 13 124001 (https://iopscience.iop.org/article/10.1088/1748-9326/aae98d)