|Fig. 1: Preliminary overall evaluation of several geoengineering techniques considering effectiveness and affordability.  (Courtesy of The Royal Society)|
Global warming is a serious and difficult challenge whose ramifications will continually be felt in both our environment and lifestyle. The effects of climate change will be experienced around the world, and its impact will be large and costly. It is possible that climate change can be controlled through adaptation and mitigated through the reduction of greenhouse gases, but up until now, not much has been done towards this reduction. Current estimates predict that mean global temperature will rise by over 2°C this century unless emissions are reduced by 50% of 1990 levels by 2050. 
Due to the seriousness of this issue, further human intervention may be warranted, and even required, in the form of geo-engineering. Geo-engineering is the large-scale engineering of the earth's environment in order to combat the effects of change in the chemistry of the atmosphere.  Many geo-engineering options involve reflecting a fraction of the sun's radiation, termed Solar Radiation Management (SRM), in order to compensate for the global rise in average temperature caused by greenhouse gases. Other geo-engineering options include cloud seeding, tropical reforestation, ocean iron fertilization, and building thicker sea ice, among many others. Geo-engineering approaches exhibit vast unpredictability and potentially great risk, and should not be enacted without careful study and evaluation of both the direct and side effects and ethical and moral issues.  Fig. 1 displays several geoengineering techniques corresponding to their affordability and effectiveness
One of the major classes of geo-engineering climate managing techniques is SRM. This technique aims to reduce the incidence and absorption of the sun's rays by reflecting back a small portion of solar radiation.  Various proposals seek to do this by increasing the earth's albedo, or it's reflecting power, thus providing a cooling effect to mitigate the warming influence caused by an ever-increasing amount of greenhouse gases.  Techniques proposed include introducing reflective matter into the atmosphere, inserting a space mirror in between the earth and the sun, and making the Earth more reflective among others. 
The main focus of this paper is to describe the theory, risks, uncertainties, and ethical questions behind using the SRM method: Stratospheric Aerosol Injection (SAI). Aerosol's are found everywhere in the atmosphere and affect the energy balance of the planet through their reflective and scattering properties.  They affect human life on earth by transporting diseases, and nutrients all across the globe. Anthropogenic aerosols have changed our atmosphere through pollution emissions and through natural land and water management.  According to the National Academy of Science's study, aerosols in the upper atmosphere take longer to remove because they are located away from most clouds and typical weather phenomena which would remove and recycle these aerosols within days, making this method more feasible. The general consensus in the scientific community is that atmospheric aerosols cool the earth. They accomplish this by interacting with sunlight entering the atmosphere; when scattering sunlight back into space, aerosols cool the earth, and when absorbing sunlight their immediate area is warmed, but the atmosphere below them is cooled.  Models and theory suggest that increasing the amount of aerosols in the atmosphere would cool the earth, mitigating some of the effects of greenhouse gases on our planet.
This proposed idea involves increasing the Earth's albedo by injecting small reflecting particles into the stratosphere. This increase in albedo is not accomplished by injecting aerosols themselves into the stratosphere, but usually by injecting a chemical precursor such as sulfur dioxide (SO2), which then, through physical and chemical processes, converts into aerosols.  Fig. 2 shows a simple diagram as to how stratospheric aerosol injection might work.
|Fig. 2: Diagram of how SAI could work if deployed. (Source: Wikimedia Commons)|
Much of the stratospheric sulfate aerosols (i.e. sulfur dioxide (SO2), carbonyl sulfide (COS), and sulfuric acid (H2SO4)) are formed through both natural and anthropogenic processes originally from the Earth's surface such as volcanic eruptions.  The process involves adding oxygen atoms to these gases through a series of chemical reactions, which eventually leads to the formation of sulfate aerosols.  Most stratospheric aerosols are formulated in the troposphere where they are transported by wind or sedimentation and eventually reach the stratosphere by rapidly mixing, mainly through weather events or turbulence.  In the stratosphere these particles may either nucleate to form newer smaller particles or condense onto existing particles.  The concentration of these particles, and their relative amounts, is determined mainly by physical (Brownian motion, diffusion etc.), chemical, and mixing processes due to molecular and turbulent motions.  Much of the basics of particle formation, nucleation, and agglomeration are well understood, however many of the finer details in particle mass and number, as well as their affect on the climate system are not. The National Academy of Sciences recommends more work in characterizing these natural processes through experimental measures and modeling before science can produce accurate models and predictions as to how stratospheric aerosols will behave and affect Earth's climate and environment.
Given the uncertainty and risks associated with launching nanoparticles of aerosol dust into the atmosphere, the side effects should be studied further extensively. One side effect that would be readily seen is the effect on sunrises and sunsets; another one, explains the National Academy of Science, is that it would heat the stratosphere where the dust is located. This would have a major impact on stratospheric chemistry, specifically that of the ozone layer, something that must be examined before this option is considered and implemented. The Royal Society also explains that there could be regional effects on the hydrological cycle as well as effects on high altitude tropospheric clouds and biological productivity which must be studied further.  A more refined understanding of particle characteristics, size distribution, and scattering patterns and predictability is needed before such an endeavor is undertaken. 
The Royal Society and National Academy of sciences each developed three ethical questions that should be asked when undertaking research and considering the use of geoengineering and stratospheric aerosol injection in regards to climate mitigation issues.
The Royal Society asked the following three questions. 
Would deliberate geoengineering be unethical and are some geoengineering techniques more ethically acceptable than others - if so, which and why?
Is a higher standard of proof or confidence needed for geoengineering interventions than for other mitigation actions?
What are the main ethical considerations that the design of a regulatory framework for geoengineering research or deployment would need to take into account?
The National Academy of Science devised the following three questions: 
Who decides if the benefits of albedo modification outweigh the risks, and what are the criteria?
Who gets to decide when and in what way albedo modification will be undertaken?
Would society ever know enough to responsibly decide to deploy albedo modification?
It is clear that all of these questions need to be discussed in depth with further research provided on the topic. It is possible, due to these moral and ethical reasons, that the topic of geoengineering galvanizes the populations of the world to demand for increased action and attenuation, such as imposing a carbon tax, planting more trees, etc., rather than needing to implement geoengineering strategies.  Due to the fact that geoengineering affects the entire planet, international laws and treaties must also be further expanded and explored in order to decrease the risk of damage to international relationships.
It is established that global warning is a clear and visible threat in the not too distant future and that reduction steps will have to be taken in order to minimize emissions and alleviate the greenhouse gas effect. Carbon dioxide emissions reduction and mitigation through direct human intervention (using less energy, planting more trees, etc.) should be favored and advised, but SRM methods offer an attractive, but unproven and ultimately risky proposition to consider. SRM techniques should be technically feasible and could provide a way to directly lower global mean temperatures in a relatively short timeframe should it prove necessary. It is also important to note that they carry several inherent risks; the first being that they do not actually reduce greenhouse gases in the atmosphere and the second that the termination phase of SRM is widely untested and not well understood using our current models and simulations. With SRM, and more specifically sulfate based aerosols (SAI), the impact is highly dependent on where in the atmosphere the particles are injected, the location on ground, and the method of delivery.
Further laboratory and field-testing would need to be conducted before SRM methods are ready for deployment and furthermore a growth in the fundamental understanding of the problem needs to occur. Models may provide useful insights into how SRM may affect global climate, regional climates, stratospheric ozone, and the hydrological cycle. They should be employed when possible in tandem with small-scale field tests.
Another consideration must be the role that ethics and morality play in the discussion of SRM and on a larger scale, geoengineering. Questions must be asked and answered by the global community such as locations for deployment, method of delivery, amount delivered, and cost. Those ultimately making these decisions must also be sure of the technical feasibility and of the side effects that SRM may have on the environment. It is because of this last consideration that I cannot recommend stratospheric based aerosol injection with absolute certainty. Not only are the uncertainties and risks too great, but also the cooperative nature that must be present for this endeavor to work has not been seen on a truly global scale in recent memory. The ethics and moral issues are too complicated for a simple panel of global powers to make. Further research should be conducted on alternative methods in order to reduce the completely man-made problem that is global warming.
© David Llanos. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 Geoengineering the Climate: Science, Governance and Uncertainty. (Royal Society of London, 2009), p. 1
 Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (National Academies Press, 1992), pp. 433-464.
 Climate Intervention: Reflecting Sunlight to Cool Earth (National Academies Press, 2015), pp. 47-148.