Stratospheric Aerosol Injection
Policy recommendation
The United States Government should urgently develop a national solar geoengineering research program with international partners, tasked with testing and evaluating the viability of Stratospheric Aerosol Injection (SAI) as a response to climate change. Experiments must be carried out to investigate the practicality and long-term effectiveness of SAI, and the potential risks and unintended consequences that may reveal that SAI is not an effective solution. Given the contentious nature of SAI and its transnational effects, researchers, environmental organizations, indigenous groups, and the international community must be involved at all stages to ensure safe and responsible experimentation. Due to the known risks and possibility of termination shock, SAI cannot be approached as a 'silver bullet' fix to the climate crisis, but rather a tool to be used to minimize some of the worst effects, whilst large scale decarbonization is pursued.
Why Stratospheric Aerosol Injection?
Since the 1960s, scientists and governmental organizations have acknowledged the potential of controlled releases of aerosols into the atmosphere as a way of regulating global temperatures in the face of human-induced climate change. The physical mechanisms of light and aerosol interaction are well understood and can be seen naturally in the large injections of aerosols into the atmosphere from volcanic eruptions. Most notably, the eruption of Mt. Pinatubo in 1991 reduced global surface temperatures by around 0.9°F for the following year, with cooling effects persisting for two to three years. Aerosols have also formed in the atmosphere for decades from the burning of sulfur dioxide in fossil fuels, and particularly maritime shipping fuel, which contributes both locally and globally to quantifiable temperature reductions.
Some scientists and researchers have suggested the use of controlled and sustained releases of aerosols into the stratosphere, known as stratospheric aerosol injection (SAI), as a form of climate change adaptation in response to rapidly rising global temperatures. It is widely accepted that SAI could quickly, efficiently, and cost-effectively reduce global temperatures down to pre-industrial levels. However, SAI is not without significant risks and concerns, and its potential impacts on the environment, climate systems, and agriculture are as of yet not well understood.
Although the hypothesized benefits of SAI are enormous, there have only been a small handful of real-world experiments to test the technology and evaluate its impact and risks. Given the climate emergency, it is critical that the United States Government take a leading role in funding and conducting SAI experiments to better understand its potential and risks in the event that it has to be implemented. If SAI is deemed to be a safe and viable solution through experimentation, the United States should begin developing the capabilities to deploy SAI on a large scale to assist in preventing the world from reaching irreversible tipping points if global temperatures continue to rise unabated.
Potential risks
The severity of the risks related to SAI has a dependence on the level of cooling that is desired, i.e., a larger desired level of cooling will contain more risk. A strong framework for research and governance regarding SAI is needed to better understand the following risks:
Experiment design
In order to conduct high-quality research on SAI safely and responsibly, a national solar geoengineering research program should be developed to coordinate and oversee the experimentation. Any SAI experimental research program's priority should be the development of relevant knowledge for policymakers and researchers, with a secondary goal of developing a path to deployment. Governmental SAI programs should be transdisciplinary, transparent, and should engage with international partners. Due to the large knowledge gaps and fragmented nature of past SAI research, research funding should be limited to short-term experiments to better understand the potential risks and benefits of SAI.
Open and transparent engagement with the public and stakeholders is vital to the success of an SAI research program. However, the pressing urgency of the climate crisis will require that preliminary research is completed in the face of pushback. Many of the risks associated with SAI are only hypothesized, and the quantification of these risks through experimentation is necessary to move forward with SAI or to rule out SAI as a viable option.
SAI experiments will require the release of aerosols or other inert particles that mimic the effects of sulfate aerosols into the stratosphere. As SAI is hypothesized to work most effectively in the stratosphere, which lies 20 km above sea level, and experimentation will require the use of unmanned drones or high-altitude balloons. Once released, the dispersal of the aerosol will need to be carefully monitored, as well as their interactions with other particles in the stratosphere such as ozone. Additionally, the environmental effects will need to be analyzed, such as cooling potential and other unintended consequences. The released aerosols during initial experiments must be a quantity that is multiple orders of magnitude lower than an amount that would have a detectable influence on global temperatures.
Long-term local and global monitoring of the aerosol will be required to determine the retention time of the particles in the stratosphere and their dispersion throughout the globe. The effectiveness of an SAI program relies on particle retention time greater than a year, so monitoring will need to continue for this duration.
The preliminary goal of a governmental SAI experimental research program is to quantify the effectiveness and safety of SAI as a tool to adapt to climate change, with the ultimate goal of developing the means to deploy SAI on a mass scale should the need arise.
Public communication
SAI research and experimentation is highly contentious in the scientific community and the discourse surrounding climate change. As recently as March of 2024, the Harvard University research program investigating the potential of SAI named the Stratospheric Controlled Perturbation Experiment (SCoPEx) was canceled after being put on hold for over 4 years due to public outcry and controversy. Concerns around SAI experimentation involve general issues regarding the environmental safety of the experiment, as well as the transnational area of effect that SAI experiments intrinsically involve. However, another key public concern regarding SAI is the perceived 'moral hazard' relating to how a 'quick fix' might reduce the urgency of efforts to reduce greenhouse gas emissions and transition to sustainable energy and how this could lead to over-reliance on geoengineering rather than addressing the root causes of climate change.
In order to address this public blowback, public education regarding the magnitude of the existential threat of climate change to human society must be improved. Unfortunate realities, such as the imminent approach of climate tipping points and the fact that even if all fossil fuel emissions ceased today, global temperatures would most likely remain the same and could potentially continue to rise for centuries due to the planet's thermal inertia, must be more widely recognized. Within this context, arguments regarding the risk analysis of SAI vs. climate change can be had. That withstanding, it will still be vital to emphasize that SAI is only a potential tool to address the climate crisis and that reducing greenhouse gas emissions and transitioning to sustainable energy is the most effective long-term solution.
Understanding stratospheric aerosol injection
The Earth's albedo and energy balance
Global temperatures can be generalized as a simple function of the balance between heat into the Earth (light from the sun) and heat out of the Earth (infrared radiation to space), what is known as the Earth's energy balance. Greenhouse gases in the atmosphere, such as CO₂, water vapor, and methane, act as a blanket around the planet, trapping heat that would otherwise escape to space. It is this imbalance caused by the burning of fossil fuels that has triggered global temperatures to rise. Another aspect of the energy balance is the Earth's albedo, or how much sunlight is reflected back into space. The Earth's albedo is determined by the reflectivity of the planet's surface and occurs naturally through the reflectivity of the ice caps and the top surface of clouds. In fact, all surfaces on Earth have a natural albedo, with oceans reflecting around 1% of incoming sunlight, while vegetation and deserts reflect between 1% - 4% of sunlight. Man-made structures also contribute to the Earth's albedo, with large dark surfaces such as car parks and roads absorbing more sunlight than bright surfaces.
Increasing the Earth's albedo can have a net cooling effect on the planet through the energy balance by directing more sunlight back into space, thereby reducing the amount of heat that is contained by the planet.
Aerosols and light scattering
Volcanoes and natural aerosol injections
Aerosols can naturally enter the atmosphere in different ways, from dust clouds to salt particles from evaporated water. The most significant natural aerosol injections come from volcanic eruptions. The gas ejections from volcanic eruptions consist mainly of water vapor and carbon dioxide, with the third most common gas being sulfur dioxide. Sulfur is found in various forms in volcanic magma, and when the magma reaches the Earth's surface where the pressure is relatively lower, the sulfur is released and reacts with oxygen in the atmosphere to form sulfur dioxide. The force of the volcanic eruption propels the sulfur dioxide into the stratosphere, where it undergoes another chemical reaction. Oxygen molecules (O₂) and water molecules (H₂O) react with the sulfur dioxide to form sulfuric acid (H₂SO₄). This sulfuric acid then condenses from its gaseous to a liquid state, forming into an aerosol.
It is this sulfuric acid aerosol that remains suspended in the atmosphere where it scatters incoming sunlight and contributes to the Earth's albedo. Another consequence of the presence of these aerosols is the depletion of the ozone (O₃) layer, which is vital for its role in reducing the amount of harmful ultraviolet radiation that reaches the Earth's surface. Sulfuric aerosols do not react directly with ozone, but they provide a surface for other chemicals such as chlorine and bromine to react and deplete the ozone in the atmosphere.
Volcanic eruptions are one of the main sources of inspiration for SAI experimentation as past large-scale eruptions such as Mt. Pinatubo in 1991 have demonstrated the potential of SAI to rapidly cool the planet. However, the infrequency of these events means that there has been limited opportunity to study their long-term effects on climate.
Anthropogenic aerosol emissions
Human activities such as burning fossil fuels and slash-and-burn agriculture also release aerosols into the atmosphere. The burning of fossil fuels, especially those with high concentrations of sulfur, forms a large portion of anthropogenic atmospheric aerosols. This is most notable in the use of maritime shipping fuels, which generally have a sulfur content of around 3.5%, compared to the global average of 0.6% for on-road diesel and gasoline. The sulfur dioxide released into the atmosphere reacts to form sulfuric acid aerosols in the same way that sulfur dioxide does in volcanic eruptions, with the main difference being that it is at a lower altitude. This lower altitude means that the aerosols have a lower retention time in the atmosphere as they can get washed out by wind and rain. Heatmaps taken of the Earth show clear areas with relatively lower temperature along busy intercontinental shipping routes where the sulfate aerosols form, which is effectively a form of solar geoengineering. In 2020, changes to fuel regulation by the International Maritime Organization (IMO) reduced the legally allowed concentration of sulfur in shipping fuel due to health concerns. Many researchers fear that this abrupt reduction of sulfate aerosols, and therefore the Earth's albedo, is significantly increasing the levels of global heating and is an effective example of a 'termination shock', showing the potential benefits and dangers of future SAI projects.
Stratospheric aerosol injection
Taking inspiration from volcanoes and knowledge from past anthropogenic aerosol emissions, scientists have proposed the use of controlled injections of aerosols into the stratosphere as a method of geoengineering. The most commonly proposed method of SAI is to release sulfur dioxide (SO₂) or other aerosols from high-altitude aircraft or balloons. In addition to sulfur dioxide and the resulting sulfate aerosols, other aerosols have been proposed for use in SAI, and although some carry benefits, they are often discounted due to lack of cost effectiveness and feasibility. There have been no controlled studies to affirm or refute the effectiveness of this proposal, but there is a general consensus that it will effectively cool the planet. But it is the unintended consequences of SAI that are the subject of much debate and concern.
Conclusion
Solar geoengineering via SAI is a contentious topic with a potential for enormous benefits, but also a wide range of risks and uncertainties that are poorly understood. The US, along with other government and world representative bodies may one day be required to implement SAI in the face of rapidly worsening climate change and cascading tipping points. Without prior experimentation, a rushed or uncoordinated approach could lead to unforeseen consequences that may prove difficult to reverse. A robust experimental program, with a focus on public engagement and transparency, is required to safely investigate the future viability of SAI as a tool to adapt to climate change.