Stratospheric Aerosol Injection for Climate Cooling?

Exploring a Controversial Climate Intervention Amidst Urgency and Uncertainty

Jul 01, 2025

Given the commotion around the UK’s funding of outdoor geoengineering experiments, one type of technology called stratospheric aerosol injection (SAI) is garnering increased attention and proposes intentionally creating emissions to combat rising global temperatures. With looming climate tipping points on the horizon and the possibility of current emission reduction efforts falling short, researchers are considering whether controversial climate intervention approaches such as SAI are feasible, scalable, and safe.

Since PSN focuses on aviation non-CO2 emissions, here we will explore a paradoxical approach to addressing climate change: adding particles to the atmosphere to cool the planet. We’ll cover what SAI is, its potential benefits, risks, and challenges, what role the aviation industry would play, and how it would intersect with aviation’s goals to reduce emissions.

A Brief Overview of Geoengineering

Geoengineering has roots stretching back decades and includes a litany of proposed methods to cool global temperatures by deliberately altering the Earth's climate system. It goes by many names including climate intervention and climate repair and generally falls into two broad categories: 1) reflecting sunlight to create a cooling effect (solar radiation management or SRM) and 2) actively removing CO2 from the atmosphere.

Proposed SRM methods include marine cloud brightening, space mirrors, cirrus cloud thinning, and SAI to reflect sunlight from the earth. Carbon dioxide removal methods aim to absorb carbon dioxide to limit warming like direct air capture (DAC), ocean fertilization, and reforestation.

Different Types of Geoengineering | CFR Education

While early climate policy focused solely on emissions cuts, the growing recognition that mitigation alone might not eliminate climate risks led to the consideration of other responses. In the 1960s, researchers considered the idea of SRM after observing the cooling effects of volcanic eruptions and the burning of sulfur-containing fuels. In the early 1970s, Russian climatologist Mikhail Budyko proposed intentionally injecting reflective aerosols into the stratosphere to mimic these effects.

What is Stratospheric Aerosol Injection?

Stratospheric aerosol injection – a type of SRM – involves injecting tiny reflective particles or aerosols into the stratosphere, about 11–50 km (7–31 miles) above the ground. The goal is to reflect a small portion of incoming sunlight back into space, thereby reducing global temperatures and offsetting effects of greenhouse gases. Imagine putting a sun hat on the Earth to shield some of the Sun’s rays.

SAI propose emulating the cooling effects observed after large volcanic eruptions which naturally release sulfur into the stratosphere. However, unlike the pulse injection of a volcanic eruption, SAI would require continual and regular injections to maintain a sustained cooling effect.

The Famous Mount Pinatubo Eruption of 1991 | FOX Weather

The most researched material for injection is sulfur dioxide (SO2), which forms sulfate aerosols in the stratosphere. Other materials like finely powdered salt, calcium carbonate (calcite), black carbon, metallic aluminum, aluminum oxide, and barium titanate are also being considered or investigated. Proposed methods for delivering materials to the stratosphere include aircraft, tethered balloons, high-altitude blimps, or artillery, with aircraft injection the most discussed (and controversial).

Why is SAI Being Considered?

The conversation around SAI is being driven by concerns that relying solely on reducing carbon emissions may be insufficient to meet the 1.5°C threshold set by the Paris Agreement. Some see it as a potential additional response option, another tool in the toolbox. Others think it should only be a last resort to moderate climate hazards and reduce risks, especially if paired with aggressive mitigation efforts like those being explored across the aviation sector. Still others believe the unknown risks and unintended consequences of SAI render it a non-starter.

What are the Potential Benefits?

The research suggests SAI could effectively reduce global mean temperature and mitigate some risks associated with climate change. Some climate model simulations suggest SAI could prevent further atmospheric warming and maintain a relatively stable climate. Potential benefits include:

Reducing temperature and precipitation extremes compared to unmitigated warming.
Reducing ice loss and helping to restore Arctic sea ice.
Delaying or preventing sea-level rise caused by thermal expansion and ice melt.
Increasing agricultural yields in some regions by stabilizing temperatures and potentially reducing tropospheric ozone.
Positively influencing atmospheric circulation and monsoon patterns.

Proponents also argue SAI is technically feasible, at least at small scale, with existing technologies and could potentially be implemented relatively quickly and at a lower cost compared to the direct costs of mitigation strategies.

What are the Risks and Challenges?

Despite these potential benefits, SAI is mired by uncertainty and presents significant risks and challenges. An even-handed assessment of these risks – both to the environment and to human health – must be considered and addressed prior to considering any type of deployment. Potential environmental risks include:

Disruption of the global hydrological cycle, potentially leading to a decrease in global mean precipitation and significant extreme regional shifts such as droughts and floods.
Changes in atmospheric chemistry and biogeochemical cycles, including nitrogen, sulfur, and carbon cycling.
Increased acid deposition from aerosols, which could impact ecosystems, particularly in pristine areas.
Changes in soil pH and toxicity due to wet and dry deposition.
Impacts on vegetation due to changes in sunlight exposure (diffuse vs direct radiation).

Injecting particles into the atmosphere could disrupt human health as well, conferring the following risks:

Adverse health impacts to humans and animals from aerosol inhalation, as toxicity of potential SAI particles is not yet fully studied.
Changes in tropospheric air pollution, such as changes in ozone and particulate matter (PM2.5), which can affect respiratory and overall health.
Impacts on food and water security if SAI leads to agricultural disruptions, droughts, or affects water quality.

SAI Aims to Reflect Solar Radiation | NASA

Beyond the purely scientific unknowns, there are also significant societal and governance challenges to ponder. The frequently debated "moral hazard" concern, for example, suggests that researching or discussing SAI could reduce motivation for essential greenhouse gas emissions reductions. Like applying a temporary fix which makes people less inclined to address the underlying problem. Then there are the geopolitical complexities:

SAI deployment could result in unequal impacts globally, with some regions potentially benefiting less or facing greater risks than others, which could lead to geopolitical tensions.
Developing effective and just governance frameworks is a major challenge, encompassing questions of international representation and oversight mechanisms.
There are concerns about the potential for weaponization of geoengineering technologies, though current evidence suggests SAI is not suitable for this purpose at a sustained, significant scale.
The risk of "termination shock", a rapid and potentially devastating warming if SAI deployment were to abruptly stop, highlights the need for a resilient and sustained deployment strategy.
Challenges related to intellectual property and patents could potentially hinder future innovation or access to the technology.
Ensuring public acceptance through transparent and inclusive processes is crucial, as awareness remains low and reactions vary.

How Would SAI Be Governed?

Speaking of governance, there is currently no international governance framework specifically for SAI. However, the prospect and feasibility of attempting to control global temperatures raises serious governance questions.

Several existing international treaties and agreements are considered relevant to SAI governance. These include the Convention on Biological Diversity (CBD) which has a de facto moratorium on climate-related geoengineering activities that may affect biodiversity in the absence of science-based, global, transparent, and effective control mechanisms; the Environmental Modification Convention (ENMOD) which prohibits hostile use of environmental modification techniques; and the Montreal Protocol, aimed at protecting stratospheric ozone.

Environmental Governance can Protect the Planet | Anritsu

As mentioned, developing effective and just governance is a major challenge. Some argue achieving consensus among all members of the UN is necessary for any steps towards realizing these capabilities. The debate involves various proposals, including expanded research and governance consultations at national and international levels. In 2023, the Climate Overshoot Commission released a report proposing a moratorium on SRM deployment large enough to risk significant trans-boundary harm, paired with support for research and governance consultations. However, the feasibility of establishing a fully-democratic, multilateral, legally-binding, and long-term agreement seems minimal with much of the discussion yet to be had.

Would SAI Impact the Aviation Industry?

Implementing SAI on a large scale using aircraft would require significant technological development, expanded operational capacity, industry logistical involvement, further research activities, potential network disruptions, and economic costs. These implications each come with their own considerations:

Development of Specialized Aircraft: SAI would necessitate the creation of new, specially designed high-altitude aircraft. These aircraft would need to operate at altitudes of around 20 km (approximately 12.4 miles) in the tropics or subtropics, which is nearly double the typical cruising height of large military and commercial jets.
New Operational Sector: SAI proposes a continuous injection of particles. This would mean a sustained, long-term commitment to aerial operations for aerosol delivery, implying the establishment of a dedicated fleet and infrastructure for global coverage.
Industry Involvement and Feasibility: The aviation industry is expected to be able to provide logistical input and confident performance estimates for various combinations of aircraft and sulfur delivery methods.
Research and Monitoring Activities: Aircraft and drones are already used in current research efforts related to atmospheric processes, such as studying how particles affect cirrus clouds and monitoring volcanic emissions. Further research would be needed to understand the small-scale physics of injection plumes and other processes to improve modeling accuracy.
Disrupting Existing Flight Networks: SAI flights could affect current flight patterns, requiring the involvement and coordination of ANSPs and airlines to navigate implementation. This would add another layer of complexity to an already delicate operational ecosystem.
Economic Implications: The direct implementation costs of SAI, including the necessary aircraft, are estimated to be relatively low compared to the costs of mitigating climate change or the damages from unmitigated warming. However, the aviation industry would have to consider those additional costs and who would pay for them.

Current Status and Outlook

As it stands, research on SAI primarily relies on climate model simulations, such as those conducted under the Geoengineering Model Intercomparison Project (GeoMIP). These models explore different injection strategies and their effects. However, model-based projections inherently include uncertainties related to scenarios, model structure, and natural variability. Quantifying and communicating these uncertainties is an increasing focus of research.

Limited, small-scale outdoor experiments related to atmospheric processes and potential SAI materials are also being conducted as part of foundational research initiatives. Programs like the UK’s Advanced Research + Invention Agency’s (ARIA) "Exploring Climate Cooling" explicitly state they are not funding the deployment of climate cooling technologies, but rather focusing on research, modeling, and carefully controlled, small-scale outdoor experiments to gather scientific data.

These small-scale experiments are distinct from large-scale climate modification and are designed to have effects that dissipate quickly or are fully reversible. Any outdoor experiments are subject to stringent governance requirements, including independent environmental impact assessments, legal reviews, and community engagement.

Despite significant opposition to field experiments in some cases, which has limited research primarily to modeling, SAI is still considered by some sources as the most viable SRM option. There is increasing support for expanded research and governance consultations on SRM at national and international levels. However, there are also concerns about smaller, unauthorized test flights being conducted by private entities. The debate surrounding SAI is intensifying, with calls for balanced research that considers both risks and potential benefits.

More Research into Climate Interventions is Needed | Imperial College

Long Story Short

SAI is a complicated and controversial geoengineering approach being explored as a potential response to the escalating risks of climate change. While research, largely through climate modeling, suggests it could be technically feasible and effective at reducing global temperatures and mitigating some associated hazards like extreme heat and sea-level rise, it also presents significant potential environmental, health, and societal risks. These risks include disruptions to vital systems such as precipitation patterns and the ozone layer, potential impacts on ecosystems and human health, and complex governance challenges related to international cooperation, equitable impacts, and the possibility of misuse.

Current efforts focus primarily on foundational research and modeling to better understand the potential outcomes and reduce uncertainties. Small-scale, controlled outdoor experiments are being explored to gather crucial real-world data, operating under stringent governance principles emphasizing transparency, risk minimization, and community engagement.

Whether or not to research or deploy SAI remains a subject of intense debate, balancing the potential to avoid dangerous climate thresholds against the introduction of new, uncertain risks. Experts emphasize the need for continued holistic research and inclusive governance discussions to inform future decisions about whether and how SAI might be part of a broader climate response strategy, recognizing that such choices would have far-reaching, global consequences.

As global temperatures rise, the debate around SAI is heating up. If SAI is pursued, the aviation industry will have a significant role to play from a paradoxical position as it endeavors to lower non-CO2 emissions while considering intentionally injecting more. It’s a complex puzzle, and there is much that needs to be considered before trying on that nebulous sun hat.