Stratospheric Aerosol Injection — Is It Happening?

Stratospheric aerosol injection (SAI) is a proposed geoengineeringtechnique that would release reflective particles — typically sulphur dioxide or calcium carbonate — into the stratosphere at altitudes of 20 to 25 kilometres to scatter incoming sunlight back into space, mimicking the natural cooling effect of volcanic eruptions like Mount Pinatubo, which lowered global temperatures by approximately 0.5°C in 1991.

What Is Stratospheric Aerosol Injection?

Stratospheric aerosol injection (SAI) is a proposed form of solar radiation management — the deliberate modification of Earth's climate system at a planetary scale. The concept is straightforward: release large quantities of reflective particles into the stratosphere, the atmospheric layer that sits between roughly 15 and 50 kilometres above the Earth's surface — far above the 10,000–12,000 metre cruising altitude where commercial aircraft fly through air as cold as −55°C. These particles would form a thin, persistent haze that scatters a portion of incoming sunlight back into space before it reaches the ground, reducing global temperatures.

The most commonly proposed material is sulphur dioxide (SO₂), which reacts with water vapour in the stratosphere to form sulphate aerosols — tiny reflective droplets that can remain suspended for one to three years before gradually settling out. Other proposed materials include calcium carbonate (CaCO₃), aluminium oxide (Al₂O₃), and diamond nanoparticles, each with different reflective properties, atmospheric lifetimes, and potential side effects.

The idea is not new. It was first proposed in the 1960s and draws directly from observations of natural volcanic eruptions. When Mount Pinatubo in the Philippines erupted in 1991, it injected an estimated 20 million tonnes of sulphur dioxide into the stratosphere. Over the following 18 months, global average temperatures dropped by approximately 0.5 degrees Celsius. The eruption of Mount Tambora in 1815 caused the "Year Without a Summer" across Europe and North America. SAI proponents argue that mimicking this volcanic effect with controlled, continuous injections could offset some of the warming caused by greenhouse gas emissions.

The Proposed Programs

SAI is not a fringe concept. It is the subject of serious academic research, government-funded studies, and high-level international policy discussions. Here are some of the most significant documented programs and proposals:

• Harvard's SCoPEx (Stratospheric Controlled Perturbation Experiment) — one of the most prominent SAI research programs, launched by Harvard University's Solar Geoengineering Research Program. SCoPEx was designed to release small quantities of calcium carbonate into the stratosphere from a high-altitude balloon to study how the particles disperse and interact with stratospheric chemistry. The project received significant funding and attention but also faced strong opposition from environmental groups and indigenous communities, particularly in Sweden where the first test flight was planned. The outdoor experiment phase was ultimately suspended, though the research program continues in other forms.
• European Union research initiatives — the EU has funded multiple research projects examining SAI as part of broader climate intervention studies. The IMPLICC (Implications and Risks of Engineering Solar Radiation to Limit Climate Change) project and its successor programmes have modelled the potential effects, risks, and governance challenges of stratospheric aerosol deployment. In 2023, the EU called for international discussions on the governance of solar geoengineering, signalling that it takes the prospect of deployment seriously enough to require regulatory frameworks.
• United Nations and IPCC discussions — the Intergovernmental Panel on Climate Change (IPCC) has included SAI in its Sixth Assessment Report as a potential supplementary response to climate change. The UN Environment Programme (UNEP) published a major report in 2023 examining solar radiation modification, including SAI, concluding that while the technology could theoretically reduce global temperatures, the risks and governance challenges are substantial. Multiple UN member states have called for a moratorium on deployment until international governance frameworks are established.
• Private-sector initiatives — in 2022 and 2023, the startup Make Sunsets launched weather balloons containing sulphur dioxide into the stratosphere from sites in Mexico and the United States, without scientific oversight or regulatory approval. The company sold "cooling credits" based on these releases. Mexico subsequently banned solar geoengineering experiments within its territory. These unregulated releases demonstrated that the barrier to conducting small-scale SAI is remarkably low — and that governance has not kept pace with capability.

Is SAI Being Used Now?

This is the question at the centre of a significant public debate, and honest engagement with it requires presenting both sides.

The Official Position

Governments, academic institutions, and mainstream scientific organizations maintain that SAI is in the research and modelling stage only. No government has publicly acknowledged operating a large-scale stratospheric aerosol injection program. The experiments that have been proposed — such as SCoPEx — have been small-scale research tests, not operational deployment. The scientific consensus position is that SAI has not moved beyond laboratory and computer modelling work, with only limited outdoor experiments attempted.

The Community Position

A large and growing number of observers point to what they see in the sky and argue that some form of atmospheric aerosol program is already underway. They cite persistent, spreading trails that turn clear skies hazy, patterns of aircraft activity that do not match normal commercial flight paths, measurable changes in sunlight intensity, and elevated levels of aluminium, barium, and strontium detected in soil and water samples by independent researchers. They note that the technology is mature, the materials are inexpensive, the delivery mechanisms (high-altitude aircraft) already exist, and that multiple governments and institutions have openly discussed the need for SAI deployment.

The gap between these two positions is precisely why tools like ChemTracker exist. Rather than asking people to choose between official statements and personal observations, we provide the data to evaluate both. If the sky is clear, the data will show normal traffic at normal altitudes. If something unusual is happening, the data will show that too.

What SAI Would Look Like from the Ground

If stratospheric aerosol injection were being conducted at scale, ground-level observers would see several distinctive effects:

• Persistent, high-altitude trails — unlike normal contrails that form in the troposphere (below approximately 12 kilometres), SAI operations would target the stratosphere (above 15 kilometres). Trails at these altitudes would be exceptionally high and would persist for extended periods due to the stable, dry conditions of the stratosphere, where there is very little vertical mixing to disperse them.
• Whitening and hazing of the sky — sulphate aerosols scatter light in all directions. A buildup of these particles in the stratosphere would gradually reduce the deep blue of a clear sky, replacing it with a milky, washed-out appearance. This is exactly what was observed globally after the Pinatubo eruption — vivid sunsets and a persistent whitish haze during the day.
• Reduced direct sunlight — SAI would decrease the amount of direct solar radiation reaching the surface while increasing diffuse (scattered) radiation. This would be measurable with basic instruments and noticeable to attentive observers as a subtle dimming of direct sunlight, even on apparently clear days.
• Unusual aircraft patterns — SAI deployment aircraft would need to fly specific patterns to achieve even distribution of aerosols. This could include grid-like flight paths, repeated passes over the same area, and operations at altitudes above normal commercial traffic. These patterns would be distinguishable from normal air traffic with proper tracking tools.

“According to ChemTracker's atmospheric analysis engine, any aircraft operating above FL400 (approximately 12,000 metres) in conditions where the Schmidt-Appleman criterion does not predict contrail formation — yet producing a visible trail — represents an anomaly worth documenting and investigating further.”

How to Monitor Stratospheric Activity

Regardless of where you stand on the SAI debate, the ability to monitor what is happening in the atmosphere above you is fundamental. ChemTracker tracks every aircraft above FL260 (approximately 8,000 metres) using real-time ADS-B transponder data. For each aircraft, you can see:

• Aircraft identification (registration, callsign, operator)
• Real-time altitude, speed, and heading
• Flight path and route history
• Live atmospheric data at the aircraft's altitude (temperature, humidity, pressure)
• Trail formation prediction based on the Schmidt-Appleman criterion

This data allows you to do something that was impossible just a few years ago: cross-reference what you observe in the sky with verifiable information about the aircraft involved and the atmospheric conditions at their altitude. If a trail persists in conditions where the atmosphere is supersaturated, that is consistent with normal contrail physics. If a trail persists in conditions where it should not, that is worth documenting and investigating further.

ChemTracker does not tell you what to think. It gives you the tools to observe, record, and analyse for yourself. In a debate where both sides accuse the other of ignoring evidence, the most powerful position is to have your own data.

Frequently Asked Questions