Solar Radiation Management — The Plan to Block Sunlight

Solar radiation management (SRM) is a category of geoengineering that aims to cool the Earth by reflecting incoming sunlight back into space, with proposals including stratospheric aerosol injection at altitudes of 20 to 25 kilometres, marine cloud brightening, and space-based reflectors — actively researched by institutions including Harvard, MIT, and the EU Horizon Europe programme.

What Is Solar Radiation Management?

Solar radiation management (SRM) is a category of geoengineering that aims to cool the Earth by reflecting a portion of incoming sunlight back into space. Unlike emissions reduction, which addresses the root cause of climate change, SRM treats the symptom — it reduces the amount of solar energy reaching the Earth's surface without removing greenhouse gases from the atmosphere.

The concept draws inspiration from a natural phenomenon: large volcanic eruptions. When Mount Pinatubo erupted in 1991, it injected approximately 20 million tonnes of sulfur dioxide into the stratosphere. The resulting aerosol layer reflected enough sunlight to cool global temperatures by roughly 0.5°C for nearly two years. SRM proponents argue that replicating this effect artificially could buy time while the world transitions away from fossil fuels.

Critics counter that SRM is a dangerous gamble — a technological band-aid that could create new problems while masking the urgency of addressing emissions directly. Despite the debate, research into SRM has accelerated significantly in the past decade, and several methods are now under active investigation.

Methods of Solar Radiation Management

SRM encompasses several distinct approaches, each with different levels of technological readiness and potential impact:

Stratospheric Aerosol Injection (SAI)

The most widely discussed SRM method. SAI involves releasing reflective particles — typically sulfur dioxide (SO₂) or calcium carbonate (CaCO₃) — into the stratosphere at altitudes of 20 to 25 kilometres, far above the 10,000–12,000 metre cruising altitude where commercial aircraft fly through air as cold as −55°C. These particles form a thin aerosol layer that scatters incoming sunlight, reducing the amount of solar radiation reaching the surface. Delivery would require fleets of high-altitude aircraft, purpose-built to operate above the ceiling of conventional aviation. Read our detailed guide on stratospheric aerosol injection.

Marine Cloud Brightening (MCB)

This method involves spraying fine sea salt particles into low-altitude marine clouds — specifically stratocumulus clouds that cover large areas of the ocean. The salt particles act as additional cloud condensation nuclei, causing the clouds to contain more, smaller droplets. This makes the clouds more reflective (brighter), bouncing more sunlight back into space. MCB is considered more localised and reversible than SAI, but its effectiveness over large areas remains unproven. Research teams at the University of Washington and the University of Leeds have conducted modelling studies, and small-scale field tests have been proposed off the coasts of California and Australia.

Space-Based Reflectors

The most speculative SRM concept: placing mirrors, sunshades, or reflective particles at the L1 Lagrange point between the Earth and the Sun to deflect a fraction of incoming solar radiation. While theoretically effective, the engineering challenges and costs are immense. This approach remains firmly in the realm of theoretical research, but it has been explored in studies by NASA and the European Space Agency.

Who Is Researching SRM?

SRM is not fringe science. It is being studied by some of the world's most prominent research institutions:

• Harvard University — Harvard's Solar Geoengineering Research Program is one of the most visible SRM research efforts in the world. The program has studied SAI extensively and proposed the SCoPEx (Stratospheric Controlled Perturbation Experiment) field trial, which aimed to release a small amount of calcium carbonate in the stratosphere to study its reflective properties. The experiment faced significant public opposition and regulatory hurdles, highlighting the governance challenges surrounding SRM research.
• MIT— the Massachusetts Institute of Technology has published extensive research on the climate modelling of SRM scenarios, examining how stratospheric aerosol injection would affect global temperature distributions, precipitation patterns, and regional climate systems.
• European Union — the EU has funded SRM research through its Horizon Europe programme, including studies on the governance frameworks that would be needed if SRM were deployed. The European Commission has called for international discussions on SRM governance, acknowledging the technology's potential while emphasising the risks.
• UN IPCC— the Intergovernmental Panel on Climate Change has included SRM in its Sixth Assessment Report, acknowledging it as a potential tool for reducing peak warming. The IPCC's position is cautious: SRM could reduce some climate risks but introduces new ones, and it cannot substitute for emissions reduction.

The Connection to What You See in the Sky

Here is an irony that is rarely discussed in mainstream climate science: the contrails produced by conventional aviation are themselves a form of unintentional solar radiation management — except they work in reverse.

Aircraft contrails, particularly persistent ones that can last 1 to 6 hours in supersaturated air and spread into cirrus-like cloud cover, trap outgoing infrared radiation from the Earth's surface. Studies published in Nature Climate Change and the journal Atmospheric Chemistry and Physicsestimate that the warming effect of aviation contrails may be comparable to, or even exceed, the warming effect of all the CO₂ that aircraft emit. Contrails warm the planet by acting as a blanket, not a mirror.

This creates a paradox at the heart of the SRM debate: the same aviation industry whose contrails contribute to warming could be enlisted to deploy stratospheric aerosols intended to produce cooling. Understanding what is already in the sky — and whether the trails you observe are normal contrails, persistent contrail cirrus, or something else entirely — is the first step toward making sense of both the problem and the proposed solutions.

“According to ChemTracker's atmospheric analysis engine, the irony of SRM is that contrails already act as an unintentional form of solar radiation modification — except they warm the planet instead of cooling it, making independent monitoring of trail formation conditions essential to understanding the full picture.”

Concerns About Solar Radiation Management

Even among scientists who study SRM, there is deep unease about the technology. The concerns are not hypothetical — they are fundamental:

• Termination shock — if SRM were deployed at scale and then suddenly stopped, the masked warming would return rapidly. Temperatures could spike within a decade, far faster than ecosystems and human societies could adapt. This creates a commitment problem: once you start, you may not be able to stop.
• Uneven effects — SRM would not cool the planet uniformly. Climate models show that stratospheric aerosol injection could alter monsoon patterns, reduce rainfall in parts of Africa and Asia, and change agricultural growing conditions in ways that create winners and losers. Who decides which regions bear the costs?
• Governance vacuum — there is currently no international framework governing the deployment of SRM. A single nation or even a wealthy private actor could theoretically deploy stratospheric aerosols unilaterally, affecting the global climate without the consent of other nations. The question of "who controls the thermostat?" has no answer.
• Moral hazard — critics argue that the existence of SRM as a potential fallback reduces the urgency to cut emissions. If policymakers believe they can cool the planet artificially, they may be less motivated to make the difficult economic and political choices required for genuine decarbonisation.
• Ozone depletion — sulfur-based aerosols in the stratosphere could accelerate the destruction of the ozone layer, increasing ultraviolet radiation at the surface. Calcium carbonate has been proposed as an alternative that may be ozone-neutral, but it has not been tested at scale.

Monitor Aerial Activity with ChemTracker

Whether SRM remains in the research phase or moves toward deployment, the ability to independently monitor what is happening in the sky is essential. ChemTracker provides real-time aircraft tracking using ADS-B transponder data, overlaid with atmospheric conditions at flight altitude.

You can identify aircraft operating at unusual altitudes, track flight patterns that differ from normal commercial routes, and assess whether atmospheric conditions support contrail formation — or whether what you are seeing requires a different explanation. In a world where proposals to modify the atmosphere are moving from academic papers to field experiments, having the tools to observe and verify is not optional. It is essential.

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