Solar geoengineering: the risks and the reality of dimming the sun

01 What solar radiation management actually does

Solar radiation management (SRM), also known as solar geoengineering, proposes to counteract global warming by reflecting a fraction of incoming sunlight back into space before it can warm the Earth's surface. Unlike emissions reduction, which addresses the root cause, SRM treats the symptom: it lowers the temperature without removing greenhouse gases from the atmosphere.

The mechanism is physically straightforward. Sunlight entering the atmosphere is partially absorbed by the Earth's surface, warming the planet, and partially reflected back into space. The fraction reflected is called albedo. SRM aims to increase the Earth's albedo by introducing reflective particles into the upper atmosphere or brightening clouds over the oceans. Even a 1-2 percent reduction in incoming sunlight could offset several degrees of warming.

The most studied method is stratospheric aerosol injection (SAI): spraying sulfur dioxide particles into the stratosphere at altitudes of 15-25 kilometers. The particles scatter sunlight, creating a cooling effect similar to what happens after major volcanic eruptions. The 1991 eruption of Mount Pinatubo injected roughly 20 million tons of sulfur dioxide into the stratosphere, and global temperatures dropped by approximately 0.5 degrees Celsius for over a year.

02 The historical precedent volcanic eruptions

Volcanic eruptions are the natural proof of concept for SRM. When Mount Pinatubo erupted in the Philippines in 1991, it produced the largest stratospheric sulfur dioxide injection of the 20th century. The cooling effect was measurable worldwide: global mean temperatures fell by about 0.5 degrees Celsius within months and remained depressed for roughly two years. The effect then faded as the particles settled out of the stratosphere.

Earlier eruptions tell a similar story. The 1815 eruption of Mount Tambora caused the Year Without a Summer in 1816, when temperatures across the Northern Hemisphere dropped enough to cause crop failures from New England to Europe. Snow fell in June in New England, and famine spread across parts of Switzerland and Ireland. The PBS Terra video draws a direct parallel: if volcanic aerosols caused famine historically, deliberate aerosol injection could do the same.

The key insight from the volcanic record is that the cooling is not uniform. Pinatubo altered tropical circulation patterns, shifting precipitation and causing drought in some regions while increasing rainfall in others. This is the central concern with SRM: a global temperature reduction would not translate into uniform, beneficial effects across all regions. Some regions would see less warming relief than others, and some would experience disruptive changes in rainfall.

03 Potential side effects on weather patterns

The most robust climate model findings show that SRM would reduce average global temperatures but would not perfectly reverse the effects of greenhouse warming. The reason is fundamental: greenhouse gases trap heat uniformly, while SRM reduces incoming sunlight primarily in the tropics and during daytime. The spatial and temporal mismatch means that even if average temperature is restored, regional climate patterns would differ from a pre-warming baseline.

Precipitation changes are the most sensitive variable. Models consistently show that aggressive SRM would reduce global mean precipitation, because a cooler atmosphere holds less moisture. The Indian monsoon, which feeds over a billion people, is particularly sensitive to stratospheric aerosol loading. Some models suggest that moderate SRM could stabilize monsoon patterns disrupted by warming; others show that heavy SRM would weaken them, risking agricultural collapse in South Asia.

Ozone depletion is another concern. Sulfate aerosols provide surfaces for chemical reactions that destroy ozone, particularly in polar regions. The Montreal Protocol has spent decades repairing the ozone hole; large-scale SRM could partially reverse that progress. Additionally, SRM does nothing to address ocean acidification, the other major consequence of carbon dioxide emissions. Even with temperatures reduced, the oceans would continue to acidify, threatening marine ecosystems and food webs.

04 The governance and ethics problem

SRM's greatest challenge is not technical but political. There is no international treaty governing geoengineering deployment. The closest existing framework is the Convention on Biological Diversity, which adopted a de facto moratorium on geoengineering in 2010, but this is non-binding and has limited enforcement power. The Paris Agreement does not address SRM at all.

The ethical problem is profound: who gets to set the global thermostat? A deployment that benefits the United States or China might harm India or Brazil. A country experiencing catastrophic heatwaves might deploy SRM unilaterally, imposing climate changes on the rest of the world without consent. There is no mechanism — not in the UN, not in any existing international body — to adjudicate such a conflict.

The termination problem adds urgency to the governance gap. If SRM is deployed at scale and then stopped abruptly — due to war, economic collapse, or political decision — the warming that was masked would return within months, at a rate far faster than any natural climate shift. Ecosystems and human systems that had adapted to the reduced temperatures would face temperature increases of 0.5-1.0 degrees Celsius per decade, far exceeding their adaptive capacity. This means that once SRM is started, stopping it is potentially more dangerous than continuing.

05 Who is researching geoengineering

Research remains modest in scale but is growing. The largest programs are in the United States, where the National Oceanic and Atmospheric Administration (NOAA) and the Department of Energy have funded SRM modeling and small-scale measurement studies. The UK's Natural Environment Research Council supports geoengineering research through programs like the SPICE (Stratospheric Particle Injection for Climate Engineering) project, which conducted theoretical work and proposed (but ultimately canceled) a small-scale field test.

Harvard University's SCoPEx (Stratospheric Controlled Perturbation Experiment) was the highest-profile proposed field experiment: a balloon-borne platform that would release small amounts of calcium carbonate and other materials into the stratosphere to measure their scattering properties. The experiment was delayed repeatedly and ultimately suspended in 2024 after opposition from indigenous groups in Sweden, where the launch was planned. The controversy illustrated the governance challenge: even small-scale research was politically untenable.

China has invested in geoengineering research, with modeling work at Beijing Normal University and the Chinese Academy of Sciences. The European Commission has funded geoengineering governance studies through programs like Geoengineering Assessment and Research. Philanthropic funders, including the ClimateWorks Foundation and the Simons Foundation, have also supported research, but total global funding for SRM research remains under $100 million — a tiny fraction of what is spent on emissions reduction.

06 The risk of unilateral deployment

The most alarming scenario is unilateral deployment: a single nation, or even a wealthy individual, deploying SRM without international agreement. The technology is surprisingly accessible. Sulfur dioxide is cheap, the delivery systems (high-altitude aircraft or balloons) are commercially available, and the cost of a sustained global deployment has been estimated at $2-10 billion per year — within the reach of many nations and some private actors.

The PBS Terra video raises the question of whether geoengineering has already been deployed covertly. While there is no credible evidence of large-scale SRM deployment, the concern highlights how little visibility exists. Unlike nuclear weapons, which require uranium enrichment and detectable test signatures, SRM deployment could begin with modified commercial aircraft and would be difficult to distinguish from normal aviation emissions in its early stages.