Solar geoengineering: the case for and against blocking sunlight to cool Earth

01What solar radiation management is

Solar radiation management, or SRM, is a form of geoengineering that aims to reduce global temperatures by reflecting a fraction of incoming sunlight back into space. Solar radiation modification (SRM), also called solar geoengineering, is a group of large-scale approaches to reduce global warming by increasing the amount of sunlight that is reflected away from Earth and back to outer space. Unlike emissions reduction, which addresses the root cause, SRM treats the symptom — it cools the planet without removing greenhouse gases from the atmosphere.

The concept draws inspiration from volcanic eruptions. When Mount Pinatubo erupted in 1991, it injected roughly 20 million tonnes of sulfur dioxide into the stratosphere, and global temperatures dropped by about half a degree Celsius for two years. SRM seeks to replicate this effect deliberately and continuously, maintaining a reflective aerosol layer in the upper atmosphere.

02How stratospheric aerosol injection works

Stratospheric aerosol injection, or SAI, is the most studied SRM method. Stratospheric aerosol injection (SAI) is a proposed method of solar geoengineering to reduce global warming. Aircraft or specialized delivery systems would spray sulfur dioxide or other aerosol precursors into the stratosphere at altitudes of 18 to 30 kilometers, where the particles would disperse and form a reflective haze.

The amount of cooling is roughly proportional to the amount of aerosol injected. Climate models suggest that injecting one to two million tonnes of sulfur per year could offset roughly one degree of warming. The injection would need to be continuous — aerosols settle out of the stratosphere within a few years, so stopping the treatment would cause warming to resume rapidly.

03The potential benefits and cooling effect

The appeal of SRM is speed and cost. While emissions reduction takes decades to show climate effects and carbon removal is expensive and slow, SRM could lower global temperatures within months of deployment at a fraction of the cost. Climate models consistently show that moderate SRM could reduce temperature extremes, slow sea-level rise, and decrease the frequency of the most severe heat events.

For regions facing existential climate threats — small island states, countries in the Sahel, and areas experiencing lethal wet-bulb temperatures — the speed of SRM is not an abstraction. It is a potential lifeline that could buy time for emissions reduction and carbon removal to take effect. The ethical argument for research is that the alternative — allowing warming to proceed unchecked — may be worse.

04The risks and unknown consequences

The risks are substantial and poorly understood. SRM would alter regional precipitation patterns, potentially disrupting monsoons that billions of people depend on for agriculture. It would not address ocean acidification, which is driven by carbon dioxide regardless of temperature. The effects on the ozone layer, stratospheric chemistry, and high-altitude ecosystems remain areas of active — and contentious — research.

Perhaps the greatest risk is political. If a single country or even a wealthy individual decides to deploy SRM unilaterally, there is no international mechanism to stop them. The global nature of stratospheric aerosols means that the consequences — intended and unintended — would be distributed worldwide, with no way to confine the effects to the deploying nation's territory.

05The governance and who decides

The governance gap is the field's defining challenge. Geoengineering is the deliberate large-scale interventions in the Earth’s climate system intended to counteract human-caused climate change. There is no international treaty governing SRM deployment, no body authorized to approve or reject it, and no mechanism for compensating nations that experience adverse effects. The UN Framework Convention on Climate Change does not explicitly address geoengineering, and the Convention on Biological Diversity has called for restraint without enforcement power.

Some researchers advocate for a moratorium on deployment paired with permitted research. Others argue that governance must be developed before, not after, the technology matures — because the first deployment will set precedents that are difficult to reverse. The question of who gets a voice — affected nations, indigenous communities, future generations — has no settled answer.

06What termination shock means

Termination shock is the scenario that haunts every SRM discussion. If aerosol injection is sustained for decades and then stopped abruptly — by war, economic collapse, or political change — the cooling effect would disappear within a few years. Global temperatures would rise far faster than under any realistic emissions scenario, devastating ecosystems and human systems that had adapted to the cooled climate.

The risk of termination shock means that once SRM is deployed, the world becomes committed to maintaining it. This creates a dependency: the aerosol layer must be replenished continuously, and the infrastructure to do so must survive political upheaval, conflict, and economic stress. A technology intended as a temporary fix could become a permanent obligation.

07Where geoengineering research stands

Research remains in early stages. Outdoor experiments — small-scale sulfur releases, marine cloud brightening trials — have faced public opposition and regulatory hurdles. The Harvard-backed SCoPEx project was delayed and ultimately suspended following opposition from indigenous groups in Sweden, where the first test was planned. Indoor research — climate modeling, laboratory studies, and social science investigations — continues to advance.

The field is split between those who argue that more research is urgently needed to inform future decisions and those who worry that research normalizes deployment and creates a slippery slope. What both sides agree on is that the current trajectory of warming makes the question of whether to deploy increasingly urgent — and increasingly difficult to defer.