Geoengineering risks: did it already cause damage and what we need to know

01The evidence for unintended geoengineering effects

Solar radiation modification—injecting reflective particles into the stratosphere to reduce incoming sunlight—is the most discussed form of geoengineering. It could lower global temperatures relatively quickly and cheaply, but the evidence is growing that the side effects would not be uniformly benign. Regional weather patterns, particularly monsoons that billions of people depend on for agriculture, could shift in ways that are hard to predict.

There is also evidence that past aerosol pollution, which was not intentional geoengineering, already altered weather systems. The cleaning of sulphur from ship fuel in 2020 reduced maritime aerosol emissions, and some researchers argue this sudden reduction may have contributed to record ocean warming by removing a masking effect. If true, this is a case where inadvertent geoengineering already had measurable consequences.

02What happened when geoengineering was attempted

Deliberate, large-scale geoengineering has not been deployed. However, smaller-scale experiments and natural analogues provide evidence. Volcanic eruptions that inject sulphur dioxide into the stratosphere cool the planet for a year or two, but they also disrupt precipitation patterns. The 1991 Mount Pinatubo eruption cooled global temperatures by about 0.5 degrees Celsius while reducing global rainfall and affecting monsoon systems across Asia and Africa.

A few small-scale outdoor experiments have been proposed or attempted, including stratospheric balloon tests that released small amounts of particles to measure dispersion. These were limited in scope and did not produce climate effects, but they revealed the depth of public and scientific concern about even modest field experiments. The backlash to several proposed tests illustrates that governance and consent are as significant a challenge as the technology itself.

03The risks of unilateral action

One of the most alarming features of solar geoengineering is that a single country—or even a wealthy individual—could deploy it unilaterally. The cost of stratospheric aerosol injection is estimated in the billions, not trillions, which puts it within reach of many national budgets and some private actors. This low barrier to entry means the world could face a deployed geoengineering programme without any international agreement on whether it should exist.

Unilateral deployment would create winners and losers. A deployment optimised to cool one region might shift rainfall away from another. The affected countries would have no recourse except diplomatic pressure or counter-deployment. This is not a hypothetical: countries have already disagreed publicly about whether geoengineering research should even proceed.

04How weather patterns were disrupted

Climate models consistently show that stratospheric aerosol injection would not simply reverse warming uniformly. It would cool the tropics less than the poles, shift the position of the jet stream, and alter monsoon systems. The Indian monsoon, on which hundreds of millions of people depend, is particularly sensitive. Some models show it weakening under certain deployment scenarios, while others show it shifting geographically.

The disruption is not just about total rainfall. It is about timing, intensity, and geographic distribution. A monsoon that arrives two weeks late or drops its rain in different locations can devastate agriculture even if the total annual precipitation is unchanged. These are regional effects that global average temperature masks, and they are the effects that would matter most to the people living under them.

05The scientific uncertainty and what we do not know

The scientific uncertainty around geoengineering is substantial. Climate models disagree on the magnitude and direction of regional effects. The response of stratospheric ozone to sustained aerosol injection is not fully characterised. The ecological impacts of reduced and diffused sunlight, which would change how plants photosynthesise, are poorly understood. The interaction between geoengineering and other climate feedbacks, such as methane release from permafrost, is largely unexplored.

This uncertainty cuts both ways. It could mean the side effects are less severe than models suggest, or it could mean they are worse. The responsible position is not that geoengineering is too dangerous to study, but that the gap between what we know and what we would need to know before deployment is very large. Closing that gap requires research that some argue is itself risky because it normalises deployment.

06The governance vacuum and who could act alone

There is no international treaty governing solar geoengineering deployment. The Paris Agreement does not address it. The Convention on Biological Diversity has a de facto moratorium that is non-binding and unenforceable. The absence of governance means that if someone decides to deploy, there is no legal framework to stop them, no mechanism to coordinate the response, and no institution to adjudicate disputes.

Several initiatives are working to fill this gap. The Climate Overshoot Commission recommended a moratorium on deployment alongside expanded research. Academic groups have proposed governance frameworks. But governance requires consensus among sovereign states, and the incentives are misaligned: countries that would benefit from cooling have different interests from those that would suffer from disrupted rainfall. The governance challenge may be harder than the technical one.

07What responsible geoengineering research looks like

Responsible research means several things. First, transparency: all outdoor experiments should be publicly registered with details of what is being released, where, and what is being measured. Second, independent oversight: experiments should be reviewed by bodies that include affected communities, not just scientists. Third, clear boundaries between research and deployment: small-scale experiments to understand atmospheric processes are different from operational cooling, and the line between them must be maintained.

Most importantly, geoengineering research must not become a substitute for emissions reduction. The risk of even discussing solar radiation modification is that it reduces the political pressure to cut emissions—a form of moral hazard. The responsible framing is that geoengineering might one day be needed as a supplement to, not a replacement for, decarbonisation. If that distinction is lost, the discussion itself becomes dangerous.