Could SRM Be Weaponised?

In 1972, news broke that the United States military had been trying to induce rainfall using cloud seeding¹ to wash out and disrupt the Ho Chi Minh Trail, a key supply line for communists fighting in South Vietnam. Operational results were mixed, but the furore that accompanied revelations about these activities ultimately led countries to sign the Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques, or ENMOD, in 1977, which outlawed such practices. 

This episode and others like it have helped fuel a lingering concern that SRM, which aims to reduce global warming by reflecting sunlight back to space, might somehow be used as a weapon. How serious is this concern? Answering this requires understanding what makes a weapon a weapon. 

What makes for an effective weapon? 

An essential characteristic of a weapon is precision.² To be effective, a weapon must be capable of producing a specific destructive or disruptive effect, at a specific time and place. 

Imprecise targeting can lead to unpredictable and undesirable results like excessive collateral damage, “friendly fire” incidents, or mistaken strikes. One reason armies rarely use chemical weapons is because they are difficult to control – once released into the air, poison gas drifts wherever the wind takes it, which can include over friendly troops. 

Uncertain timing can also undermine the dependability of weapons. Among the reasons militaries avoid using biological weapons is that their effects may take days or weeks to materialise. Infections, diseases, and illnesses spread at different speeds along unpredictable trajectories that are difficult to match to changing battlefield conditions. 

And unexpected outcomes can compromise usefulness. One of the reasons the United States stopped using herbicides like Agent Orange during the Vietnam War was due to their unforeseen side effects – in addition to defoliating jungles, these chemicals also resulted in widespread health problems.

Stratospheric aerosol injection – too imprecise 

SAI would involve releasing tiny particles in the upper atmosphere to reflect a small fraction of incoming sunlight and reduce temperatures. The climate effects of SAI have been studied in some depth and it is clear that it would be imprecise in space, in time, and in its potentially destructive effects.³ 

Aerosols (tiny reflective particles) released as part of an SAI intervention would not stay fixed in place, but instead would quickly encircle the planet and drift toward the poles.⁴ Any attempt to target a single country or group of countries would end up affecting all countries – enemies and allies alike – as well as whichever state used SAI. 

The fact that SAI aerosols would persist in the atmosphere for one to two years means that shorter interventions would not be possible.⁴ It also means that an intervention would continue for one to two years following a decision to halt it. 

SAI effects themselves would have uncertain consequences. SAI would affect the climate – long-term temperature and rainfall trends – but, due to the long timescale and large spatial scale of the intervention, it would not allow for control over specific weather events.³ This means that if SAI were able to increase the likelihood of flooding in a target region, it would not lead to the predictable occurrence of a specific flood, let alone particular flood impacts (like making specific routes impassable) that an attacker could count on to help it in a conflict. 

In sum, the inability to pinpoint targets, complete strikes over short periods, and inflict damage in a predictable way would make SAI ineffective as a weapon. 

Marine cloud brightening and cirrus cloud thinning – more localised but unpredictable 

Compared to SAI, MCB and CCT are less well understood. MCB would involve spraying seawater into low-lying marine clouds to increase their reflectivity. CCT would entail seeding certain high-altitude cirrus clouds to facilitate increased heat flow out of the atmosphere.  

Both MCB and CCT would be more localised than SAI, in the sense that their effects would be much more confined in space and time. They could cover much smaller areas (from kilometres across) and operate over much shorter timescales (days).⁵ At least in principle, greater precision in space and time would mean that both MCB and CCT have more potential for weather control. 

However, the immediate, local cooling effects of MCB and CCT would be limited in their scope, only working where certain conditions are met, e.g., where susceptible clouds are present.⁶ More problematic is the fact that the localised direct effects of MCB and CCT would also trigger remote, indirect effects via “teleconnections”, where a change in the climate of one place can lead to a change in climate thousands of miles away.⁵ 

Teleconnections can be difficult to identify and their consequences challenging to predict. Using MCB or CCT to influence local weather conditions could have unforeseen and possibly undesirable effects far away from the intervention site. In practice, this unpredictability would make both MCB and CCT unsuitable for use as weapons. 

Not weaponisable, but still a potential source of instability 

It is difficult to see how any leading SRM idea could be used as a weapon of war. Unlike guns, bombs, missiles, and a staggering array of other technologies, SRM simply lacks the precision, accuracy, and reliability that are essential elements of effective weapons. 

Yet while SRM may not be weaponisable, it could still cause international instability. For instance, if one country felt that its rights were violated by another country using SRM, the “victim” country might threaten to attack the “aggressor” country.  

However, its potential effects on international relations are uncertain. To the extent that climate change acts as a “threat multiplier”, exacerbating international tensions over economic, political, and security issues, offsetting some climate impacts through SRM might even enhance security.