Cirrus Cloud Thinning

CCT is an idea to decrease the lifetime of some cirrus clouds to allow more heat to escape to space. CCT would work by seeding regions where cirrus clouds would have formed without seed particles. In principle, seeding would produce thinner, shorter-lived cirrus clouds, reducing their warming effect and cooling the planet.¹ There are uncertainties in the feasibility and risks of this cooling strategy, and there are currently no practical ideas for its implementation. 

The climate is changing due to the build-up of greenhouse gases, especially carbon dioxide, in the atmosphere, primarily from the burning of fossil fuels. These greenhouse gases warm the planet by preventing heat from escaping to space.  

Most sunlight reflection methods, e.g., stratospheric aerosol injection and marine cloud brightening, would counteract the warming effects of greenhouse gases by increasing the amount of sunlight reflected to space.  

In contrast, CCT would lower temperatures by increasing the amount of heat escaping to space. This cooling mechanism would more directly offset the heat-trapping effect of elevated greenhouse gas concentrations.¹ 

Researchers have hypothesised that this approach to cooling might be preferable as it might counteract some of the changes in rainfall associated with the heat-trapping effect of greenhouse gases.² 

Seeding cirrus clouds to shorten their lifespan

All clouds reflect some sunlight back to space and prevent some heat from leaving the atmosphere. Cirrus clouds are wispy, high clouds that form from ice at very cold temperatures (below −35°C), and trap heat more effectively than they reflect sunlight, leading to a net warming effect.³

Since cirrus clouds trap heat and reflect some sunlight, their heating effect is greatest in darker places. This means CCT would be most effective at darker times of the year and at high latitudes. Limiting CCT to high-latitude clouds during winter would maximise its cooling effect.⁵ 

CCT would only work on certain cirrus clouds. Some cirrus clouds form when ice crystals grow around “seed” particles, such as dust or other solid particles suspended in the air. These cirrus clouds form at higher temperatures and lower altitudes, and they are relatively thin and short-lived​​​​.⁶ CCT would not work on these cirrus clouds​​.⁷ 

Other cirrus clouds form without seed particles. Without these seed particles, the air must get much colder to start forming ice crystals. However, when this process does occur, smaller ice crystals form, leading to thicker, longer-lived cirrus clouds. As a result, unseeded cirrus clouds are thicker and more effective at trapping heat.⁸

CCT would target these unseeded cirrus clouds, adding tiny particles of silver iodide or bismuth tri-iodide which would stimulate the formation of ice crystals.⁷ Just as in the naturally seeded clouds, these modified cirrus clouds would form fewer, larger ice crystals, leading to thinner, shorter-lived cirrus clouds, allowing more heat to escape to space.

Substantial uncertainties around CCT’s effectiveness and consequences 

Climate model studies suggest that, at best, cooling from CCT might offset most of the warming seen to date, but not more,⁹ though the cooling would be patchy, stronger in some places and weaker in others.⁵ However, there are substantial uncertainties, and if done wrong, it may even cause warming.¹⁰ 

Researchers are unsure what fraction of cirrus clouds form with and without seed particles in today’s climate.¹¹ This fundamental uncertainty makes it difficult to know how effective CCT could be, which regions should be seeded, and how much material is needed. 

Adding particles to the wrong places or adding too many particles – “overseeding” – could cause cirrus clouds to form in places they would not have or could make them thicker. This would be counterproductive, producing warming, rather than cooling.¹² 

Even if it works, CCT would produce an uneven cooling effect. This would lead to shifts in rainfall patterns and could cause increases in extreme rainfall in some regions and increases in droughts in others.^13,14 

It is unclear if CCT would work 

Cirrus clouds have a net warming effect, and CCT may offer a way to reduce that warming effect. It is unique among sunlight reflection methods¹⁵ in that it would decrease the amount of heat the Earth’s atmosphere traps, more closely offsetting the warming effect of greenhouse gases. 

All studies of CCT to date have been idealised climate modelling studies, providing insights into cirrus cloud formation and processes.¹ There are currently no detailed engineering ideas for how to achieve CCT in practice, nor have there been any field tests.  

Whether and how CCT could be implemented has yet to be studied in detail, so it is not clear if this idea would work in practice.