Cirrus cloud thinning (CCT) is a suggested method of climate engineering. Cirrus clouds are high, cold clouds made of ice. Like other clouds, they reflect sunlight and absorb heat radiation. However, cirrus clouds differ because, on average, they absorb more heat radiation than they reflect sunlight. This leads to a warming effect on Earth's climate. Thinning or removing these clouds could reduce their ability to trap heat, creating a cooling effect. This might help reduce global warming. CCT is a type of climate engineering, separate from solar radiation management.

In 2021, the IPCC described CCT as a proposal to reduce cirrus clouds by adding substances that help ice form in the upper part of the atmosphere. However, the IPCC reported low confidence in CCT's cooling effect because scientists do not fully understand how cirrus clouds behave, how they interact with tiny particles in the air, and how to best use seeding methods. CCT might also increase global rainfall.

Basic principles

Cirrus clouds, which are high in the atmosphere, can be changed to make them last shorter and appear less thick. This change would reduce how much heat they trap in the atmosphere, unlike lower clouds that hold more heat. Scientists think that cirrus clouds form when water freezes without help from special particles. If special particles called ice nuclei are added, the clouds might form when water freezes with help from these particles instead. If the number of ice nuclei is lower than usual, the ice particles in the clouds would grow larger because there would be less competition for water vapor. This would make the ice particles fall faster. Adding tiny particles called aerosols could make ice crystals grow quickly, using up water vapor and stopping the formation of new ice crystals. This would make the clouds less thick and shorter-lived, allowing more heat to escape from the top of the atmosphere as the ice particles fall. Less water vapor and heat in the upper atmosphere would help cool the Earth.

A material called bismuth tri-iodide (BiI3) has been suggested for this process. It works well as an ice nucleus when temperatures are below -10 °C, is not harmful, and is cheaper than other materials like silver iodide. The seeding particles would need to be added regularly because they would fall to the ground along with the large ice crystals.

Current research

Cirrus cloud thinning is most effective at high latitudes and when the sun is at a high angle in the sky, unlike solar radiation management techniques, which work best during the day at lower latitudes. These techniques are less effective in areas with high levels of aerosol particles in the air.

Scientists do not fully understand how cirrus clouds interact with aerosols and climate, which is important for thinning cirrus clouds. Processes like heterogeneous freezing, where ice forms on surfaces, are not well studied. The speed at which ice grows is also unclear. Vertical air movement plays a key role in activating ice nuclei, but observations are limited, making this uncertain. Heterogeneous freezing may already be common in cirrus clouds, which could reduce the cooling effect of thinning. Scientists are unsure about how ice forms in cirrus clouds, whether it happens through heterogeneous or homogeneous freezing, and how these processes are represented in climate models. Adding too many ice nuclei, called "over-seeding," might cause warming instead of cooling. Studies have debated the potential and effectiveness of cirrus cloud thinning.

Some models simplify cirrus cloud thinning by increasing the speed at which ice crystals fall, but this is only done below the temperature of -38 °C, where ice forms without needing aerosols.

Cirrus clouds may form in response to secondary organic aerosols, which are tiny particles created by natural plant activity.

Some climate models suggest that seeding cirrus clouds could reduce climate harm caused by rising carbon dioxide levels.