Cirrus cloud thinning (CCT) is a proposed method of climate change solutions. Cirrus clouds are high, cold clouds made of ice. Like other clouds, they reflect sunlight and absorb heat-trapping radiation. However, cirrus clouds differ because they absorb more heat-trapping radiation than they reflect sunlight, which leads to warming the climate. Making these clouds thinner or removing them could reduce their ability to trap heat, which might cool Earth's climate. This could be a way to help reduce global warming. CCT is a type of climate change solution, separate from methods that manage sunlight.
In 2021, the IPCC described CCT as a plan "to reduce the amount of cirrus clouds by adding substances that help ice form in the upper part of the atmosphere." However, the IPCC noted that scientists have limited understanding of how cirrus clouds work, how they interact with tiny particles in the air, and how to best use seeding methods. CCT might also increase rainfall worldwide.
Basic principles
Cirrus clouds can be changed to shorten their lifespan and reduce how much light they block, which lowers their effect on warming Earth compared to lower, warmer clouds. Materials to help this change could be sent by drones or planes. Scientists think cirrus clouds high in the atmosphere form when water freezes without help from other materials, creating many tiny ice crystals. If special materials called ice nuclei are added, the clouds might form when freezing happens with help from these materials instead. If the number of ice nuclei is kept low, the ice particles in the clouds would grow larger because there is less competition for water vapor, making them fall faster. Adding aerosols could make ice crystals grow quickly, using up water vapor and stopping new ice crystals from forming. This would make the clouds less thick and shorter-lived, allowing more heat to escape from Earth’s atmosphere as ice particles fall. Less water vapor and heat in the upper atmosphere would cool the climate.
Bismuth tri-iodide (BiI3) has been suggested as a material to help change clouds because it works well as an ice nucleus at temperatures below -10 °C, is not harmful, and is cheaper than materials like silver iodide. The seeding materials would need to be added often because they would fall out of the atmosphere 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 methods rely on reducing the thickness of cirrus clouds, which are high in the atmosphere. However, scientists do not fully understand how cirrus clouds interact with aerosols and other factors that influence their formation and behavior.
One important process, called heterogeneous freezing, is not well studied. This involves ice forming on surfaces other than pure water, but details about how quickly ice grows in these conditions are unclear. Vertical air movement, which helps ice nuclei become active, is also not well understood because of limited observations. If heterogeneous freezing is already common in cirrus clouds, it might reduce the cooling benefits of thinning these clouds. 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, known as "over-seeding," might cause warming instead of cooling. Studies on cirrus cloud thinning show mixed results about its potential and effectiveness.
Because climate models do not accurately represent how ice crystals form in cirrus clouds, some research uses a simplified method by increasing the speed at which ice crystals fall below -38°C, the temperature where ice forms without needing a surface.
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 damage caused by rising carbon dioxide levels.