Cirrus cloud
Effects on climate

Cirrus clouds cover up to 25% of the Earth (up to 70% in the tropics at night) and have a net heating effect. When they are thin and translucent, the clouds efficiently absorb outgoing infrared radiation while only marginally reflecting the incoming sunlight. When cirrus clouds are 100 m (330 ft) thick, they reflect only around 9% of the incoming sunlight, but they prevent almost 50% of the outgoing infrared radiation from escaping, thus raising the temperature of the atmosphere beneath the clouds by an average of 10 °C (18 °F) —a process known as the greenhouse effect. Averaged worldwide, cloud formation results in a temperature loss of 5 °C (9 °F) at the earth's surface, mainly the result of stratocumulus clouds.

Cirrus clouds are likely becoming more common due to climate change. As their greenhouse effect is stronger than their reflection of sunlight, this would act as a self-reinforcing feedback. Metallic particles from human sources act as additional nucleation seeds, potentially increasing cirrus cloud cover and thus contributing further to climate change. Aircraft in the upper troposphere can create contrail cirrus clouds if local weather conditions are right. These contrails contribute to climate change.

Cirrus cloud thinning has been proposed as a possible geoengineering approach to reduce climate damage due to carbon dioxide. Cirrus cloud thinning would involve injecting particles into the upper troposphere to reduce the amount of cirrus clouds. The 2021 IPCC Assessment Report expressed low confidence in the cooling effect of cirrus cloud thinning, due to limited understanding.
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