Cloud feedback

Cloud feedback is a type of climate change feedback that has been difficult to quantify in climate models. Clouds can either amplify or dampen the effects of climate change by influencing Earth's energy balance. This is because clouds can affect the magnitude of climate change resulting from external radiative forcings. On the other hand, clouds can affect the magnitude of internally generated climate variability. Climate models represent clouds in different ways, and small changes in cloud cover in the models have a large impact on the predicted climate. Changes in cloud cover are closely coupled with other feedbacks, including the water vapor feedback and ice–albedo feedback.

Climate change is expected to change the distribution and type of clouds. This can relate to "the spectrum of cloud types, the cloud fraction and height, the radiative properties of clouds, and finally the Earth’s radiation budget".^: 2224 If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet.

Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect. But seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Differences in planetary boundary layer cloud modeling can lead to large differences in calculated values of climate sensitivity. A model that decreases boundary layer clouds in response to global warming has a climate sensitivity twice that of a model that does not include this feedback. However, satellite data shows that cloud optical thickness actually increases with increasing temperature. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud; details that are difficult to represent in climate models.

In preparation for the 2021 IPCC Sixth Assessment Report, a new generation of climate models have been developed by scientific groups around the world. The average estimated climate sensitivity has increased in Coupled Model Intercomparison Project Phase 6 (CMIP6) compared to the previous generation. Values range from 1.8 to 5.6 °C (3.2 to 10.1 °F) across 27 global climate models. The cause of the increased equilibrium climate sensitivity (ECS) lies mainly in improved modelling of clouds. Temperature rises are now believed to cause sharper decreases in the number of low clouds, and fewer low clouds means more sunlight is absorbed by the planet and less reflected to space.