Cloud feedback
A cloud feedback is a climate change feedback where some aspects of cloud characteristics (e.g. cloud cover, composition or height) are altered due to climate change, and these changes then further affect the Earth’s energy balance.: 2224 On their own, clouds are already an important part of the climate system, as they consist of liquid droplets and ice particles, which absorb infrared radiation and reflect visible solar radiation. Clouds at low altitudes have a stronger cooling effect, and those at high altitudes have a stronger warming effect. Altogether, clouds make the Earth cooler than it would have been without them.: 1022
If climate change causes low-level cloud cover to become more widespread, then these clouds will increase planetary albedo and contribute to cooling, making the overall cloud feedback negative (one that slows down the warming). Vice versa, if they change in such a way that their warming effect increases relative to their cooling effect then the net cloud feedback, then the net cloud feedback will be positive and accelerate the warming, as clouds will be less reflective and trap more heat in the atmosphere.
There are many mechanisms by which cloud feedbacks occur. Most substantially, evidence points to climate change causing high clouds to rise in altitude (a positive feedback), the coverage of tropical low clouds to reduce (a positive feedback) and polar low clouds to become more reflective (a negative feedback). Aside from cloud responses to human-induced warming through greenhouse gases, the interaction of clouds with aerosol particles is known to affect cloud reflectivity, and may modulate the strength of cloud feedbacks. Cloud feedback processes have been represented in every major climate model from the 1980s onwards. Observations and climate model results now provide high confidence that the overall cloud feedback on climate change is positive.: 95
Cloud feedbacks are estimated using both observational data and climate models. Uncertainty in both these aspects - for example, incomplete observational data or uncertainty in the representation of processes in models mean that cloud feedback estimates differ substantially between models. Thus, models can simulate cloud feedback as very positive or only weakly positive, and these disagreements are the main reason why climate models can have substantial differences in transient climate response and climate sensitivity.: 975 In particular, a minority of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models have made headlines before the publication of the IPCC Sixth Assessment Report (AR6) due to their high estimates of equilibrium climate sensitivity (ECS). This had occurred because they estimated cloud feedback as highly positive. Although those particular models were soon found to contradict both observations and paleoclimate evidence, it is suggested to be problematic if ruling out these 'hot' models solely based on ECS and care should be taken when weighting climate model ensembles by temperature alone.
One reason why constraining cloud feedbacks has been difficult is because humans affect clouds in another major way besides the warming from greenhouse gases. Small atmospheric sulfate particles, or aerosols, are generated due to the same sulfur-heavy air pollution which also causes acid rain, but they are also very reflective, to the point their concentrations in the atmosphere cause reductions in visible sunlight known as global dimming. These particles affect the clouds in multiple ways, mostly making them more reflective through aerosol-cloud interactions. This means that changes in clouds caused by aerosols can be confused for an evidence of negative cloud feedback, and separating the two effects has been difficult.