Cloud feedback

During daytime, clouds scatter incoming shortwave radiation from the Sun due to their albedo, which results in substantial cooling

Water vapor in the clouds also absorbs longwave radiation from the Earth's surface and reemits it back. This effect is weaker than the albedo cooling, but it is active day and night

Cloud feedback is a type of climate change feedback, where the overall cloud frequency, height, and the relative fraction of the different types of clouds are altered due to climate change, and these changes then affect the Earth's energy balance.^[1]: 2224 On their own, clouds are already an important part of the climate system, as they consist of water vapor, which acts as a greenhouse gas and so contributes to warming; at the same time, they are bright and reflective of the Sun, which causes cooling.^[2] 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.^[3]: 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). But if clouds become higher and thinner due to climate change, 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.^[2] These processes have been represented in every major climate model from the 1980s onwards.^[4][5][6] Observations and climate model results now provide high confidence that the overall cloud feedback on climate change is positive.^[7]: 95

However, some cloud types are more difficult to observe, and so climate models have less data about them and make different estimates about their role. 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.^[3]: 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.^[8][9] This had occurred because they estimated cloud feedback as highly positive.^[10][11] Those particular models were soon found to contradict both observations and paleoclimate evidence,^[12][13] and the AR6 used a more realistic estimate based on the majority of the models and this real-world evidence instead.^{[7]: 93 [14]}

One reason why it has been more difficult to find an exact value of cloud feedbacks when compared to the others 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.^[15] These particles affect the clouds in multiple ways, mostly making them more reflective. 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.^[16]