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. 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.
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.
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. 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.
How clouds affect radiation and climate feedback
Clouds have two major effects on the Earth's energy budget. Firstly, they reflect shortwave radiation from sunlight back to space due to their high albedo - a cooling effect for the Earth. Secondly, the condensed and frozen water contained inside them absorbs longwave radiation emitted by the Earth's surface. Clouds themselves also emit longwave radiation, both towards the surface and to space. Clouds are usually colder than the surface, so that they emit less energy upward. The net longwave effect is that the presence of clouds reduces the radiation emitted to space, i.e. a warming effect.
In meteorology, the difference in the radiation budget caused by clouds, relative to cloud-free conditions, is described as the cloud radiative effect (CRE). This is also sometimes referred to as cloud radiative forcing (CRF). However, since cloud changes are not normally considered an external forcing of climate, CRE is the most commonly used term.
This can be described by the equation
Where CRE is cloud radiative effect (W m−2), Rall-sky is the radiation flux (W m−2) under actual sky conditions, and Rclear-sky is a hypothetical radiation flux (W m−2) computed for the identical temperature and moisture conditions but omitting the optical effects of clouds.
Cloud feedback is one of a number of climate feedbacks. Cloud feedback sums up the influence of all aspects of the cloud field on radiation, weighted by the sensitivity of each aspect to global average temperature change. In equation form,
where N is the Earth's net radiation (W m−2), is the change in some aspect or characteristic of cloudiness (e.g. cloud cover, thickness, particle sizes, height), and T is the global mean near-surface air temperature (K).
On a hypothetical cloud-free Earth, water vapor would contribute 67% and CO2 24% of the greenhouse effect keeping the planet warmer than it would be without an atmosphere. In actual (all-sky) conditions, clouds contribute 25%, and their screening effect lowers the vapor and CO2 contributions to 50% and 19% respectively., D20106. Web page According to 1990 estimates, the presence of clouds reduces the outgoing longwave radiation by about 31 W/m2. However, it also increases the global albedo from 15% to 30%, and this reduces the amount of solar radiation absorbed by the Earth by about 44 W/m2. Thus, the observed global population of clouds contributes a net cooling of about 13 W/m2.table 3.1 If all clouds were removed with all else remaining the same, the Earth would lose this much cooling and the global temperatures would increase.
Climate change increases the amount of water vapor in the atmosphere due to the Clausius–Clapeyron relation, in what is known as the water-vapor feedback. It also affects a range of cloud properties, such as their height, the typical distribution throughout the atmosphere, and cloud microphysics, such as the amount of water droplets held, all of which then affect clouds' radiative forcing. Differences in those properties change the role of clouds in the Earth's energy budget. The name cloud feedback refers to this relationship between climate change, cloud properties, and clouds' radiative forcing.IPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.)]. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022. Clouds also affect the magnitude of internally generated climate variability.
Cloud feedback mechanisms
Low clouds
Low clouds include the cumulus, stratocumulus and stratus cloud types. Scientifically they tend to be defined as those clouds with cloud top pressure higher than 680 hPa, according the to International Satellite Cloud Climatology Project. The feedback of low clouds primarily arises from effects on shortwave radiation.
Tropical marine low-cloud feedback Multiple lines of evidence, including global climate models, observations and high resolution process modelling, agree that the tropical low cloud amount is likely to decrease, which corresponds to a positive feedback.
Possible break-up of equatorial stratocumulus clouds
In 2019, a study employed a large eddy simulation model to estimate that equatorial stratocumulus clouds could break up and scatter when levels rise above 1,200 ppm (almost three times higher than the current levels, and over 4 times greater than the preindustrial levels). The study estimated that this would cause a surface warming of about globally and in the subtropics, which would be in addition to at least already caused by such concentrations. Stratocumulus clouds would not reform until the concentrations drop to a much lower level.
It was suggested that this finding could help explain past episodes of unusually rapid warming such as Paleocene-Eocene Thermal Maximum. In 2020, further work from the same authors revealed that in their large eddy simulation, this tipping point cannot be stopped with solar radiation modification: in a hypothetical scenario where very high emissions continue for a long time but are offset with extensive solar radiation modification, the break-up of stratocumulus clouds is simply delayed until concentrations hit 1,700 ppm, at which point it would still cause around of unavoidable warming.
However, because large eddy simulation models are simpler and smaller-scale than the general circulation models used for climate projections, with limited representation of atmospheric processes like subsidence, this finding is currently considered speculative. Other scientists say that the model used in that study unrealistically extrapolates the behavior of small cloud areas onto all cloud decks, and that it is incapable of simulating anything other than a rapid transition, with some comparing it to "a knob with two settings". Additionally, concentrations would only reach 1,200 ppm if the world follows Representative Concentration Pathway 8.5, which represents the highest possible greenhouse gas emission scenario and involves a massive expansion of coal infrastructure. In that case, 1,200 ppm would be passed shortly after 2100.
Mid-latitude marine low-cloud feedback There is both observational and modelling evidence that a positive mid-latitude low-cloud feedback is feasible. In part, such a feedback could arise for similar reasons to the tropical marine low-cloud feedback. In addition, a poleward shift of mid-latitude Storm tracks would reduce the solar radiation interacting with low cloud and result in a positive feedback.
High-latitude low-cloud optical depth feedback The optical depth (or opacity) of cloud can increase if the number of cloud particles increases for given water content, or the water content increases. Related to this, a shift from liquid cloud particles to ice cloud particles tends to correspond to a shift from more numerous smaller particles to fewer larger particles, and therefore can decrease optical depth. A number of studies have explored the potential for high-latitude cloud optical depth to contribute to climate feedback. However, there is not clear evidence that a non-zero feedback exists for this cloud type. Land clouds Land clouds can include clouds types of differing heights.