Cloud feedback
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Cloud feedback is the coupling between cloudiness and surface air temperature where a surface air temperature change leads to a change in clouds, which could then amplify or diminish the initial temperature perturbation. Cloud feedbacks can affect the magnitude of internally generated climate variability or they can affect the magnitude of climate change resulting from external
radiative forcing Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured by watts / metre2. It is a scientific concept used to quantify and compare the external ...
s. Global warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Cloud representations vary among global climate models, and small changes in cloud cover have a large impact on the climate. Differences in planetary boundary layer cloud modeling schemes can lead to large differences in derived values of
climate sensitivity Climate sensitivity is a measure of how much Earth's surface will cool or warm after a specified factor causes a change in its climate system, such as how much it will warm for a doubling in the atmospheric carbon dioxide () concentration. In te ...
. 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 Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
. However, satellite data show that cloud
optical thickness In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through ...
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.


Other effects of cloud feedback

In addition to how clouds themselves will respond to increased temperatures, other feedbacks affect clouds properties and formation. The amount and vertical distribution of water vapor is closely linked to the formation of clouds. Ice crystals have been shown to largely influence the amount of water vapor. Water vapor in the subtropical upper troposphere has been linked to the convection of water vapor and ice. Changes in subtropical humidity could provide a negative feedback that decreases the amount of water vapor which in turn would act to mediate global climate transitions. Changes in cloud cover are closely coupled with other feedback, including the
water vapor feedback A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
and ice–albedo feedback. Changing climate is expected to alter the relationship between cloud ice and supercooled cloud water, which in turn would influence the microphysics of the cloud which would result in changes in the radiative properties of the cloud. Climate models suggest that a warming will increase fractional cloudiness. The albedo of increased cloudiness cools the climate, resulting in a negative feedback; while the reflection of infrared radiation by clouds warms the climate, resulting in a positive feedback. Increasing temperatures in the polar regions is expected in increase the amount of low-level clouds, whose stratification prevents the convection of moisture to upper levels. This feedback would partially cancel the increased surface warming due to the cloudiness. This negative feedback has less effect than the positive feedback. The upper atmosphere more than cancels negative feedback that causes cooling, and therefore the increase of CO2 is actually exacerbating the positive feedback as more CO2 enters the system. A 2019 simulation predicts that if greenhouse gases reach three times the current level of atmospheric carbon dioxide that stratocumulus clouds could abruptly disperse, contributing to additional global warming.


Cloud feedback in IPCC report

The
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) a ...
(IPCC) assessment reports contain a summary of the current status of knowledge on the effect of cloud feedback on climate models. The IPCC Fourth Assessment Report (2007) stated:IPCC Fourth Assessment Report: Climate Change 2007: ''Working Group I: The Physical Science Basis'', sectio
8.6.3.2 Clouds
(2007). Accessed 18 June 2016.
By reflecting solar radiation back to space (the albedo effect of clouds) and by trapping infrared radiation emitted by the surface and the lower troposphere (the greenhouse effect of clouds), clouds exert two competing effects on the Earth’s radiation budget. These two effects are usually referred to as the SW (shortwave) and LW (longwave) components of the cloud radiative forcing (CRF). The balance between these two components depends on many factors, including macrophysical and microphysical cloud properties. In the current climate, clouds exert a cooling effect on climate (the global mean CRF is negative). In response to global warming, the cooling effect of clouds on climate might be enhanced or weakened, thereby producing a radiative feedback to climate warming (Randall ''et al''., 2006; NRC, 2003; Zhang, 2004; Stephens, 2005; Bony ''et al''., 2006).
In the most recent, the
IPCC Fifth Assessment Report The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014.IPCC (2014The IPCC’s Fifth Assessment Report (AR5) leaflet/ref> As h ...
(2013), cloud feedback effects are discussed in the Working Group 1 report,Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.), 5th Assessment Report of the Intergovernmental Panel on Climate Change, Working Group 1 report
''Climate Change 2013: The Physical Science Basis''
(2013). Accessed 18 June 2016.
in Chapter 7, "Clouds and Aerosols",Boucher, O., D. Randall, P. Artaxo, C. Bretherton, G. Feingold, P. Forster, V.-M. Kerminen, Y. Kondo, H. Liao, U. Lohmann, P. Rasch, S.K. Satheesh, S. Sherwood, B. Stevens and X.Y. Zhang
"Clouds and Aerosols"
Chapter 7, pp. 571-657, ''Climate Change 2013: The Physical Science Basis'' (2013). Accessed 18 June 2016.
with some additional discussion on uncertainties in Chapter 9, "Evaluation of Climate Models". The report states "Cloud feedback studies point to five aspects of the cloud response to climate change which are distinguished here: changes in high-level cloud altitude, effects of hydrological cycle and storm track changes on cloud systems, changes in low-level cloud amount, microphysically induced opacity (optical depth) changes and changes in high-latitude clouds." The net radiative feedback is the sum of the warming and cooling feedbacks; the executive summary states "The sign of the net radiative feedback due to all cloud types is less certain but likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds." They estimate the cloud feedback from all cloud types to be +0.6 W/m2°C (with an uncertainty band of −0.2 to +2.0), and continue, "All global models continue to produce a near-zero to moderately strong positive net cloud feedback." The closely related effective climate sensitivity has increased substantially in the latest generation of global climate models. Differences in the physical representation of clouds in models drive this enhanced sensitivity relative to the previous generation of models.


See also

Fixed anvil temperature hypothesis Fixed anvil temperature hypothesis is a physical hypothesis that describes the response of cloud radiative properties to rising surface temperatures. It presumes that the temperature at which radiation is emitted by anvil clouds is constrained b ...


References

{{Reflist Climate forcing