Cloud feedback

Cloud feedback is the coupling between cloudiness and surface air temperature in which a change in radiative forcing perturbs the surface air temperature, leading to a change in clouds, which could then amplify or diminish the initial temperature perturbation.

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.^[1] Cloud representations vary among global climate models, and small changes in cloud cover have a large impact on the climate.^[2][3] Differences in planetary boundary layer cloud modeling schemes can lead to large differences in derived 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.^[4] However, satellite data show that cloud optical thickness actually increases with increasing temperature.^[5] 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 addition to how clouds themselves will respond to increased temperatures, there exist other feedbacks that will 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.^[6] 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 would act to mediate global climate transitions.^[7]

Changes in cloud cover are closely coupled with other feedback, including the water vapor feedback 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. More clouds cools the climate, resulting in a negative feedback.^[8] 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.^[9]

References

1. ^ Hartmann, D.L., M.E. Ockert-Bell, and M.L. Michelsen (1992). "The Effect of Cloud Type on Earth's Energy Balance: Global Analysis". J. Climate 5: 1281–1304. Bibcode 1992JCli....5.1281H. doi:10.1175/1520-0442(1992)005<1281:TEOCTO>2.0.CO;2. 
2. ^ Cess, R. D., et al. (1990). "Intercomparison and Interpretation of Climate Feedback Processes in 19 Atmospheric General Circulation Models". J. Geophys. Res. 95 (D10): 16,601–16,615. 
3. ^ Stocker, T.F., et al. (2001). "Physical climate processes and feedbacks". In J.T. Houghton, et al.. Climate Change 2001: The Scientific Basis, Contributions of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, U.K.: Cambridge University Press. 
4. ^ National Research Council (2004). Understanding Climate Change Feedbacks. Panel on Climate Change Feedbacks, Climate Research Committee. National Academies Press. ISBN 0309090725. http://www.nap.edu/catalog.php?record_id=10850. 
5. ^ Tselioudis, G., W.B. Rossow, and D. Rind (1992). "Global Patterns of Cloud Optical Thickness Variation with Temperature". J. Climate 5: 1484–1495. Bibcode 1992JCli....5.1484T. doi:10.1175/1520-0442(1992)005<1484:GPOCOT>2.0.CO;2. 
6. ^ Donner, L. J., C. J. Seman, B. J. Soden, R. S. Hemler, J. C. Warren, J. Ström, and K.-N. Liou (1997). "Large-scale ice clouds in the GFDL SKYHI general circulation model". J. Geophys. Res. 102 (D18): 21,745–21,768. Bibcode 1997JGR...10221745D. doi:10.1029/97JD01488. 
7. ^ Pierrehumbert, R. T., and R. Roca (1998). "Evidence for Control of Atlantic Subtropical Humidity by Large Scale Advection". Geophys. Res. Lett. 25 (24): 4537–4540. Bibcode 1998GeoRL..25.4537P. doi:10.1029/1998GL900203. http://geosci.uchicago.edu/~rtp1/papers/GRL_Roca98/P_RocaGRL98.pdf. 
8. ^ Fowler, L.D., and D.A. Randall (1996). "Liquid and Ice Cloud Microphysics in the CSU General Circulation Model. Part III: Sensitivity to Modeling Assumptions". J. Climate 9: 561–586. Bibcode 1996JCli....9..561F. doi:10.1175/1520-0442(1996)009<0561:LAICMI>2.0.CO;2. 
9. ^ Wetherald, R., and S. Manabe (1988). "Cloud Feedback Processes in a General Circulation Model". J. Atmos. Sci. 45: 1397–1416. Bibcode 1988JAtS...45.1397W. doi:10.1175/1520-0469(1988)045<1397:CFPIAG>2.0.CO;2.