Abstract
Cloud albedo enhancement is one of several possible methods of solar radiation management by which the rate of increase in world temperatures could be reduced or even reversed. It depends on a well-known phenomenon in atmospheric physics known as the Twomey effect. Twomey argued that the reflectivity of clouds is a function of the size distribution of the drops in the cloud top. In clean mid-ocean air masses, there is a shortage of the condensation nuclei necessary for initial drop formation in addition to high relative humidity. This means that the liquid water in a cloud has to be in relatively large drops. If extra nuclei could be artificially introduced, the same amount of liquid water would be shared among a larger number of smaller drops which would have a larger surface area to reflect a larger fraction of the incoming solar energy back out to space.
This chapter, which has been modified slightly for the purposes of this volume, was originally published as part of the Encyclopedia of Sustainability Science and Technology edited by Robert A. Meyers. DOI:10.1007/978-1-4419-0851-3
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Abbreviations
- Albedo:
-
Whiteness or reflectivity. The ratio of outgoing to incoming radiation power.
- Cloud condensation nuclei:
-
The small fragment of material needed to start the growth of a cloud drop when the local relative humidity exceeds 100%.
- MODIS:
-
An acronym for moderate resolution imaging spectroradiometer. This is an instrument carried by two NASA satellites Aqua and Terra. The systems measure 36 spectral bands of radiation of the entire earth surface. Results are available free and are an invaluable aid to research into climate change.
- Permafrost:
-
Ground which remains frozen throughout the year. It can trap very large amounts of methane which would be released if it were to melt.
- Positive feedback:
-
A change in the value of a variable which leads to a further change in the same direction. It can often produce explosive effects and oscillations.
- Relative humidity:
-
The ratio of the amount of water vapor in a parcel of air to the maximum amount that it could hold if nucleating particles were present. Values can exceed 100% and even reach 300% if all nuclei are absent.
- Solar constant:
-
The rate at which solar energy of all wavelengths crosses unit area at the top of the atmosphere. It is about 1,366 W/m2 but is strictly not quite constant. There are cyclical variations with an amplitude of about 3.5% due the ellipticity of the earth’s orbit around the sun, 11 year cycles and faster random variations of about 0.1%.
- Twomey effect:
-
The relationship of cloud albedo to cloud depth, liquid water content and concentration of cloud condensation nuclei elucidated by Sean Twomey. His work began following observation of increased albedo of ships tracks in satellite photographs.
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Salter, S.H. (2013). Solar Radiation Management, Cloud Albedo Enhancement. In: Lenton, T., Vaughan, N. (eds) Geoengineering Responses to Climate Change. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-5770-1_4
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