Bright surfaces reflect more heat and light than darker ones. The fraction of radiation (in the form of heat and light) reflected by on object is called its ALBEDO. Across the earth, albedo varies. The dark blue of the deep ocean has a much lower albedo than the white ice caps of the poles. Lower albedos (darker surfaces) absorb more heat, and this is important in regulating the planet's heat budget. The earth's albedo varies mainly through changes in cloudiness, snow and ice cover, vegetation cover and land use. The most significant surfaces where the level of albedo influences the degree of heat absorption are the land surface and clouds.

Albedo varies from about 0.9 for fresh snow and the tops of some clouds (i.e. 90 per cent of incoming radiation is reflected back) to 0.03 for the deep ocean when the sun is high in the sky. Asphalt used for road surfaces has an even lower albedo, which is why it gets so hot in the summer.

Changes in albedo are an important factor in FEEDBACK effects. The classic example is with snow on dark ground. If a snow-covered area warms and the snow melts, the dark ground starts to become exposed and the albedo decreases, more sunlight is absorbed by the ground, and the temperature tends to increase still further. The converse is true: if more ground becomes snow-covered, it reflects incoming solar radiation and a cooling cycle happens.

If darker material settles on snow or ice it will reduce its albedo, and thereby increase the heat absorbed. Such dust could be natural or anthropogenic in origin—e.g. from volcanoes, dust storms, forest fires, smoke from forest clearing, and unscrubbed emissions from coal-fired power stations.

As demonstrated by the example above, changes in reflectivity represent an important feedback mechanism in the climate system. Events that contribute both greenhouse gases and particulate matter to the atmosphere can decrease the albedo of the land surface through deposition of the particulate matter, which increases the heat absorbed thereby enhancing the greenhouse warming effect of the greenhouse gas emissions. The opposite effect can also occur, if aerosols emitted from human activities or volcanic eruptions are distributed through the atmosphere they can increase the albedo of the atmosphere and provide an overall cooling effect that to some extent counters the warming effect of greenhouse gases.

Further reading:

Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing.


15 November, 2010

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