Albedo

Introduction

The albedo of an object or material is its tendency to reflect light. A bright, highly reflective surface, such as fresh snow, has a high albedo; a dark surface, such as asphalt, absorbs light and thus has a low albedo. Albedo is generally expressed as the fraction of incident light— the amount of light coming from the sun or some other source—that a surface reflects, with values ranging from zero (no reflection) to one (total reflection). Decreases in Earth's albedo brought on by melting snow, vegetation shifts, and other factors can exacerbate global warming, especially at high latitudes.

Historical Background and Scientific Foundations

Earth's albedo can be measured locally on the ground and used in computer models to predict large-scale effects, or measured on a broader scale using satellites and observations of earthshine, that is, light reflected

from Earth onto the dark side of the moon. As little as a 1% decrease in Earth's overall albedo may be enough to spur significant climatic warming.

However, studies of Earth's global albedo have varied widely in their findings over the past several decades, complicating scientists' efforts to build the effects of albedo into a broader understanding of global warming. Part of this uncertainty likely stems from changes in the type and amount of cloud cover. The albedo-climate picture is generally clearer at smaller scales, such as on snow-covered tundra or sea ice, where albedo measurements can be more localized.

Impacts and Issues

Albedo may be involved in a number of different feedback mechanisms that amplify global warming. The best-known is perhaps the snow/ice albedo feedback, by which increasing temperatures shrink snow and ice cover at high latitudes and altitudes, exposing darker stretches of water or ground that more readily absorb solar radiation. This lowered surface albedo promotes warming that melts more snow and ice, promoting still more warming, and so on. Shrinking snow cover has played a significant role in the increasingly early onset of spring at high latitudes over the past several decades.

Changes in vegetation communities brought on by global warming, such as the advance of treeline to higher altitudes and latitudes and the loss of tundra to forests and shrublands, may also be darkening formerly bright, snowy expanses at high latitudes, increasing regional warming.

WORDS TO KNOW

EARTHSHINE: Sunlight reflected from Earth and illuminating some other body, such as a spacecraft or the moon. The side of the moon that always faces Earth is illuminated by earthshine; by measuring the brightness of portions of the moon that are lit only by earthshine, the reflectivity (albedo) of Earth can be measured.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

INCIDENT LIGHT: Light arriving at the surface of an object (for example, Earth).

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

SEA ICE: Ice that forms from the freezing of ocean water. As the salt water freezes, it ejects salt, so sea ice is fresh, not salty. Sea ice forms in relatively thin layers, usually no more than 3–7 ft (1–2 m) thick, but it can cover thousands of square miles of ocean in the polar regions.

TREELINE: The highest-altitude (or highest-latitude) line along which trees can grow. As climate warms, treelines advance to higher altitudes and latitudes.

TUNDRA: A type of ecosystem dominated by lichens, mosses, grasses, and woody plants. It is found at high latitudes (arctic tundra) and high altitudes (alpine tundra). Arctic tundra is underlain by permafrost and usually very wet.

Human land-use practices can also change albedo. Particulate matter like dust kicked up from heavily grazed areas, deforested areas, farmland, growing deserts, new housing developments, and other impacted environs, as well as soot from the burning of fossil fuels, can collect on snow and ice and cause it to melt more quickly. Scientists from the National Snow and Ice Data Center in Boulder, Colorado, found that a single event spreading dust over the surface could cause snow to melt from a Rocky Mountain basin 18 days earlier than it would if the snow was dust-free. Changes to snowmelt regimes in mountain ranges around the world may lead to destructively powerful spring runoff as well as water shortages.

See Also Arctic Melting: Greenland Ice Cap; Arctic Melting: Polar Ice Cap; Global Warming; Solar Illumination; Soot.

BIBLIOGRAPHY

Books

Lemke, P., et al. “Observations: Changes in Snow, Ice and Frozen Ground.” In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon, et al. New York: Cambridge University Press, 2007.

Periodicals

Chapin, F. S. III, et al. “Arctic and Boreal Ecosystems of Western North America as Components of the Climate System.” Global Change Biology 6 (2000): 211-223.

Curry, Judith A., and Julie L. Schramm. “Sea Ice-Albedo Climate Feedback Mechanism.” Journal of Climate 8 (1995): 240-247.

Karl, Thomas R., and Kevin E. Trenberth. “Modern Global Climate Change.” Science 302 (2003): 1719-1723.

Nijhuis, Michelle. “Dust and Snow: High in the Snowy San Juan Mountains, Tiny Particles Have Big Implications.” High Country News (May 29, 2006).

Soja, Amber J., et al. “Climate-induced Boreal Forest Change: Predictions Versus Current Observations.” Global and Planetary Change 56 (2007): 274-296.

Web Sites

“Albedo.” Arctic Coastal Ice Processes, October 26, 2006. < http://www.arcticice.org/albedo.htm> (accessed October 31, 2007).

Britt, Robert Roy. “Baffled Scientists Say Less Sunlight Reaching Earth.” Live Science, January 24, 2006. < http://www.livescience.com/environment/060124_earth_albedo.html> (accessed October 31, 2007).

“Scientists Watch Dark Side of the Moon to Monitor Earth's Climate.” American Geophysical Union News, April 17, 2001. < http://www.agu.org/sci_soc/prrl/prrl0113.html> (accessed October 31, 2007).

Sarah Gilman

Albedo

The reflecting power of a surface, expressed as a ratio of reflected radiation to incident or incoming radiation; it is sometimes expressed as a percentage. Albedo is also called the "reflection coefficient" and derives from the Latin root word albus, which means whiteness. Sometimes expressed as a percentage, albedo is more commonly measured as a fraction on a scale from zero to one, with a value of one denoting a completely reflective, white surface, while a value of zero would describe an absolutely black surface that reflects no light rays.

Albedo varies with surface characteristics such as color and composition, as well as with the angle of the sun. The albedo of natural earth surface features such as oceans, forests, deserts, and crop canopies varies widely. Some measured values of albedo for various surfaces are shown below:

The albedo of clouds in the atmosphere is important to life on Earth because extreme levels of radiation absorbed by the earth would make the planet uninhabitable; at any moment in time about 50% of the planet's surface is covered by clouds. The mean albedo for the earth, called the planetary albedo, is about 30–35%.

[Mark W. Seeley ]

Albedo

Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. It is used to describe areas of the Earth (terrestrial albedo) or the overall brightness of reflective astronomical objects such as planets and moons (astronomical albedo). An object’s or area’s albedo is its ability to reflect a proportion of the electromagnetic radiation (including visible light) that falls upon it. High albedo equals high brightness or reflectivity; low albedo equals low brightness. A perfect mirror has an albedo of 100%; smooth, unoxidized surfaces of white metals like aluminum or silver comes close to that figure. Some metals, like brass or copper, however, are colored, and do not reflect all visible light equally well.

In astronomy and meteorology, albedo describes the proportion of sunlight reflected back into outer space. Without reflection all the planets and their satellites would be invisible to us since, unlike the Sun, they are not luminous (with some exceptions, such as Jupiter, which radiates more heat from its interior than it absorbs from the Sun). An object with zero albedo would be perfectly black and therefore perfectly undetectable, except when it passed in front of a radiation source and blocked it temporarily.

The albedo of Earth is around 30–35%. It is higher over snow-covered surfaces or where there is a cloud cover, and lower over clear oceans or land. After 30–35% of the sunlight is reflected back to space, the remaining 65–70% is first absorbed by Earth and its atmosphere and is re-emitted at the much longer wavelengths corresponding to the average temperature of our planet, 60°F (15.5°C). This re-emitted radiation is in the infrared part of the spectrum, and while we feel it as heat, it is not visible. Earth is known as the blue planet because the albedo reflects the particular wavelength that lies in the blue area of the spectrum. Mars, on the other hand, appears reddish to us, probably because its surface formations contain a large proportion of iron oxide, which reflects red light.

The albedo plays a crucial role in determining Earth’s climate, as the average temperature at its surface is closely tied to the 65–70% of absorbed sunlight or solar energy. The light which is directly reflected back does not contribute to the warming of our planet. If, therefore, the albedo were to increase, the temperature at the Earth’s surface would drop. If the albedo were to decrease, temperature would rise.

A moon of Saturn, Enceladus, has the highest albedo of any body in the Solar System: approximately 99%. Its surface is nearly pure water ice. Some asteroids have albedos as low as .05%

See also Weather.

Albedo

Albedo means reflecting power and comes from the Latin word, albus, for white or whiteness. The scientific meaning of albedo is the ability of a surface to reflect a certain proportion of visible light . A perfect mirror has an albedo of 100%; the polished surface of white metals like aluminum or silver comes close to that figure. Some metals like brass or copper , however, are colored, and they do not reflect all visible light equally well. This shows that albedo is dependent on the wavelength of the light being reflected.

In astronomy and meteorology , albedo describes the proportion of sunlight reflected back into outer space , for example, by a planet or a satellite . Without this reflection all the planets and their satellites would be invisible to us since, unlike the Sun , they are not self-luminous. The light from the Sun is white or yellowish because it is emitted from a star whose temperature is very high, close to 9,900°F (5,482°C).

The albedo of Earth is around 30-35%. It is higher over snow-covered surfaces, or where there is a cloud cover, and lower over clear oceans. After 30-35% of the sunlight is reflected back to space, the remaining 65-70% is first absorbed by Earth and its atmosphere and is re-emitted at the much longer wavelengths corresponding to the average temperature of our planet, 60°F (15.5°C). This re-emitted radiation is in the infrared part of the spectrum , and while we feel it as heat , it is not visible. Astronauts in outer space would not be able to see Earth if it had no albedo. Earth is known as the blue planet because the albedo reflects the particular wavelength that lies in the blue area of the spectrum. Mars , on the other hand, appears reddish to us, probably because its surface formations contain a large proportion of iron oxide, which reflects red light.

The albedo plays a crucial role in determining Earth's climate, as the average temperature at its surface is closely tied to the 65-70% of absorbed sunlight or solar energy . The light which is directly reflected back does not contribute to the warming of our planet. If, therefore, the albedo were to increase, the temperature at the Earth's surface would drop. If the albedo were to decrease, temperature would rise.

See also Weather.

albedo The proportion of insolation that is reflected back from the Earth, from the tops of the clouds, and from the atmosphere, without heating the receiving surface. It averages about 30 per cent, but varies widely according to the substance and texture of the surface, and the angle and wavelength of the incident radiation. The value for green grass and forest is 8–27 per cent (over 30 per cent for yellowing deciduous forest in autumn); for cities and rock surfaces 12–18 per cent (over 40 per cent for chalk and light-coloured rock and buildings); for sand up to 40 per cent; for fresh, flat snow up to 90 per cent; and for calm water only 2 per cent in the case of vertically incident radiation but up to 78 per cent where there is a low angle of incidence. The albedo for cloud surfaces averages 55 per cent, but can be up to 80 per cent for thick stratocumulus.