The Greenhouse Effect and the Albedo
We all know what a greenhouse is. It is a structure with the roof and sides made of glass. It is permeable to light energy and retains heat. This warming of the air contained in it allows plants to grow and flourish. Much like a greenhouse the Earth's atmosphere acts in a similar fashion. Instead of a layer of glass, it is a layer of gas that is permeable to light waves and opaque to UV and infrared waves. Naturally occurring greenhouse gases include ozone, nitrous oxide, methane, carbon dioxide, and water vapor.
The greenhouse effect is an important phenomenon to our existence. Solar light waves pass through the atmosphere and are reflected or absorbed by the Earth's surface. Reflected light is scattered back to space but the absorbed energy is reradiated as infrared energy. This infrared energy again does not easily pass through the atmosphere. This energy remains trapped within the atmosphere causing an increase of the Earth's average temperature. This warming is important because it normally keeps the mean temperature of the Earth at a temperate 15ºC. Without the atmosphere and the greenhouse effect, our Earth would be an average -25ºC, an environment perfect for great big ball of ice, void of any multicellular life forms. This is not the case. Currently our Earth is experiencing a decrease in the ice outcrops around the world, while temperatures are on the rise.
As a child I can remember that my mother would suggest wearing white shirts during the summer if I wanted to remain cool. I rebelled and had a collection of black shirts with laminated pictures of my childhood Heavy Metal heros. I sweated those summers out until the onset of glam rock and it was no longer cool. At this point I was old enough to accept my mother's advice. She was right, but I did not know why? Was it a freak of nature or could a strong foundation in science answer this question? Later on in my high school physics class I learned that the color white is a combination of all the colors of the light spectrum reflecting off a surface at once. The opposite occurs with the color black. When the eye sees the color black, it actually sees the absence of light. The light waves are absorbed by the surface. This simple concept relates to the reflective and absorptive capabilities of our Earth. The albedo is a reflection coefficient that describes the reflectivity of an object. An albedo value of 1.00 describes 100% reflection of light. An albedo of 0.00 describes complete absorption of light. My concert tee's had an albedo of 0.00. Currently, the Earth has an albedo value of 0.39, perfect for a temperate greenhouse effect. (Kaufmann, 1991).
Ice, when seen from space, is the color of white. It therefore has an albedo which reflects most of the light hitting it. The ocean and land are darker and have lower albedos, which means they absorb more of the light hitting them. The albedo of the Earth is a factor in the current temperature of the planet. If the albedo changes, then the temperature of the Earth also changes. Increasing the amount of ice on the surface would decrease the temperature of the Earth because more light would be reflected from the surface. However if the percentage of ice is decreased, then the albedo would decrease and more light energy would be absorbed. This unfortunately is happening throughout the globe. Many examples such as the glaciers on Mount Kilimanjaro, Glacier National Park and the Columbia Glacier in Alaska have all receded in recent times. This recess may be correlated to the rise in average temperature that has been observed since the dawn of the industrial revolution. If this recession continues the albedo will also be affected and the increased level of the greenhouse effect may ignite a warming of our globe. (Gore 2006).
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