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The term "greenhouse effect" is the layman's term for global warming caused by an excess of certain gases (most notably carbon dioxide) in the atmosphere.
When these gasses are present in excessive concentrations, they prevent heat from escaping from the earth's atmosphere and radiating back into space. This is similar to the way that a greenhouse's roof traps the heat within the greenhouse.
In times when the concentration of these gases was lower, the heat escaped and the earth did not warm up. Nowadays, it is believed, the gases are trapping the heat and causing the earth's atmosphere to warm up.
From what I understand, this refers to the emissions of gasses by factories, cars, people, animals into our atmosphere which are said to contribute to the depletion of our ozone layer. This all boosts the carbon dioxide content which in turn also heats up our planet. In theory, once all this happens, polar caps and glaciers begin to melt. The ocean's water temperatures begin to heat up. Plant life begins to suffer as well as all human and animal life. Therefore we have the "greenhouse effect". In order to prevent all this, we must work on alternative sources of energy.
Greenhouse effect refers to the increase in earth's surface temperature by trapping of sun's energy received on earth and preventing it from radiating back. This effect is called greenhouse effect because its action is similar to the action of greenhouses, which are built with glass or plastic roof that allow sunlight to enter the greenhouse, but prevent heat radiated back from the greenhouse into the atmosphere.
Presence of some gases like carbon dioxide and ozone in upper layers of earth's atmosphere act like the transparent roof of the greenhouse. These gases allow suns radiations, which are mostly inform of light to pass through them and reach the earth's surface, but trap substantial amount of the heat radiated back from the earth in the form of infrared radiations.
Such gases have been present in the Earth's atmosphere for millions of year, causing the Earths average temperature to be about 33 degrees Celsius lower than what it would be otherwise. However since the advent of Industrial revolution and subsequent large scale and continuing industrialization the concentration in the atmosphere of polluting gases causes greenhouse effect has increased, and continues to rise. Because of this the average temperature of earth has risen during this period and is increasing further.
This rise in earth's average temperature called global warming, has resulted in major ecological balances in the world, causing major damage to its flora and fauna. One major impact of global warming is the melting of the ice caps in polar regions. Unless the trend of global warming is halted, the melting of polar caps will reach very high irreversible levels submerging under the seal large lands along the sea coasts all over the world.
The greenhouse effect is a natural process by which the Earth's lower atmosphere is warmed. The term "greenhouse effect" has a negative connotation because of its association with air pollution and global warming. However, it is only because of the greenhouse effect that the Earth's surface is warm enough to sustain life. In the absence of the greenhouse effect, Earth's average surface temperature would be the same as its outer atmosphere, about 0° Fahrenheit (-18° Celsius)—a far cry from our actual average surface temperature of 59° Fahrenheit (33° Celsius)!
"Greenhouse gases" in the atmosphere—primarily water vapor and carbon dioxide and secondarily methane, nitrous oxide, and chlorofluorocarbons (CFCs)—trap heat in the same way as the glass of a greenhouse. Specifically, solar radiation enters the atmosphere in the form of visible light, is absorbed by the Earth's surface, and is converted to heat. Some of that heat radiates upward and is absorbed by greenhouse gases in the atmosphere. The gases reradiate heat outward in all directions, including back toward the surface. In this way the Earth's lower atmosphere is heated continuously, even at night when there is no incoming sunlight.
Since the Industrial Revolution (in the mid-1700s), levels of carbon dioxide and other greenhouse gases in the atmosphere have been increasing. This is primarily due to the burning of coal, oil, gas, and wood. The increase of greenhouse gases means that more heat is being trapped and returned to Earth, creating a condition known as "global warming."
Sources: Engelbert, Phillis. The Complete Weather Resource, vol. 3, pp. 500-4; Gay, Kathlyn. The Greenhouse Effect, pp. 2, 122-25; McGraw-Hill Encyclopedia of Science and Technology, 7th ed., vol. 8, pp. 225-26.
pohnpei397's answer is the best!
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