give a brief discription of greenhouse effect.

Greenhouse effect may be defined as the process of steady increase in the temperature of the earth's atmosphere due to the presence of green house gases such as carbon dioxide, methane, chloroflorocarbons etc. These green house gases forms a blanket around the earth which allow the sun rays (short wave radiation) to pass through it but restricts the passage of reflected heat rays (long wave radiation) from going out of the atmosphere. This results in trapping the heat in the atmosphere leading to increase in temperature. 

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The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases. [1] [2]

Solar radiation at the frequencies of visible light largely passes through the atmosphere to warm the planetary surface, which then emits this energy at the lower frequencies of infrared thermal radiation. Infrared radiation is absorbed by greenhouse gases, which in turn re-radiate much of the energy to the surface and lower atmosphere. The mechanism is named after the effect of solar radiation passing through glass and warming a greenhouse, but the way it retains heat is fundamentally different as a greenhouse works by reducing airflow, isolating the warm air inside the structure so that heat is not lost by convection.[2] [3] [4]

If an ideal thermally conductive blackbody was the same distance from the Sun as the Earth is, it would have a temperature of about 5.3 oC. However, since the Earth reflects about 30% of the incoming sunlight, this idealized planet 's effective temperature (the temperature of a blackbody that would emit the same amount of radiation) would be about −18o C. The surface temperature of this hypothetical planet is 3oC below Earth 's actual surface temperature of approximately 14 oC. The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect

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• The Earth gets most of it's energy from the sun in the form of radiation.

Almost all the heat energy here on Earth, some 99.97% of it, comes from the Sun. Because the Sun is so hot that heat energy is very energetic and it is in the form of Solar Radiation, which has a very short wavelength.

• Some of this energy rebounds back into space

The lighter surfaces on Earth such as clouds, ice-caps, sand etc act like mirrors and reflect some of the incoming energy from the Sun straight back into space. The lighter a surface is the more energy gets reflected off it.

• while a little bit of it is absorbed by Earth's atmosphere

Of the heat energy that’s coming to us from the Sun, only a very tiny amount is absorbed by Earth’s atmosphere. And really, this is largely to do with other physical and chemical properties of the atmosphere; not so much the greenhouse gases.

• Greenhouse gases such as carbon dioxide, methane, and nitrous oxide,

These are three of the four most important greenhouse gases. The one that you didn’t mention is water vapour, this gas is many times more abundant than all the other greenhouse gases put together but, as greenhouse gases go, it’s pretty ineffective. Still, it is the gas that causes most of the warming.

• when emitted because of industrial, transportational, agricultural, and housing purposes, 

Human activities have virtually no effect on the amount of water vapour in the atmosphere but have significantly increased the amount of other greenhouse gases. All told there has been a 42% increase in the amount of these gases.

The main greenhouse gases come from many sources including those that you mentioned. Another important source, and the largest single contributor, is deforestation – the cutting down of forests.

• create a thick layer that absorbs the heat and reflects it back to Earth, rather than leting it be lost to space. 

As the amount of greenhouse gases in the atmosphere increases, they are able to retain a greater amount of heat energy. The important thing to know is that this heat energy is in a different form from that which comes from the Sun.

The energy from the Sun has a short wavelength; that heat is absorbed by the Earth and all the things on it. When that heat is radiated back out from the Earth, it’s now coming from a source much cooler than the Sun and so it has a longer wavelength. This is the key point, the new wavelength is the same as that of the greenhouse gases and so the gases can now absorb the heat.

• This causes the Earth's climate to change as temperatures go up and polar icecaps melt, making the oceans overflow.

The rising of the oceans is one of many consequences of climate change and each year they’re rising by an average of 3.2mm (more in some places, less in others). Half of this rise (1.6mm) is caused because the water in the seas and oceans is warming up and this is leading to expansion. Just under half (1.5mm) is being caused because the polar ice is melting; the remaining small contribution (0.1mm) is caused by something called either isostatic or post-glacial rebound – effectively the springing back up of the earth when it is no longer being depressed by the weight of ice that was once on it.

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For purposes of clarity: 

The Greenhouse Effect is the term used to describe the retention and subsequent downward transmission of heat energy that is absorbed by the molecules of greenhouse gases. It’s the mechanism that provides Earth with a habitable temperature.

Global Warming refers to the gradual and worldwide increase in the average global temperature. There are both natural and manmade components. Natural warming provides us with some 33°C of warmth, the human part of it has, so far, added just under 1°C.

Climate Change is the name given to the effect that global warming has on the climate. So as not to get the two confused you can think of global warming as being the cause and climate change as being the effect.

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thanq Rtish, but i dont need such a big discription to write in the exam :!

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