The Greenhouse Effect:
The Earth receives most of its energy from the sun in the form of short wave radiation. Much of this incoming solar radiation passes through the atmosphere to reach the Earth's surface. The Earth absorbs some of this energy and radiates the remainder back into the atmosphere in the form of infrared radiation.
The outgoing infrared radiation has a longer wavelength than incoming solar radiation. This allows it to be absorbed by certain gases in the atmosphere. The main gases that absorb infrared radiation are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and haloflourocarbons (HFCs).
These gases trap some of the infrared radiation and re-radiate it back to the earth's surface as heat, causing a warming effect known as the "greenhouse effect."
the greenhouse effect is necessary to life on earth as we know it; without it, the earth's surface would be about 35° C cooler on average.
While the greenhouse effect is necessary to maintain the earth's climate, it is possible to have too much of a good thing. For over the past 200 years, the burning of fossil fuels and destruction of forests has caused the concentrations of heat-trapping greenhouse gases to increase significantly in our atmosphere.
With more gases in the atmosphere, more radiation is absorbed and re-radiated back to earth. Scientists call this the enhanced greenhouse effect.
Claire Parkinson at Earthsky.org
As the concentrations of these gases continue to increase in the atmosphere, the earth's temperature rapidly climbs.
In the 20th Century, global temperatures have increased by 0.7° C (1.3° F). If concentrations of greenhouse gases in the atmosphere continue to increase, the average temperature at the Earth's surface could increase from 1.8 to 4°C (3 to 7°F) above 2000 temperature levels by the end of this century.
In the past, the planet has gone through cycles of warming and cooling, but the changes seen today are occurring much more rapidly than during a natural cycle.
Climate Change part 1 - What is climate change?