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Greenhouse Effect  
Global warming is linked to the release of CO2 from the combustion of fuels. Although water is the main greenhouse gas (most abundant), its effect may not be compared to the effect of CO2. The CO2 molecule may absorb radiation of different wavelenghts that water doesn't absorb. CO2 for 50% of global warming. High radiation from the sun enters the atmosphere and is radiated back to the Earth a lower-energy infrared (ideal scenario). However the gases trap this low-energy radiation, preventing it from leaving the Earth.

 Main Greenhouse Gases:
a) H20 - Source: evaporation of ocean
b) CO2 - Source: combustion of fuels and biomass
c) CH4 -Source: decay of organic matter caused by intensive farming
d) N20 - Source: artificial fertililzers and combustion of biomass
e) O3 - Source: Pollutant in photochemical smogs
f) CFC - Source: Refrigerators,propellents, foaming agents and etc.