Global warming describes an increase 
in the average temperature of the Earth's atmosphere and oceans. 
The terms global warming or anthropogenic global warming 
are also used to describe the theory that increasing temperatures 
are the result of a strengthening greenhouse effect 
caused primarily by man-made increases in carbon dioxide 
and other greenhouse gases. 

The natural greenhouse effect 
keeps the Earth 30°C warmer than it otherwise would be; 
adding carbon dioxide to an atmosphere, 
with no other changes, 
will tend to make a planet's surface warmer; 
the question is, by how much?

Greenhouse gases 

(GHG) are gaseous components 
of the atmosphere that contribute to the greenhouse effect. 
The major natural greenhouse gases are water vapor, 
which causes about 36-70% of the greenhouse effect on Earth 
(not including clouds); carbon dioxide, 
which causes between 9-26%; and ozone, 
which causes between 3-7% 
(note: the influences of the various gases are not additive)






Human activity raises levels of greenhouse gases 
primarily by releasing carbon dioxide.

The concentrations of several greenhouse gases have 
increased over time due to human activities, such as:
   * burning of fossil fuels and deforestation 
leading to higher carbon dioxide concentrations,
    * livestock and paddy rice farming, 
land use and wetland changes, 
pipeline losses, and landfill emissions 
leading to higher methane concentrations,
    * the use of CFCs in foam-blowing agents,
aerosol propellants, refrigeration systems, air-conditioning. 
Solvents for cleaning electronic microcircuits,
carbon tetrachloride, methyl chloroform.
The use of Halons in fire suppression systems 
and various manufacturing processes.
 
Carbon dioxide is the subject of the proposed Kyoto Protocol. 
Methane and nitrous oxide are also taken into account 
in the international agreements, 
Ozone Depletion is the subject of the Montreal Protocol.