NOTE: go to the following to see the viewgraph on
Evolution of Earth's Atmosphere.
There is also a listing of composition of different types of objects in our solar system, such as the Sun, Jupiter, and Rocky Planets.
Over
the past 30 years, the Sun's irradiance has varied +- 1 part in 1367, or +-
0.07%: this is not responsible
for the global climate change observed during the last
century.
http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant