Global warming occurs in cycles caused mainly by changes in the sun's energy output and the sun's relative position to the Earth.

Major Causes of Global Temperature Shifts:

• Astronomical Causes

• 11 year and 206 year cycles: Cycles of solar variability ( sunspot activity )
• 21,000 year cycle: Earth's combined tilt and elliptical orbit around the Sun ( precession of the equinoxes )
• 41,000 year cycle: Cycle of the +/- 1.5° wobble in Earth's orbit ( tilt)
• 100,000 year cycle: Variations in the shape of Earth's elliptical orbit ( cycle of eccentricity)  

• Atmospheric Causes

• Heat retention: Due to atmospheric gases, mostly gaseous water vapor (not droplets), also carbon dioxide, methane, and a few other miscellaneous gases -- the "greenhouse effect"
• Solar reflectivity: Due to white clouds, volcanic dust, polar ice caps

• Tectonic Causes

• Landmass distribution: Shifting continents (continental drift) causing changes in circulatory patterns of ocean currents. It seems that whenever there is a large land mass at one of the Earth's poles, either the north pole or south pole, there are ice ages.
• Undersea ridge activity: "Sea floor spreading" (associated with continental drift) causing variations in ocean displacement.