Various factors affect the average temperature of the Earth’s atmosphere. Other things being equal, when the earth is closer to the Sun, or when the Sun radiates more energy, then the atmosphere is warmer.
The Cloud Mystery is a documentary that explains another factor in average atmospheric temperatures: cloud coverage. Other things being equal, when there are more clouds the atmosphere is cooler. This is because clouds reflect the Sun’s radiation back into space.
The documentary explores what causes clouds to form, and discusses empirical evidence that the cloud system is one of the major factors affecting Earth’s climate.
Clouds are formed when moisture in the atmosphere coalesces around aerosol particles. These particles are created in large part by cosmic rays. So, when more cosmic rays enter the atmosphere, more clouds are formed — leading to lower temperatures.
Cosmic rays increase when the solar system passes through arms of the Milky Way galaxy, and decrease when the Sun’s protective magnetic field increases in strength. According to the documentary, these two factors cause the the long term cycles in Earth’s average temperature.
UPDATE: A more comprehensive documentary (which includes a discussion of the information in The Cloud Mystery) is called The Great Global Warming Swindle.