Why can't infrared rays go through the atmosphere? Why can't infrared rays reach the earth? Why can't they pass through the atmosphere?
 A: Gases in the atomsphere have natural resonant frequencies at which they vibrate if stimulated to do so. To stimulate the vibration, a frequency at, or near, the resonant frequency must be applied. When you push a child on a swing with the same frequency as the swing's natural frequency, the child/swing will "absorb" that energy and swing higher .
Carbon dioxide molecules, for one, have their resonance frequency in the infrared. So when light from the sun strikes the atmosphere, much of the infrared is absorbed into vibrations of these molecules. It is re-emited again but in random directions instead of the path it would have taken straight to the Earth's surface. Some of re-emision reaches Earth but the majority of the absorbed infrared is re-emited back into space.
Turn this around. Much of the shorter wave radiation that easily passes through the atmosphere strikes the earth and is re-emited as infrared. Instead of escaping into space, much of it is also absorbed by molecules such as CO2. A significant percentage of that randomly directed re-emmision heads back toward Earth to melt glaciers.
