"Carbon dioxide cools the thermosphere, even though it acts to warm the atmosphere near the Earth's surface (the troposphere). This paradox occurs because the atmosphere thins with height. Near the Earth's surface, carbon dioxide absorbs radiation escaping Earth, but before the gas molecules can radiate the energy to space, frequent collisions with other molecules in the dense lower atmosphere force the carbon dioxide to release energy as heat, thus warming the air. In the much thinner thermosphere, a carbon dioxide molecule absorbs energy when it collides with an oxygen molecule, but there is ample time for it to radiate energy to space before another collision occurs. The result is a cooling effect. As it cools, the thermosphere settles, so that the density at a given height is reduced."

http://planetary.org/blog/article/00000794/