Even though emissions of carbon dioxide warm the lower atmosphere, they have the opposite effect on the upper atmosphere. The reason has to do with the difference in density. Near the Earth's surface, carbon dioxide absorbs radiation escaping Earth, but before it 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.

Video

In this video clip, NCAR scientist Stan Solomon explains this phenomenon, which is illustrated by the animation.
(©UCAR, video and animation.) News media terms of use*

Image & Photo

New research shows that the outermost layer of the atmosphere will lose 3 percent of its density over the coming decade, a sign of the far-reaching impacts of greenhouse gas emissions. As the density declines, orbiting satellites experience less drag. (©UCAR, illustration.) News media terms of use*

Stan Solomon. (©UCAR, photo by Carlye Calvin.)News media terms of use*