Atomic greenhouse effect. Animation showing how energy being absorbed and re-emitted by an atom forms the basis of the greenhouse effect. An atom is shown with its electrons (black dots) occupying energy levels (circles) around the nucleus (red and blue). The outermost electron is shown absorbing the energy of short-wave radiation (such as ultraviolet, purple arrow). This excitation causes the electron to rise to a higher energy level, further from the nucleus. The electron then falls to a lower energy level, emitting long-wave radiation (infra-red, red arrows). This is the principle behind global warming, where the greenhouse effect leads to increased warming of a planet.