The Greenhouse Effect

Gases in the Earth's atmosphere (mostly water vapor and carbon dioxide, but a few others as well) create a natural greenhouse effect - a rise in temperature as our atmosphere traps energy from the Sun. But human activities have greatly increased the amount of carbon dioxide in the atmosphere, possibly enhancing the greenhouse effect and causing global warming.
Rays from the sun shine down on the Earth.  Some bounce off into space, others bounce around in the earth's atmosphere, trapped by the greehouse effect.