Definition

The greenhouse effect is the natural warming of Earth's surface caused by gases in the atmosphere, chiefly water vapor, carbon dioxide, methane, nitrous oxide, and ozone, that let sunlight through but absorb the infrared heat the warmed ground gives off, then re-emit part of that energy back downward. This raises Earth's average surface temperature from about $-18^\circ C$ to about $15^\circ C$, and it happens because these gases have molecular vibrations that change their dipole moment, unlike symmetric nitrogen and oxygen, which make up $99\%$ of the air but do essentially none of the heat trapping. Radiative balance requires outgoing longwave radiation to match absorbed sunlight, about $240\ \text{W/m}^2$, so adding more greenhouse gases raises the effective emission height into colder air and forces the surface to warm further to restore that balance, an effect of roughly $3.7\ \text{W/m}^2$ for a doubling of $CO_2$.