Radiative forcing is the difference between the radiant energy received by a planet from its Sun and the energy radiated back to space. This parameter is very important for the planet’s climate, because positive radiative forcing will warm the planet and negative radiative forcing will cool it. There are several factors controlling the radiative forcing. On present-day Earth, greenhouse gases, aerosols in the atmosphere, albedo, ozone, and the anthropogenic stress are more important. In a primitive Earth, greenhouse gases, methane oozes, and albedo could have played a key role in regulating the radiative forcing and controlling ancient climates.