Answer:
If the temperature of the apparent surface is too high and Earth radiates more heat than it receives, the planet will become colder and radiate less energy back to space.
Explanation:
The "greenhouse effect" is widely discussed in the media, and although its details are complicated, its principles are not difficult to understand. Without a greenhouse effect, radiation from the Sun (mostly in the form of visible light) would travel to Earth and be changed into heat, only to be lost to space. This scenario can be sketched as follows:
Sun's radiation → absorbed by Earth → Re-radiated to space as heat
The greenhouse effect is a phenomenon in which solar energy quickly enters the lower atmosphere and the surface of the Earth, transforms into heat, but cannot thereafter leave the globe freely. Here is a rough sketch of this:
Earth absorbs solar radiation, some of which is then retained by the atmosphere or re-radiated into space as heat.