In the atmosphere there are natural greenhouse gases, which is a prerequisite for life at earth.
The greenhouse gases act as a sort of roof, holding in a certain amount of energy from the sun and thereby keeping the temperature of the surface at about 15°C. Without that roof the temperature would be about -18°C.

The most important greenhouse gases are water vapour, carbon dioxide (CO2), methane, nitrous oxide, ozone and CFCs.

Emissions from human activity
Industrialisation and the growth of the world’s population has brought with it more transport needs, more energy consumption, more food and more waste, which in turn has lead to an increase in emission of CO2 and other greenhouse gases. These add to the already existing greenhouse gases in the atmosphere, causing the natural roof to become denser.

This means that a lot more of the sun’s energy is trapped under the roof, in turn raising the earth’s temperature. Global warming can therefore be seen as an enhanced greenhouse effect.