It is a vital natural process that warms Earth’s surface by trapping the sun’s energy in our atmosphere. Without the greenhouse effect, the Earth's temperature would be around -18°C (source).

Solar radiations reach our atmosphere, some bounce back to space. The rest is absorbed by land, ocean and the heat re-radiated from Earth toward space. Greenhouse effect gases in our atmosphere such as carbon dioxide, methane or nitrous dioxide will trap some of the heat in our atmosphere, preventing it from reaching space. These gases act as the glass wall of the green house, thus the name of its effect.

So, what really is the problem?

Imagine a winter day in Dubai. It’s 18°C and you wear a light jacket. This jacket is the natural greenhouse effect. Now, imagine in addition of this light jacket, you wear a ski jacket. The sun is not hotter, but the heat you trap under your jackets is too much. Thus, the additional ski jacket you put on is the extra greenhouse effect due to greenhouse gases generated by human activities.