The words are "greenhouse effect", "greenhouse gas" and "carbon dioxide".
Greenhouse effect is a natural process, is where the greenhouse gas in atmosphere will reabsorb some of the energy from sunlight and reflect them back to earth surface in order to keep the earth warm and suitable for lives.
However, the real problem is where greenhouse gas in the atmosphere is increasing. Examples of greenhouse gas are such as carbon dioxide, nitrous oxide, methane etc. The more these has exist in the atmosphere, the more sunlight them reabsorb. This reduces the amount of sunlight radiating back to space and leads to global warming.
