Some gases have the ability to trap solar radiation that causes the warming of the earths atmosphere. These are called greenhouse gases. Some of these include carbon dioxide, water vapor, methane, ozone & nitrous oxide. These gases cause the greenhouse effect.
If the amount of these gases in the atmosphere increases, it causes global warming due to which the temperature of the earth increases.
On the other hand, if the amount of these gases decreases, the earth's temperature would go below freezing.
Therefore the amount of greenhouse gases in the atmosphere should be stable. Otherwise either the earth's temperature increases too much or decreases below freezing.