What is meant by the greenhouse factor of a gas in the atmosphere? - Leaving Cert Chemistry - Question B - 2011

The greenhouse factor of a gas is a measure of its effectiveness at trapping heat in the atmosphere relative to carbon dioxide (CO₂). Carbon dioxide is assigned a baseline value of 1 for the greenhouse effect. This means that any other gas's greenhouse factor is expressed compared to that of CO₂. For example, if a gas has a greenhouse factor of 3, it means that it is three times more effective than carbon dioxide at contributing to the greenhouse effect.