Greenhouse gas is a type of gas that can absorb the heat energy emitted from the Earth's surface and redirects it back to the Earth's surface. As more GHG are emitted, it creates the greenhouse effect, driving climate change. The most common greenhouse gases are carbon dioxide, methane, nitrous oxide, and water vapour.
To mitigate and eliminate this greenhouse effect, industries need to measure and reduce their GHG emissions, to become carbon neutral or carbon positive.
The standardised way of measuring and comparing one's GHG emissions is using the carbon dioxide equivalent (CO2 eq), where other greenhouse gas effects are converted to CO2 equivalencies. This is why different activities/processes are measured and compared through CO2 emissions.
For example, energy from natural gas emits less GHG than coal due to how it is produced. Coal produced in one country can also emit different GHG since the production process may differ.