What Are They?
Greenhouse gases (GHG) are gases that contribute to the greenhouse effect on Earth by trapping gases into the atmosphere.
By order of abundance in the atmosphere, GHG are:
Water vapour (H2O)
Carbon dioxide (CO2)
Nitrous oxide (N2O)
Hydrofluorocarbons (includes HCFCs and HFCs)
Most of these gases are naturally present in the atmosphere. They are not directly harmful to humans or animals’ health (except ozone (O3) above concentrations of about 0.1 ppm). However, human activity has increased their concentration, and they are creating a warming effect with rippled effects on the environment and society.
Others are pure products of human activity (CFCs, HCFCs & HFCs).
According to the EPA, the impact of these gases on climate change depends on three key factors:
How much is in the atmosphere?
Concentration is the quantity of a particular gas in the air—larger emissions of GHG results in higher concentrations in the atmosphere.
When measuring GHG emissions, two measures are commonly used:
1. “CO2 equivalent” units of CO2 (CO2e)
2. Another measure is the atomic weight, typically as “carbon”
However, converting GHGs to kg of carbon is not useful as it does not allow a comparison between different GHGs.
Please use CO2e.
How long do they remain in the atmosphere?
These gases can stay in the atmosphere for different amounts of time, ranging from a few years to thousands of years. However, these gases remain in the atmosphere long enough to become well mixed, so the amount measured in the atmosphere is equivalent worldwide, regardless of the emissions’ location.
How strongly do they impact the atmosphere?
Some gases are more effective than others at warming the planet. For each greenhouse gas, a Global Warming Potential (GWP) has been calculated to reflect how long it stays in the atmosphere on average, and how strongly it absorbs energy. Gases with a higher GWP absorb more energy, per unit of mass, than gases with a lower GWP, thus contributing more to warming Earth.
The IPCC considers 6 GHGs to be relevant for human-caused climate change:
Global Warming Potential
Carbon dioxide (CO2)
Nitrous oxide (N20)
124 - 14,800
7,390 - 12,200
Sulfur hexafluoride (SF6)
Nitrogen trifluoride (NF3)
- “CO2e” (e standing for “equivalent”) takes into account the various greenhouse effects of these gases in terms of warming and lasting. This is a simple unit that agglomerates ALL GHG.
- Even if the individual warming effect can be mentioned in an analysis, it can be misleading to state that for example, methane (CH4) has 25 times more GH effect than CO2, as this is already accounted for in the CO2e reporting.