What are greenhouse gases?

The main GHGs are outlined below.

1. Carbon dioxide (CO₂) – Certainly the most well-known of the GHGs (we often abbreviate to talk about a “carbon reduction strategy” when we are really talking about a strategy to reduce all GHG emissions) as it makes up around 80% of all GHG emissions. This is due to it being generated when we burn fossil fuels to generate power or manufacture cement (and other common, large scale activities)
2. Methane (CH₄) – Famously generated by livestock as well as other farming activities and landfill decay. Methane makes up around 10% of the GHGs
3. Nitrous oxide (N₂O) – Generated through various agricultural activities, fossil fuel combustion and water treatment processes. Nitrous Oxide makes up around 7% of the GHGs
4. Fluorinated gases – This is a range of gases with very long names such as Hydrofluorocarbons, perfluorocarbons, sulphur hexafluoride, and nitrogen trifluoride! These make up around 3% of emissions but are the most potent form a global warming perspective.

Converting them all to a common unit

Although CO₂ is the most common GHG it is not the most powerful from a global warming perspective. In fact, it is estimated that 1 tonne of Hydrofluorocarbons is equivalent to over 1,800 tonnes of CO₂. One tonne of methane is not as bad as that but it is still estimated to be the equivalent of around 25 tonnes of CO₂ in terms of its impact as a greenhouse gas.

To make it much easier to measure and compare carbon footprints, all emissions are converted into “Carbon Dioxide Equivalent” which is written as “CO₂e”. So when people abbreviate things and say “our carbon footprint is X tonnes” or “our carbon footprint is Y tonnes of CO₂” what they are actually referring to is CO₂e.

Here’s a common diagram used to show the difference:

(Click to expand)