Greenhouse Gases

Greenhouse gases are gases that - because of their molecular structure - absorb or trap infrared radiation, leading to atmospheric warming.

1. What greenhouse gases are included in a carbon footprint?

A GHG Protocol-aligned carbon footprint accounts for the seven main greenhouse gases:

1. Carbon dioxide (CO₂): From fossil fuels, industrial processes, etc.
2. Methane (CH₄): From agriculture, landfills, natural gas systems.
3. Nitrous oxide (N₂O): From fertilisers, combustion, industrial activities.
4. Hydrofluorocarbons (HFCs): Used in refrigeration and air conditioning.
5. Perfluorocarbons (PFCs): From aluminium production and electronics manufacturing.
6. Sulphur hexafluoride (SF₆): Used in electrical insulation (very high GWP).
7. Nitrogen trifluoride (NF₃): Increasingly used in electronics (also very high GWP).

The last four – HFCs, PFCs, SF₆, and NF₃ – are synthetic gases often referred to collectively as fluorinated gases.

2. Is water vapour a greenhouse gas?

Yes – water vapour is technically a greenhouse gas. However, it’s not included in carbon footprinting. That’s because it doesn’t persist in the atmosphere in the same way as other GHGs. Water constantly cycles through evaporation and condensation over short timeframes, which means its concentration is not directly controlled by human activity.

That said, warming caused by other GHGs can lead to higher levels of water vapour – which then traps more heat. This creates a feedback loop that amplifies warming.

3. Are greenhouse gases man-made?

Some are, some aren’t.

The fluorinated gases (HFCs, PFCs, SF₆, NF₃) are entirely man-made, created through industrial processes like manufacturing refrigerants, electronics, or insulating foams.

The other GHGs – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – occur naturally. But human activity has significantly increased their concentration in the atmosphere. Burning fossil fuels, large-scale agriculture, and land use change are the main drivers of this increase.

4. What is Global Warming Potential (GWP)?‍

GWP is a way to compare the climate impact of different greenhouse gases. It measures how much heat a gas traps in the atmosphere over a specific time period (usually 100 years), relative to carbon dioxide (CO₂).

• CO₂ has a GWP of 1.
• Methane (CH₄) has a GWP of around 28–34.
• Some fluorinated gases have GWPs in the thousands.

Higher GWP means greater warming impact per tonne of gas emitted. The EPA publishes a more detailed explainer here.

5. Why do we talk about CO₂e?

‍CO₂e stands for carbon dioxide equivalent. It’s a common unit that expresses the impact of all greenhouse gases using the same scale. By converting methane, nitrous oxide, and other gases into CO₂e using their GWP, you can report a single carbon footprint figure that reflects the full climate impact of your activities.

6. Which greenhouse gases are the biggest contributors to climate change?

Globally, the main contributors are (all expressed in CO2e):

• Carbon dioxide (CO₂) – responsible for about 75% of global CO2e
• Methane (CH₄) – around 17%
• Nitrous oxide (N₂O) – roughly 6%
• Fluorinated gases – less than 2%

Check out the underlying data here.

7. How long do greenhouse gases stay in the atmosphere?‍

It depends on the gas:

• CO₂ can stay in the atmosphere for hundreds to thousands of years.
• Methane lasts around 12 years, but traps more heat in that time.
• Nitrous oxide persists for over 100 years.
• Fluorinated gases vary widely – some can remain for thousands of years.

This persistence is part of what makes cutting emissions urgent – once emitted, these gases don’t go away quickly.

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