Greenhouse Gas
Every carbon footprint, emission factor, and net-zero target ultimately traces back to one physical fact: a handful of gases in the atmosphere let sunlight in but trap the heat trying to escape. Those gases are the reason climate accounting exists.
A greenhouse gas is any gas that absorbs and re-emits infrared radiation — and the small set that human activity is loading into the atmosphere is what every GHG inventory measures.
A greenhouse gas (GHG) is a gas that absorbs and re-emits infrared radiation, trapping heat in the atmosphere and driving global warming. The main ones in emissions accounting are carbon dioxide, methane, nitrous oxide, and the fluorinated gases, compared using global warming potential and reported in CO₂e.
What a greenhouse gas is
A greenhouse gas is any atmospheric gas that absorbs and re-emits infrared (heat) radiation. This property lets the gas trap outgoing heat that would otherwise escape to space — the greenhouse effect — warming the planet’s surface.
The mechanism is specific. Incoming sunlight is mostly short-wave radiation that passes through the atmosphere and warms the surface. The surface radiates that energy back out as long-wave infrared. Greenhouse gases absorb some of this infrared and re-emit it in all directions, including back downward, so heat accumulates near the surface. Gases made of two identical atoms, like nitrogen (N₂) and oxygen (O₂) — which make up most of the air — do not do this; only molecules with the right structure, such as CO₂, methane, and water vapour, absorb infrared. That is why a handful of trace gases, present in parts per million, govern the planet’s temperature.
The greenhouse effect itself is natural and essential — without it Earth would be far below freezing. The concern is the enhanced effect: human activity has raised the concentration of these gases, trapping more heat. Greenhouse-gas accounting exists to measure that human contribution, which is why an inventory counts the gases people emit, not the natural background.
The main greenhouse gases
GHG accounting focuses on the “Kyoto basket” — the seven gases and gas groups that human activity emits and that international frameworks require companies and countries to report. They differ enormously in how strongly and how long they trap heat.
| Gas | Formula | AR6 GWP-100 | Main human sources |
|---|---|---|---|
| Carbon dioxide | CO₂ | 1 | Fossil-fuel combustion, cement, deforestation |
| Methane | CH₄ | 29.8 | Agriculture, fossil fuels, landfill |
| Nitrous oxide | N₂O | 273 | Fertiliser, combustion, industry |
| Hydrofluorocarbons | HFCs | ~100–14,800 | Refrigeration, air conditioning |
| Perfluorocarbons | PFCs | ~6,600–11,100 | Aluminium smelting, electronics |
| Sulphur hexafluoride | SF₆ | ~25,200 | Electrical switchgear |
| Nitrogen trifluoride | NF₃ | ~17,400 | Electronics manufacturing |
Carbon dioxide is the reference point at a GWP of 1, but it is not the most potent — it dominates because of the sheer quantity emitted. The fluorinated gases are emitted in tiny volumes yet punch thousands of times above their weight per tonne, which is why they matter far more than their mass suggests.
Comparing gases: GWP and CO₂e
Because the gases trap heat so differently, they cannot simply be added by mass. Each is converted to a common currency using its global warming potential (GWP) — a factor expressing how much heat a tonne of the gas traps over a chosen period (usually 100 years) relative to a tonne of CO₂. Multiplying each gas by its GWP and summing gives the total in carbon dioxide equivalent (CO₂e).
On the current IPCC AR6 100-year basis, one tonne of methane equals 29.8 tonnes of CO₂e [GreenCalculus gwp.CH4_fossil.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100 · v2026.203] and one tonne of nitrous oxide equals 273 tonnes [GreenCalculus gwp.N2O.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100]. This conversion is what makes a single-number carbon footprint possible — every gas expressed in the same unit. It also means the GWP basis chosen is part of any GHG figure, not an afterthought.
Why water vapour is not counted
Water vapour is actually the most abundant greenhouse gas and contributes the largest share of the natural greenhouse effect — yet it appears nowhere in a company’s GHG inventory. The reason is a crucial distinction between a forcing and a feedback.
Humans do not directly control atmospheric water vapour; its concentration is set by temperature. As the long-lived gases warm the planet, warmer air holds more water vapour, which amplifies the warming — a feedback. Because it responds to temperature rather than driving it independently, water vapour is treated as part of the climate system, not as a reportable emission. Inventories count the gases we add directly, which force the warming that water vapour then amplifies.
Where greenhouse gases come from
Every greenhouse gas has both natural and human sources; accounting is concerned with the human (anthropogenic) additions that are shifting the balance. The dominant human sources map closely onto the gases:
- Carbon dioxide — burning coal, oil, and gas for energy and transport, plus cement production and land-use change.
- Methane — livestock digestion, rice paddies, landfill, and leaks across the oil and gas supply chain.
- Nitrous oxide — nitrogen fertiliser on farmland, some combustion, and industrial processes.
- Fluorinated gases — refrigerants, electrical equipment, and electronics and metals manufacturing; entirely industrial in origin.
In accounting terms these are organised by where they occur relative to the reporting company — directly from owned sources, from purchased energy, or across the value chain — the basis of the scope framework used in every GHG inventory.
Worked micro-example
A site emits three gases in a year. Each is converted to CO₂e using its AR6 GWP-100 value, then summed.
| Gas | Amount × GWP | CO₂e |
|---|---|---|
| Carbon dioxide | 10 t × 1 | 10.0 tCO₂e |
| Methane | 1 t × 29.8 | 29.8 tCO₂e |
| Nitrous oxide | 0.1 t × 273 | 27.3 tCO₂e |
| Total | — | 67.1 tCO₂e |
By mass the site emitted 11.1 tonnes of gas, but by warming impact it emitted 67.1 tCO₂e — because the small amounts of methane and nitrous oxide dominate once GWP is applied. This is why non-CO₂ gases cannot be ignored.
Common mistakes
- Equating greenhouse gases with CO₂. Carbon dioxide is one of several; methane, nitrous oxide, and F-gases often dominate a footprint once GWP is applied.
- Adding gases by mass. Gases must be converted to CO₂e via their GWP before summing — a tonne of SF₆ is not a tonne of CO₂.
- Expecting water vapour in an inventory. It is the largest natural greenhouse gas but a feedback, not a directly-emitted forcing, so it is not reported.
- Dismissing F-gases because volumes are tiny. Their GWPs run into the thousands, so small leaks carry large CO₂e.
- Omitting the GWP basis. The same emissions give different CO₂e totals under AR5 versus AR6 — always state the basis.
See what a tonne of greenhouse-gas emissions actually means in everyday terms.
Frequently asked questions
A greenhouse gas is a gas that absorbs and re-emits infrared radiation, trapping heat in the atmosphere and warming the planet — the greenhouse effect. The main greenhouse gases in emissions accounting are carbon dioxide, methane, nitrous oxide, and the fluorinated gases, which are compared using global warming potential and reported in carbon dioxide equivalent (CO₂e).
The Kyoto basket covers carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF₆), and nitrogen trifluoride (NF₃). These are the gases GHG inventories are required to report; they differ by orders of magnitude in how strongly they trap heat.
No. Per tonne, carbon dioxide is the weakest of the accounted gases — it has a global warming potential of 1, the reference value. Methane, nitrous oxide, and the fluorinated gases trap far more heat per tonne. Carbon dioxide dominates emissions only because it is released in vastly greater quantities.
Water vapour is the largest natural greenhouse gas, but its concentration is controlled by temperature rather than emitted directly — it is a feedback, not a forcing. As the long-lived gases warm the atmosphere, it holds more water vapour, amplifying the warming. Because humans do not add it directly, inventories count the forcing gases, not water vapour.
Through global warming potential (GWP), which expresses how much heat a tonne of each gas traps over a chosen period — usually 100 years — relative to a tonne of CO₂. Multiplying each gas by its GWP converts it to carbon dioxide equivalent (CO₂e), allowing all gases to be added into a single comparable figure.
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