Where emission factors come from
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- Emission factorClaude (opens a new tab): explain Emission factorChatGPT (opens a new tab): explain Emission factor
- Elementary flowClaude (opens a new tab): explain Elementary flowChatGPT (opens a new tab): explain Elementary flow
- Characterisation factor (GWP100, IPCC AR6)Claude (opens a new tab): explain Characterisation factor (GWP100, IPCC AR6)ChatGPT (opens a new tab): explain Characterisation factor (GWP100, IPCC AR6)
- Reading an LCI dataset's metadataClaude (opens a new tab): explain Reading an LCI dataset's metadataChatGPT (opens a new tab): explain Reading an LCI dataset's metadata
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You multiply quantity by an emission factor. But that factor is itself the end of a long calculation. Here is what happened before it reached you.
In one line
An emission factor is a list of elementary flows, each weighted by a characterisation factor, summed into one number.
The chain behind one number
Section titled “The chain behind one number”- Process. A power plant, a steel mill, a truck. Measured or modelled.
- Elementary flows. What crosses into nature: CO₂, CH₄, N₂O to air, per unit of output.
- Characterisation. Each flow multiplied by its weight, for example IPCC GWP₁₀₀.
- Emission factor. The sum. The number you see in a database.
In a database, step 2 already includes everything upstream: the coal mine behind the power plant, the ore behind the steel. That is why one factor can stand for a whole supply chain.
What is an elementary flow?
Section titled “What is an elementary flow?”An elementary flow is a substance taken from nature or released to it, with nothing else done to it by people. Fossil CO₂ to air is one. So is water from a river, or methane leaking from a pipeline. Steel and electricity are not: they are products, made of many flows further up the chain.
Worked example: 1 kWh of grid electricity
Section titled “Worked example: 1 kWh of grid electricity”The flows below are the grid electricity process from the worked example in lesson 8. They are hypothetical teaching numbers. The GWP₁₀₀ values are real: IPCC AR6.
| Elementary flow, to air | kg | GWP₁₀₀ | kg CO₂-eq |
|---|---|---|---|
| Carbon dioxide, fossil | 0.35 | 1 | 0.3500 |
| Methane, fossil | 0.0008 | 29.8 | 0.0238 |
| Dinitrogen monoxide (N₂O) | 0.00001 | 273 | 0.0027 |
| Emission factor | 0.3766 |
So this grid’s emission factor is 0.3766 kg CO₂-eq per kWh. Methane and nitrous oxide add about 8% to the CO₂ alone, which is why a factor that counts only CO₂ understates climate change.
Read the label before you use it
Section titled “Read the label before you use it”Two factors for “steel” can differ several-fold. Every database record carries the facts that explain why:
| Field | What it tells you |
|---|---|
geography |
Which grid, which country’s mills. |
technology |
Blast furnace or electric arc, virgin or recycled. |
reference year |
Grids decarbonise; old factors overstate. |
boundary |
Cradle-to-gate, or more? |
method |
Which GWP set. Fossil methane is 29.8 over 100 years, 82.5 over 20 (AR6). |
LCA 202 lesson 2 shows how to choose between datasets once you can read these fields.
Check yourself
Is “1 kg of steel” an elementary flow?
Show answerHide answer
No. Steel is a product made by people. Its factor is built from the elementary flows of mining, coking, smelting and the energy behind them.
What you should now be able to do
Section titled “What you should now be able to do”- Explain an emission factor as a sum of elementary flows times characterisation factors.
- Tell an elementary flow from a product flow.
- Recalculate a simple emission factor by hand from its flows and IPCC AR6 GWP₁₀₀ values.
- Name the five things to check on a dataset before you use its factor.
Sources
Section titled “Sources”- IPCC (2021), Climate Change 2021: The Physical Science Basis, Working Group I contribution to AR6, Chapter 7, Table 7.15 (GWP₂₀ and GWP₁₀₀ for CH₄ and N₂O): https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
- ISO 14044:2006, definitions of elementary flow and characterisation factor (described in our own words): https://www.iso.org/standard/38498.html
Checked: 2026-10-03
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