Environment: treading lightly on the Earth
Environment: treading lightly on the Earth

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1.3.1 Converting carbon to CO2 (or CO2e) emissions

This study note will explain how to convert carbon to CO2 (or CO2e) emissions.

Study note: Converting carbon to CO2 (or CO2e) emissions

You have to be careful when looking at GHG emissions data to be clear on units. Emissions are sometimes expressed in tonnes of carbon rather than carbon dioxide or CO2 equivalents (CO2e). This expresses emissions using just the mass of the carbon (C) atom rather than the whole carbon dioxide (CO2) molecule or CO2 and other GHG emissions converted into CO2 equivalents (CO2e).

Each chemical element has an atomic mass associated with it, but these would be tiny numbers for individual atoms if expressed in grams or kilograms. So these are usually given as the relative atomic masses, taking the mass of a carbon atom to be exactly 12 units (usually abbreviated ‘amu’ for ‘atomic mass units’). Using this convention, the relative atomic mass of hydrogen is close to 1, carbon 12, and oxygen about 16.

So a water molecule H2O would have a mass of (2 × 1) + 16 = 18 amu. A carbon dioxide molecule (CO2) would have mass 12 + (2 × 16) = 44 amu.

As a carbon atom has a mass of 12 amu, whereas each CO2 molecule has mass 44 amu, the form in which emissions totals are given makes a great difference to the amounts recorded. So always check whether emissions are given in units of C, CO2 or CO2e, and convert if necessary (as shown below):

  • 1 tonne carbon emissions = 1 tonne × 44/12 = 3.67 tonnes CO2 (or CO2e) emissions (the extra 2.67 tonnes came from the oxygen atoms in the molecule).
  • 1 tonne CO2 (or CO2e) emissions = 1 tonne × 12/44 = 0.27 tonnes carbon emissions (the other 0.73 tonnes were oxygen).


  • 12 amu of carbon produces 44 amu of CO2 emissions.
  • So 12 tonnes of carbon produces 44 tonnes of CO2 emissions.
  • Therefore 1 tonne of carbon produces 44/12 × 1 = 3.67 tonnes of CO2 emissions.

However, since the mass of carbon emissions is directly proportional to the mass of CO2 or CO2e emissions, the percentage breakdown (e.g. in Figure 3) is the same whether you measure the carbon footprint using C, CO2 or CO2e.

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