Environmental Impact Guidance
Step 3: Express all changes in emissions in terms of their carbon equivalent
Greenhouse gases have different impacts in the atmosphere - some are more potent than others in causing global warming. As a result, adding the volume (or weight) of emissions of different gases will not give a good indication of the actual impact that a change in the level of emissions of different gases might have on global warming.
In order to get a comparable figure for each gas in terms of their impact, the amount of each gas can be given in terms of its carbon equivalence. To do this, first multiply the weight (in tonnes) of the emissions of each gas by its Global Warming Potential (GWP) - see table below.
Greenhouse Gas |
100 years GWP* |
Carbon Dioxide (CO 2 ) |
1 |
Methane (CH 4 ) |
23 |
Nitrous Oxide (N 2 O) |
296 |
Hydrofluorocarbons (HFCs) ** |
12 - 12,000 |
Perfluorocarbons (PFCs) ** |
6,500 - 9,200 |
Sulphur hexafluoride (SF 6 ) |
22,200 |
Source: Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis ; see http://www.grida.no/climate/ipcc_tar/wg1/index.htm
* GWPs can be calculated for different time horizons to show the effects of atmospheric lifetimes of the different gases. It is recommended that a timescale of 100 years is used for the purposes of conducting a carbon impact assessment.
** The Different HFC and PFC gases have different GWPs. Click here to see the full list of HFC and PFC gases and associated GWPs.
This gives you the impact of each of the greenhouse gases in terms of their carbon dioxide equivalent. In order to quote the emissions in terms of their carbon equivalent, multiply the (tonnes x GWP) figure for each gas by 12, and divide by 44.
John considers each gas in turn. The increase of 857 tonnes of carbon is already given in terms of carbon, so he doesn't need to do anything to this. The 0.13 tonnes of methane he calculates to be equivalent to (0.13 x 23) = 3.0 tonnes of carbon dioxide. He multiplies this by (12/44) in order to get an equivalence of 0.8 tonnes of carbon. The 1.2 tonnes of nitrous oxide is equivalent to (1.2 x 296) = 355.2 tonnes of carbon dioxide. He works this out to be equivalent to 96.9 tonnes of carbon. |
Page last modified: 16 November 2004
Page Published: 16 November 2004