Carbon footprint is a measure of the total amount of greenhouse gases (GHG) emitted by an individual, organization, or activity, expressed as the amount of carbon dioxide equivalent (CO2e) emitted. Carbon footprint is measured in tonnes (metric tons) because it is a measure of the weight of GHG emissions.

The main GHGs that contribute to climate change include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. Each of these gases has a different global warming potential (GWP), which is a measure of the relative warming effect of a GHG compared to CO2 over a specific time period.

To calculate a carbon footprint, the amount of each GHG emitted is multiplied by its GWP and then added together to get the total CO2e emissions. The result is then expressed in tonnes (metric tons) of CO2e.

For example, if an organization emits 1,000 metric tons of CO2, 50 metric tons of CH4, and 10 metric tons of N2O, the total CO2e emissions would be calculated as follows:

1,000 metric tons CO2 x 1 (GWP of CO2) = 1,000 metric tons, CO2e 50 metric tons CH4 x 28 (GWP of CH4 over 100 years) = 1,400 metric tons CO2e, 10 metric tons N2O x 265 (GWP of N2O over 100 years) = 2,650 metric tons CO2e

Total CO2e emissions = 1,000 + 1,400 + 2,650 = 5,050 metric tons CO2e

Therefore, the carbon footprint of the organization would be reported as 5,050 metric tons CO2e.