A carbon footprint is an estimate of the climate change impact of activity – such as making a product, living a lifestyle or running a company.
Typically, a carbon footprint is calculated by estimating not just the CO2 emissions that the activity in question causes, but also any emissions of other greenhouse gases (such as methane and nitrous oxide) and in some cases other types of climate impacts as well, such as vapour trails from aeroplanes. For simplicity, all these impacts are added together and expressed as a single number in terms of carbon dioxide equivalent (CO2e): the amount of CO2 that would create the same amount of warming.
There are many existing and evolving standards for calculating carbon footprints but in truth no footprint is precise. Even for simple activities such as burning a litre of petrol, which releases a known amount of CO2, there are still uncertainties about the emissions caused by extracting and refining the petrol before it was burned. For more complicated activities these uncertainties are greatly multiplied.