The estimated climate change impact of something, such as a product or service or a company's operations.
It is common shorthand for accounting for all greenhouse gases. Using the example of plastic, so-called ‘indirect’ emissions, such as extracting oil to make the plastic, need to be traced back and included alongside direct emissions, such as producing something with the plastic.
Claiming carbon-neutrality when not accounting for all potential costs can be seen as 'public relations'.
Any footprint calculation is an estimate, based on more or less reliable data – so figures need treating with caution and discernment. As ever, we notice and value what we measure, so may ignore and devalue vital intangibles we cannot measure such as biodiversity and its loss.
Despite challenges, trying to count carbon impacts focuses attention, and might help us develop 'a carbon instinct' (Berners-Lee, M. How bad are bananas? The carbon footprint of everything, 2010, p.xi), and thus direct our efforts where we might have greater effects.
When organisations take on the task of carbon footprinting, often change is progressive. It is a learning process, with the aim being successively lower impacts. Developing carbon-consciousness helps individuals and companies make strategic choices, and influences daily practices.
You could calculate the relative carbon footprints of people travelling by train or plane to a meeting, or of video-conferencing instead. Costs of accommodation, meals and other impacts would need to be taken into account.