What is a carbon footprint?

A footprint adds up emissions from energy use, travel, food, goods and services, converting gases such as methane into their carbon dioxide equivalent. The global average for fossil carbon dioxide is roughly 4.7 tonnes per person a year, while the average American is responsible for around three times that. Food choices matter: a 2018 study in the journal Science found beef from dedicated beef herds produces around 100 kilograms of greenhouse gases per kilogram of meat, far more than beans or peas. A single return flight from London to New York adds roughly a tonne of carbon dioxide per passenger. The phrase was popularised in the mid-2000s by a BP advertising campaign that encouraged people to calculate their personal footprint.