Each human emits two tonnes of carbon dioxide a year

A study by the Universidad de Almería in Spain also established for the first time that human excrement contributes to water pollution, primarily with nitrogen and phosphorus.

"Food...produces emissions of around two tonnes of carbon dioxide per person and per year and consumes 20 gigajoules of primary energy," said main study author Iván Muñoz, according to The International Journal of Life Cycle Assessment.

The gigajoule is equal to one billion joules. Six gigajoules is about the amount of chemical energy in a barrel of oil. The study analysed the link of food production and consumption chain with global warming and acidification and eutrophication (excess of nutrients), taking what a person in Spain ate in 2005 (881 kg) as a reference.

Calculations included agricultural and animal production, industrial food processing, sale and distribution, preparation and cooking at home, solid waste treatment (food remains and packaging), as well as human excretion.

According to the study, producing food from animals such as meat and dairy products,
causes the greatest impact, a Universidad de Almería release said. Agriculture, livestock, fishing and the food industry are the greatest source of carbon dioxide water pollution, but in both cases the effects of human excretion (through breathing or due to waste water treatment) are next on the list.