Humans emit two tons of CO2 a year – through eating

You can turn down the heating; you can switch to a hybrid car. But unless you can stop, um, we’d better call it excreting, your efforts won’t go a long way towards cutting your carbon emissions.

For the first time, scientists have totted up just how much carbon is released into the environment by human excrement – and, boy, it’s a lot. Two tonnes of carbon dioxide a year, say researchers at Spain’s Universidad de Almería, or more than a fifth of total emissions.

Of course, the carbon produced directly forms part of a closed system. “Human excrements have a net null effect on global warming, as they are offset by carbon fixation in photosynthesis,” says author Ivan Munoz. “As a result, they do not contribute to increasing the concentration of CO2 in the atmosphere.”

But as well as the CO2 and methane produced by the human metabolism, the study also looked at the energy consumption associated with using toilet paper, soap and basin water, together with the treatment of sewage at treatment stations.

Other calculations included agricultural and animal production, industrial food processing, sale and distribution, preparation and cooking at home and waste treatment of food remains and packaging.

And the team concludes that producing food from animals – meat and dairy products, in other words – causes the greatest impact on the environment.

“Human excretion contributes significantly to water polluting through providing organic matter and nutrients, such as nitrogen and phosphorus, which fosters the growth of algae, a decrease in the level of oxygen dissolved in the water, bad smells and other problems associated to eutrophication,” Muñoz says.