Food's Greenhouse Gases

One challenge for a consumer trying to reduce his or her carbon footprint is determining which foods release the most carbon dioxide and other greenhouse gases (GHG) like methane from ruminant digestion or rice paddies, and nitrous oxide emissions from fertilizer and manure. In America, emissions from food are almost twice those from driving, amounting to 8.1 tonnes of CO₂ equivalent for the average US household. The calculations are difficult because there are many site-specific variations, but approximations can provide a first guide to consumption choices. For example, an American switching to a vegetarian diet would save 1.5 tonnes of CO₂eq (Trivedi, 2008).

Red meat is one of the worst, with livestock accounting for 18 percent of all man-made greenhouse gas emissions: 9% of CO2, 35-40% of methane and 65% of nitrous oxide (mostly from fertilizer). Only 2 to 25% of nutrients are converted into edible meat depending on the species: 2.3kg of grain for each kg of chicken, 5.9kg for a kg of pork, and 13kg of grain plus 30kg of forage for a kg of beef. Dairy products are thus also GHG intensive, as are eggs. Most vegetables and fruits are low in GHG emissions, unless they are grown in heated greenhouses (Trivedi, 2008).

Organic grain has a lower carbon footprint than conventional grain because of the elimination of nitrogen fertilizers, but organic poultry requires 10% more energy than intensively-raised poultry. Organically-raised salmon has 30% more GHG emissions than conventionally farmed salmon because of a required higher-quality fishmeal diet. Farmed shrimp have a particularly high carbon footprint. Roaming the oceans to catch large fish for human consumption is very energy-intensive. Farmed herbivorous fish like tilapia, carp, bream and catfish have the lowest footprint (Trivedi, 2008).

Buying local may only make a marginal difference, as 83% of GHG emissions are associated with food production, and only 4% with food miles or the transport from foreign countries, and another 5% from moving the food from wholesaler to retailer and from the consumer taking it home. An American buying everything local would only cut their carbon footprint by 4%, the same savings as not eating red meat one day a week (Trivedi, 2008).

- Trivedi, Bijal. 2008. Dinner's Dirty Secret. New Scientist, 13 September 2008, p. 28-31.