The next food label on the horizon: Greenhouse Gas Emissions. When it appears, there is no guarantee that local food is going to get the best score.

An analysis of the environmental toll of food production concludes that transportation is a small portion of the total “carbon footprint” of various foods. Beef and dairy are responsible for far more greenhouse gases than produce, whether they are locally raised or transported from the other side of the country.

“If you have a certain type of diet that’s indicative of the American average, you’re not going to do that much for climate while eating locally,” says Christopher Weber, a researcher at Carnegie Mellon University in Pittsburgh. Weber used an input–output life-cycle assessment, which counts not only the CO2 produced when food is shipped but also all greenhouse gases, including methane (CH4) and nitrous oxide (N2O), emitted from farm production. This means counting all the way back to the fossil fuels used to manufacture fertilizer and tractors.

“There is more [total] greenhouse gas impact from methane and nitrous oxide than from all the CO2 in the supply chain,” Weber says. In large part, he adds, this is because N2O and CH4 emission in the production of red meat “blows away CO2”. Cows burp methane, and growing their feed uses large amounts of fertilizers that are converted to N2O by soil bacteria..

Final delivery “food-miles” make up just 1% of the greenhouse emissions of red meat, and 11% for fruits and vegetables. Weber calculated that a completely local diet would reduce a household’s greenhouse emissions by an amount equivalent to driving a car 994 miles fewer per year, assuming 25 m.p.g. Switching from red meat to veggies just one day per week would save the equivalent of 1,155 miles of driving.

Edgar Hertwich, an expert on life-cycle analysis who is at the Norwegian University of Science and Technology, calls the results “quite convincing” but notes that consumers should still keep an eye on food flown on airplanes, which have very high greenhouse gas emissions.

Whatever the source of greenhouse gas emissions from food, many are now calling for labeling that lets shoppers know how much carbon went into their goods. In the UK, the government-supported Carbon Trust offers a voluntary carbon label, and a proposed California law aims to regulate such labeling, much like organic food standards.

“Our goal is to get the most accurate information that’s available in the hands of consumer so they can make informed purchasing decisions,” says Matthew Perry, head of Carbon Label California.

(Environmental Science and Technology,April 16, 2008,  http://pubs.acs.org/journals/esthag/index.html