"We're not trying to tell people they need to go vegan now," he added. "We're saying there are a lot of alternatives."
Greenhouse Gas Diet
Weber and Matthews aimed to make their study comprehensive by focusing not only on the main greenhouse gas, carbon dioxide (CO2), but also on methane and nitrous oxide.
These gases are much less common than CO2, but they're far more effective at trapping heat in the atmosphere, so they also make a big difference.
"The non-CO2 greenhouse gases make up about half the impact," Weber said.
Vast amounts of methane—the main component of natural gas—comes out of both ends of animals such as cows and sheep and from decomposing manure, Weber says.
(Read related story: "New Weapon Against Warming: 'Flatulence Cards' Offset Dog, Human Emissions" [March 6, 2007].)
And when human-made nitrogen-containing fertilizers break down naturally, fields give off nitrous oxide.
But the climate impacts of foods vary widely by region and by season, so Weber is skeptical about efforts to label foods with their greenhouse gas emissions.
"It's kind of ludicrous that you could put a number on a product like this and have it make sense to people," he said.
People who support eating locally say the study does not undermine the locavore movement.
They're concerned about more than climate impacts, such as the effect of farming on communities and the conditions of farm workers, they say.
The new study "hasn't really changed my ideas drastically," said Sage Van Wing, a Seattle-based writer.
She coined the term locavore with her friend Jessica Prentice, and they helped found a group of the same name.
"We're looking for locally sourced foods that have less inputs, either from transportation or production," Van Wing said.
"The easiest sound bite is 'food miles,'" she added. "That's not to say that's the only thing locavores is about, but it's the starting point."
David Morris is vice president of the Institute for Local Self-Reliance, based in Minneapolis, Minnesota.
"The local food movement did not develop because of concerns about climate and greenhouse gas emissions," Morris said.
"It developed because people don't trust multinational companies," he said. "They have a desire to know the supplier [of their food].
"People feel that [local food] is safer, and it's fresher," he added.
Van Wing and Morris aren't content to advise people on how to minimize their climate impacts, but they want to change the way food is raised.
"The whole idea behind locavores is to try to create a market behind people farming the way we prefer and a distribution system for small-scale farms," Van Wing said.
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