In 2008, two Carnegie Mellon researchers analyzed the reduction in carbon emissions that might come from moving to local food. They found that American food consumption produces greenhouse gas equivalent to 8.9 tons of carbon dioxide per household per year. Food delivery represents .4 tons of that total; all agricultural transportation up and down the food chain creates one ton of carbon dioxide per household annually. … … One recent UK report found that the greenhouse gas emissions involved in eating English tomatoes were about three times as high as eating Spanish tomatoes. The extra energy and fertilizer involved in producing tomatoes in chilly England overwhelmed the benefits of less shipping. Even New Zealand lamb produced less greenhouse gases than English lamb. …

But the most important environmental cost of metropolitan agriculture is that lower density levels mean more driving. … If just a twentieth of an acre of metropolitan farm land per person could (implausibly) eliminate half of food delivery emissions, this would typically be associated with 41 more gallons of gas per household. Those driving-related greenhouse gas increases would be 2.4 times higher than the emissions savings from reduced food transport.