Edward L. Glaeser (Professor of Economics at Harvard) article on density vs. urban farming

http://articles.boston.com/2011-06-16/bostonglobe/29666344_1_greenhouse-gas-carbon-emissions-local-food

"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."