From the FT:
On the issue of carbon emissions, Maxwell contends, citing research by the UK’s Department for Environment, Food and Rural Affairs, that half the total environmental impact of food transport is caused by individual consumer shopping trips – in fact, Maxwell says, more than the impact of transporting it from overseas. And there is the issue of jobs in developing countries. In Kenya alone, more than 1m jobs depend on exporting fresh produce to the UK.
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..two Carnegie Mellon researchers recently broke down the carbon footprint of foods, and their findings were a bit surprising. 83 percent of emissions came from the growth and production of the food itself. Only 11 percent came from transportation, and even then, only 4 percent came from the transportation between grower and seller (which is the part that eating local helps cut).