Highlights from an article at Delicious Earth:

• Shipping is responsible for 4.5% of all global emissions of CO2 – almost three times higher than previously believed.
• Marine heavy fuel oil, which is burned by all large ships, is the residue of the world's oil refineries.
• Alarming levels of cancer-causing air pollution are being released.
• Shipping stuff by airplane generates even more emissions.
• Food imports from China have tripled over the past decade.
• China is now Canada's fifth largest supplier of fresh fruits and vegetables.

Ships exploit a ready supply of the world's cheapest, most polluting "bunker" fuel.

Despite the trail of climate change emissions and pollution associated with shipping, there are notable examples where local foods are worse than imported alternatives. When evaluating food, you have to consider the entire life cycle of the product. For example:

• Importing beans from Africa – where the farms are small, tractor use is limited, and natural fertilizers are used – tends to be more efficient than growing beans in Western countries, due to our reliance on energy-intensive chemical fertilizers and irrigation. 
• Greenhouse-grown roses in Holland require six times more energy than roses flown from North Africa to Europe.
• Because of methane emissions and feed crops, local meat has a much higher climate change footprint than imported vegetarian alternatives.

While you have to consider the entire life cycle of the food, some basic guidelines still hold true: Choose foods that are low on the food chain, locally-grown and seasonal, locally-grown and preserved, and/or organic.