Just this Saturday I went to the local 'farmer's market' where I had the choice of locally grown tomatoes (it's winter here, darnit!) or organic ones from Spain... which ones would you buy -those that are grown conventionally in heated green-houses or those that come from thousands of kilometers away?
Well here pretty much all tomatoes are grown in greenhouses because we don't really have the climate for it, but that's by the by. In general, I would prefer something grown locally, even under artificial conditions, to something shipped in from thousands of kilometres away. The carbon cost of packaging, transport at both ends, the main journey, preserving the food during the trip, resulting increased waste etc. is enormous, and I'd have a hard time believing that running a greenhouse is worse than repeating that process over and over again to replenish stocks.
Obviously, this can't always be followed. However hard you try, there are some things the West Country will just not support. But, for example, in Sainsbury's I am faced with a choice of mushrooms from down the road in Somerset or mushrooms from Spain. You'll be pleased to know the Somerset ones are even organic