One important point made in the passage is that ----.

It cannot be denied that buying locally grown food
really does offer big advantages, not just in
freshness, but also in environmental savings.
Imported foods, especially those flown in from the
opposite hemisphere, use up huge amounts of jet
fuel – 127 calories of fuel per calorie of Californian
lettuce flown to Britain, and 66 calories of fuel per
calorie of South African carrot; that at least is what a
green research group based in London has
estimated. Much of that cost is hidden from
consumers, because air fuel attracts no tax – an
advantage guaranteed by international treaty. And
under the Kyoto Protocol, carbon emissions from
international transports aren't added to national
carbon-emission tallies, because nobody can agree
whose account to charge them to. But the fuel used
to import food and drink to Britain continues to
account for four million tonnes of CO2 emissions
annually, which is about 2.5 per cent of the national
total.