All that said, I think Christine's also right to emphasise the long term question mark over the sustainability (in the literal rather than normative sense) of intercontinental supply chains for low value-added bulk products: see CIBC's analysis on this at http://yaleglobal.yale.edu/about/pdfs/oil.pdf.

(True, commodity prices have fallen a lot over the last few weeks - but without a really significant shift in the underlying supply and demand fundamentals, we'll be back staring at the problems of high oil and food prices as soon as we emerge from the recession. More on this at http://globaldashboard.org/wp-content/uploads/2008/08/WFP_speech.pdf)

You pose the question in terms of a trade-off between the environmental damage and the benefit in terms of income for poor farmers.

But for many products, there is no such trade-off because the products grown in Africa lead to less environmental damage overall than if they were produced in Europe. The environmental damage done by flying roses to Europe is generally reckoned to be less than the environmental damage done by growing roses in European hothouses. So buying roses from Africa not only helps the poor farmer, it is also less damaging to the environment than buying roses grown locally.

"Food miles" should be a discredited concept: we should not be interested in the "food miles" that products have incurred but the total tonnes of CO2 emmissions required to put the product on our table. Very often, the African products will score as more environmentally friendly overall than their local competitor, even if they involve more food miles.

But consumers can't be expected to weigh all this up in a complex calculation as they walk down the supermarket aisle. Fortunately, a simple solution is at hand. If emmissions of greenhouse gases were capped at an environmentally sustainable level (which may well be at zero net emmissions) and the permission to emit those greenhouse gases were tradable, then the price of each good or service would reflect the full cost to the planet of producing it. Consumers could then make their choices based on price alone.

Owen