Mythical food miles?

Choosing pre-packed salad

29 March 2013 by Adrian Williams

Everyone knows food miles are bad. The further something travels to reach us, the more it must harm the environment. But is this really true? As so often, the reality is more complicated - Adrian Williams explains.

The term 'food miles' was coined to represent the complexity of the whole food system. But the media soon turned it into a simplistic measure of distance alone. Do lots of food miles mean a higher carbon footprint? What is a carbon footprint, anyway, and can it help us make better decisions?

We can calculate a food's carbon footprint (CF) using Life Cycle Assessment (LCA). It is the sum of all greenhouse gas (GHG) emissions produced by making and delivering that food. For much industrial or domestic activity, the CF depends largely on fossil-fuel use - burning fossil carbon emits CO2. But in agriculture, two major gases change things: nitrous oxide from artificial fertilisers and manure management, and methane from animals' digestive systems and manure. Further along the food chain, refrigerants can leak from sources like mobile chillers on lorries and in shops.

To find a product's carbon footprint, we need to trace its GHG emissions all the way back to natural resources in the ground. So a lorry's carbon footprint must include emissions from extracting, refining and delivering the diesel, not just from burning it. Similarly, for crops we include the energy used in making fertilisers and pesticides, vehicle fuel for cultivation and harvest, the energy cost of making farm vehicles and other capital items, drying and storage energy and even the energy used to produce seeds.

Is local always best?

'Local is always best' is a commonly-heard mantra, but is it true? There is no single answer. Crops - including grass - vary in yield because of soil, climate, water availability and management approaches. Some crops are perishable, while others have long storage lives, though some of these need refrigeration and other preservative processes. So we can only start to answer 'local is best' in the context of our demands for different kinds of food.

Carbon footprint comparisons

If a crop's carbon footprint is the same at the farm gates of locations A and B, then moving the crop from A to B and consuming it there must be worse because of the transport emissions. But life is rarely so simple!

Take tomatoes. We can grow them outdoors in the UK, but with a small yield and very limited season; most commercial production needs heated greenhouses with extra CO2 to boost photosynthesis. We can grow tomatoes all year, but need extra energy for lighting in winter. Or we can buy tomatoes in winter from, say, Spain. There, little or no heat is needed and the cheapest tomatoes grow in simple plastic-covered structures.

Overall, producing tomatoes in the UK emits more GHG than transporting them from Almeria in Spain, but again the truth is complex. Tomato quality may not be the same, and production techniques are improving all the time; an increasing proportion of UK tomato-growers use combined heat and power - that is, they generate their own electricity and use the 'waste' heat that would otherwise go into the atmosphere to warm their greenhouses - or they import waste heat and CO2 from elsewhere, instead of using stand-alone heaters. This can mean their products emit less than they would if grown in Spain - provided both heat and CO2 would truly otherwise go to waste.

The comparison makes clear that being transported a long way does not contribute as much to GHG emissions as a heated production system. But for outdoor crops, the differences are smaller. For potatoes, the comparison is more about importing the crop from abroad versus storing those grown in the UK. The eastern Mediterranean is a common source, but here the emissions from transport outweigh those of storage, and the emissions from potato farms there and here are similar.

So, these two examples show there's no simple rule - more food miles isn't always a bad thing. Of course, the method of transport is crucial. Air emits most GHGs - about 10 to 25 more than heavy lorries for a given weight.

Beyond carbon footprints

Agriculture's environmental burdens go beyond GHG emissions alone, and the concept of the carbon footprint doesn't address this. Other burdens such as energy and resource use, land occupation and potential for water pollution are commonly included in wider LCAs. This makes comparisons more complex, but also possibly confusing. For example, beef from Brazil may have a higher carbon footprint than UK beef, but it uses less energy.

Beef from Brazil may have a higher carbon footprint than UK beef, but it uses less energy.

One area of increasing concern, though, is the amount of water that different foods consume. The subject has been raised by the Water Footprint Network and much has been made of the water footprint of, say, bread (1.6m3 per kilo) and beef (15.4m3). But these stark numbers alone do not address the real problems.

The crucial difference is the need for 'blue water' taken from rivers or aquifers for crop production, and what conflict there is for that water, which is also needed for human health and industry. If crops are watered by the rain, there is no conflict; if many users are competing for too little water, the conflict can be intense. So depending on demand, the same water footprint can cause much more harm in one place than in another. For a water footprint to be valuable to decision-makers, we need to find out how much stress the relevant water supply is under, and our methods to do this are still developing.

Other factors also need considering to put a water footprint figure into context, such as how quickly aquifers recharge. Current work towards an international standard on water footprints aims to use such an approach.

This brings us back to Spanish tomatoes. Almeria has winter sunshine, so we can have tomatoes - and other fresh vegetables - in the winter with lower GHG emissions than we could get by growing our own. But the water stress of growing tomatoes in Spain is very high. Rainfall is low and both horticulture and humans demand a lot of water. A recent national debate was resolved by opting for local desalination rather than a long pipeline from Catalonia.

Our future use of these precious resources must be managed with care to extend our sustainable life of the planet. Food miles do not tell the whole story, but they lead us to the methods that do - methods which we must develop and apply better: Life Cycle Assessment and impact-weighted Water Footprinting.

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Dr Adrian Williams is a principal research fellow at Cranfield University.