Calculating food miles

The greenhouse gas emissions caused by baked beans have amused schoolboys for decades. Now they are proving a headache for experts at Oxford University.

Recently the United Kingdom supermarket chain Tesco announced it would introduce labels on its products, detailing their carbon footprint. The information, it said, would go beyond the mere question of food miles — how far the produce has been transported — to include indirect greenhouse emissions given off during its production and processing. Tesco freely admitted that it doesn’t know how to measure this yet, and has effectively outsourced the problem to scientists at Oxford’s Environmental Change Institute in southern England, along with the promise of £5-million funding to help them along.

The problem for Tesco’s grand announcement is that Brenda Boardman, who leads the institute, is in the dark too. ‘I don’t know how we’ll do this either yet. We haven’t started and it’s not going to be an easy project,” she says. ‘Some ways of doing it are contested and there are accuracy issues. The first stage of the Tesco project is to get people together to talk about whether there is a standard way we can do it.”

In principle, the concept is easy. A so-called ‘life-cycle analysis” tots up the energy used to extract raw materials and turn them into products. The greater the energy use, the greater the carbon footprint, and the worse for the environment a product is. Tesco says such information would allow consumers to shop according to their environmental conscience. As demand for more damaging products falls, the thinking goes, so will the stocking of that product. The supermarket is not alone in coveting carbon labels: another UK food company, Duchy Originals, set up by Prince Charles, is among those investigating similar schemes.

The problems start in deciding exactly what emissions should be counted.

Direct carbon use is easy to measure, but indirect emissions are far more difficult. Should supermarkets include the electricity used to refrigerate products in their stores? What about the fuel in the tractors on a farm thousands of kilometres away? And if you think the answer is obvious, what about the fuel in the cars the farmworkers drive to get to work? ‘Boundaries are hugely difficult and, of course, the boundaries may not be in this country,” says Boardman. Some experts even argue the audited supply chain should extend as far as the ultimate source of energy — the sun.

There are other problems too. Most experts argue that renewable sources of electricity should be treated differently from energy drawn from fossil fuels, which could give some French products a much lower carbon footprint because of that country’s heavy reliance on nuclear power, which produces almost carbon-free electricity. But will shoppers share the view that such products are truly green? And some vegetables transported from abroad could still have lower carbon footprints than those home-grown inside heated polytunnels with bags of fertiliser. ‘There are offsetting reasons why one may not be better than the other,” says Boardman.

Such difficulties have not stopped some industries trying to work outthe ’embodied energy” of their products. ‘Embodied energy is becoming a much more important aspect to take into account,” says Ken Double, head of evaluation at the Energy Saving Trust. ‘And when we talk about embodied energy we often mean embodied carbon.”

The building trade is ahead of the rest in calculating such embodied emissions, partly because improvements in energy efficiency have steadily eaten into the reductions that can be achieved in energy use, so attention has switched to reducing the embodied impact of the materials used. Geoff Hammond and Craig Jones, mechanical engineers at Bath University, have compiled an inventory of the embodied energy of common building materials including bricks, carpets, toilets, glass and paint. Measured in megajoules/kg of material, the highest embodied energies come from materials that require high-temperature processing, such as aluminium (154MJ/kg), rubber (102MJ/kg) and plastics (81MJ/kg). Natural materials such as clay and plaster have values less than 3MJ/kg. The Bath scientists have also allocated an embodied carbon value: manufacturing each kilo of the ceramic lavatory in your house, for example, sent 1,4kg of carbon dioxide into the atmosphere.

The results neatly demonstrate the problem of developing advanced technology to tackle climate change. Photovoltaic cells, which turn sunlight into electricity and are increasingly common on buildings, have an embodied energy of 1 305 to 4 752 MJ/m2, depending on design. Even a PVC framed double-glazed window has an embodied energy of about 2 300MJ/m2 — with about 110kg of embodied carbon dioxide.

Boardman says such life-cycle calculations are often used to decide whether it would be more energy efficient to carry on using older products such as cars and buildings, or to replace them with cleaner versions. ‘You put together the carbon that has gone into making something with the lower levels of carbon that come from making it more efficient, and at some point there’s a crossover with your existing product.”

Such calculations are complicated, and without a standardised method, open to abuse. A report last year that claimed life-cycle analysis of new cars showed that hybrid cars consume more energy over their lifetime than gas-guzzling 4×4 vehicles. But Bob Saynor, a transport specialist with the Energy Saving Trust, says it was badly flawed. ‘There is a kernel of truth, in that it takes more energy to make a hybrid vehicle. But this adds only 2,5% to the total energy used in its lifetime, a figure dwarfed by savings of about 20% of total energy that come from hybrids’ better fuel consumption.”

The report’s authors, Saynor says, blundered in dividing the energy used to create a new model by the numbers of vehicles sold. ‘This explains their ludicrous conclusion that a Jeep is greener than a Honda Accord hybrid, since the Honda has so far only sold tens of thousands whereas old designs like the Jeep have sold millions.”

Dodgy embodied energy calculations can clearly produce emissions as bad as baked beans. Tesco has been warned. —

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David Adam
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