LOCAL or imported? Conventional or organic? Can you make choices that will keep your diet healthy and reduce your carbon footprint? Is it possible to eat green? Does it even matter?
It may surprise you to learn that our diets account for up to twice as many greenhouse emissions as driving. One recent study suggested that the average US household's annual carbon food-print is 8.1 tonnes of "equivalent CO2 emissions" or CO2eq (a measure that incorporates any other greenhouse gases produced alongside the CO2). That's almost twice the 4.4 tonnes of CO2eq emitted by driving a 25-mile-per-US gallon (9 litres per 100 kilometres) vehicle 19,000 km - a typical year's mileage in the US.
As greenhouse gas emissions attract ever greater scrutiny and criticism, the fields of sustainable consumption and life-cycle carbon accounting have prompted academics to tally the greenhouse gas emissions of hundreds of products and manufacturing processes so that we can make more environmentally friendly food choices.
In the UK some supermarkets have already begun pilot programmes to label foods with their carbon footprint. One potato crisp producer is now labelling some lines with their CO2eq footprint - the makers calculated that each 34.5-gram packet that leaves the factory accounts for 75 grams of CO2eq. The Carbon Trust, a campaign group based in London, is working on a standardised system that companies can follow to work out the CO2eq footprint of any product.
So how do you calculate your stomach's CO2eq footprint? It's far from simple. For a start, you have to analyse every joule of energy used, from farm to fork, to measure its greenhouse gas contribution. Food produced using wind or solar power will produce lower emissions than food reliant on gas or coal, for example. For meat and dairy produce you also have to account for methane and nitrous oxide emissions - both potent greenhouse gases.
Methane remains in the atmosphere for 9 to 15 years and traps heat 21 times as effectively as CO2. Fertilisers and manure release nitrous oxide, which is 296 times as good as CO2 at trapping heat and remains in the atmosphere for 114 years on average. A food's emissions total also depends heavily on where it grew and how it was transformed from raw ingredients into your dinner. This includes gases generated by tilling the land, sowing the crops, making fertilisers and pesticides, harvesting the food and shipping it to processing plants, as well as electricity for cleaning, processing and packing your food, and then transporting it to your store. Finally, the loss of carbon sinks when forests are cleared for grazing or crops has to be accounted for.
The calculations can become "fiendishly complicated", says Astrid Scholz, an ecological economist at Ecotrust, a think tank based in Portland, Oregon. Scholz led the development of a carbon calculator for the Bon Appétit Management Company Foundation, which developed a Low Carbon Diet for its 400 plus cafeterias in the US.
For example, to calculate the CO2eq impact of eating an industrially raised chicken breast, you would factor in the following. First, there's the emissions from preparing the feed pellets. This would include the fertiliser, growing and processing the grain, and finally transforming it into bite-sized pellets that will feed the chicken while it sits in a hut with 250,000 other birds. Add to that the energy for heating the structure, the fuel for transporting the chicken to the slaughter facility, and the emissions from running the slaughtering facility and manufacturing the packaging.
Then there are emissions from transporting the animal to the wholesaler, the refrigeration costs of storing the meat, the trip to the retailer, and further refrigeration in the shop. Then you drive to the store, buy your chicken, drive home and cook it - all those emissions count too. Chicken is a relatively simple example, but the more stages involved in a food's production, the harder it becomes to calculate its true CO2eq footprint.
Scholz found that until recently there had been no wide scale effort to calculate CO2eq for foods in the US. In Europe, however, there are fledgling programmes that have calculated CO2eq emissions for some foods, so she used these figures to create a carbon calculator that she says gives comparable figures for the US. "We took a Dutch chicken farm and plopped it in Texas and assumed that it worked in a similar way," she explains.
She describes the resulting carbon calculator as "version 1.0 of a good idea". It doesn't give you the derivation of the figures, but it will tell you that 333 grams of CO2eq is emitted to make one hard-boiled egg. Compare that with a bowl of cereal with milk: 1224 grams of CO2eq - equivalent to driving a typical SUV 6 km.
The main culprit in the bowl isn't the cereal, it's the milk. That's because the most emissions-intensive foods are red meat and dairy products. In general, red meat emits 2.5 times as much greenhouse gas as chicken or fish, since rearing cows and other livestock requires a lot of energy. It takes 2.3 kilograms of grain to make every kilo of chicken meat, 5.9 kg of grain for a kilo of pork, and 13 kg of grain plus 30 kg of forage for a kilo of beef. Worse still, they produce methane and their manure releases nitrous oxide.
However, Peter Tyedmers, an ecological economist at Dalhousie University in Nova Scotia, Canada, warns that such calculators should be taken with a pinch of salt. Tyedmers and his students provided much of the raw data for the calculator, and while he agrees it is a good idea in principle, he says the figures they came up with are specific not just to the precise types of foods they measured, but to every detail of where and how they were produced, so cannot be generalised. For example, regional differences in farming practices can make a big impact on the final figure, he says. Simply changing an animal's feed can have a huge impact on its CO2eq footprint too. "It's all very fluid," says Tyedmers. "There's a tremendous hunger for these sorts of numbers and this has created the assumption that any existing figures are robust. They're not."
Tricky as it may be, some general rules are emerging that can guide you towards a less carbon-intensive diet. One sure-fire way of reducing your CO2eq footprint is to go vegetarian. Gidon Eshel and Pamela Martin, at the University of Chicago, calculated that switching from the average American diet to a vegetarian one could cut annual emissions by almost 1.5 tonnes of CO2eq per person.
If you can't face life without steak, there might soon be an alternative source of meat that can still dramatically cut your CO2eq emissions: in vitro meat. Animal-rights organisation People for the Ethical Treatment of Animals (PETA), which promotes vegetarianism, is offering a $1 million prize for an industrial-scale means of making in vitro chicken meat with a taste and texture indistinguishable from the real thing.
Careful dietary choices can also make a big difference to your greenhouse gas emissions. For example, you may well think that eating local or organic produce are greenhouse-friendly options, but that's not always the case. Even different fish have a wide variety of greenhouse impacts.
Christopher Weber at Carnegie Mellon University in Pittsburgh, Pennsylvania, also examined whether vegetarians eat enough nutrients. His conclusion was a resounding yes. "Plant-based diets are safe, and are probably nutritionally superior to mixed diets deriving a large fraction of their calories from animals," he wrote. The message is clear: indulge in a steak once in a while, but our planet's health would be better off if we just give meat the chop.