local Vs supermarket

John Cossham

CRAGger York

Hello Nick, ecofootprinting, and more recently carbon calculators, are fairly new concepts and due to their complexity, their accuracy is debatable. I have used several C calculators, and put in the same personal info, and had quite a wide variation of scores. I do not believe they are ‘calculators’, more like guesstimators or approximators.

You say you need to compare the CO2 difference between shopping locally and shopping in a local shop. I’m assuming you want to compare the same ‘basket’ of goods?

These are some of the issues which a carbon footprinting consultant would have to take into consideration:

Your travel: how far is the supermarket from your abode, are you travelling there and back just for your one basket, or as part of an existing trip? Size of vehicle… Rolls Royce or push bike with trailer? Type of fuel in engine? Considering your ‘local shop/s’ or farmers’ market, how far is this/are these, same questions really. A trip to the supermarket on the way back from work by bike might have a smaller travel footprint than a special Sunday trip out to the Farmers’ market in a nearby village and a visit to a greengrocers, butchers, hardware shop and clothes shop in several different parts of town. You might be able to get your whole basket delivered if bought from a supermarket (and some deliver using relatively eco-friendly vehicles, with dozens of deliveries per trip) but probably not from several smaller shops.

Considering the goods you purchase, a supermarket might be able to offer a wider range of organic goods, which MIGHT have a lower C footprint (see below) than smaller shops which can only stock a few lines and therefore only has ‘normal’ goods which use fertiliser (made from fossil gas) and weedkiller (more chemicals). However, let’s imagine the exact same basket in both outlets.

Goods themselves are incredibly difficult to give an accurate carbon footprint to. Consider tomatoes. If you buy locally grown, outdoor tomatoes (or unheated greenhouse)from the farm gate, that will definitely have a lower footprint than equivalent Spanish ones flown, driven or shipped in. However, many local tomatoes are grown in heated glasshouses, giving them a higher footprint than outdoor toms from further afield. But, if the heat is provided by a nearby power station, offering the greenhouses ‘waste’ heat, that will again give a different value.

If goods are flown in, it is not known how much carbon footprint to assign to this activity. Some calculators just assign one unit of carbon dioxide for every unit of jet fuel used… others have a ‘multiplier’ which takes into consideration the fact that when a plane emits in the upper atmosphere, some of the emissions (NOx) cause more greenhouse effect than these same gases emitted at ground level. The Government calculator, ActOnCO2, just uses the simplest system as the government says ‘we don’t yet understand the chemistry, so we won’t guess a multiplier’. Other calculators assign air transport a much greater impact. So that’s one area where ‘calculators’ are just guessing or estimating…

Many products have dozens of ingredients (go look at a packet of biscuits) and to calculate an ‘accurate’ footprint for these, each of the ingredients has to be assessed. A year or two ago, a well-known brand of crisps started to footprint one of it’s lines, and the job was considerably more complicated and costly than they had bargained for. Also, Tesco said they were going to put a C footprint on all of it’s lines… but this hasn’t happened and I think they’ve quietly dropped it as unworkable, and probably not worth doing as the figures would be almost meaningless, and customers probably wouldn’t use the info anyway.

So, my final thoughts are that you are not going to be able to get any sort of accurate info on this as there are too many variables. You can safely say that a vegan basket has a lower impact than a meaty one, that UK goods have travelled less than tropical ones (but may have been grown in heated greenhouses)and that farmers’ markets give more money to farmers than supermarkets do. Buy organic and in season to have a lower footprint, but organic agriculture uses more land per weight of crops… it’s ALL SO COMPLICATED!!!

If you want a more informed answer, contact The Stockholm Environment Institute at the University of York. They were one of the originators of ecofootprinting, and are continuing to work on carbon calculators. John Barrett is the most knowlegeable… but he is a very busy chap! Good luck!!

John Cossham, York
johncossham (at) tiscali (dot) co (dot) uk