It was another one of those simple questions: what is the carbon cost of what we eat? How can we choose products which have low carbon footprints?

Inevitably the answer was neither easy nor, it appears, something that we should be encouraged to know. Or perhaps we should.

Whatever your stance on climate change, it makes sense to minimise our use of energy, and there should be a link with cost. Currently in the UK we pay an annual ‘car tax’ on vehicles which is based on their carbon emissions. As most cars with low emissions are also those with low fuel consumption, driving a frugal car ensures that you pay less for fuel and less tax.

But what about something equally everyday, like soup?

Let’s say we intend having soup for lunch. There are three ways that I could envisage us doing that:

1. Buy a tin of soup from the supermarket, and heat that up, recycling the can.
2. Buy a packet of dried soup from the supermarket, make that up and heat it, recycling the packet it came in.
3. Buy vegetables, meat, etc, from the supermarket, and prepare those, cooking them slowly to create fresh soup, putting the trimmings etc. into our organic recycling bin.

Each of those has many variations, such as whether we buy tins from our local convenience store, fresh produce from a farm shop, and so on. Or we could forgo all the hassle and go out to a local pub and pay them to provide us with soup.

Intuitively – a dangerous concept, of course – 3 should have the lowest carbon cost, or product carbon footprint as it is termed, and 1 the highest. But is there much difference? And if we were to buy vegetables which had been produced overseas, as we probably would during the winter months here, how would that affect it?

Although domestic consumption of products is thought to account for about 20% of our carbon emissions, it seems remarkably hard to get a straight answer to this. Food and other product use is ignored by carbon footprint calculators such as that here, which goes for the simple things like direct energy use and transport.

Wonderful organisations such as The Carbon Trust can explain here how to measure product carbon footprints, but don’t seem to offer any. Similarly for the BSI (the UK standards authority), although at least their guide for industry here has some fascinating insights.

The idea of eating with respect to carbon emissions has even reached Wikipedia, in its article here on low carbon diets.

Most telling, perhaps, is Tesco’s stand on product carbon footprints. Although it announced its intention to provide them for all its products, and offers an impressive but incomplete tabulation here, it decided to abandon plans to put such information on product packaging back in 2012.

The data provided by Tesco makes fascinating reading. There is little difference in carbon cost between bottled fizzy drinks like cola (soda) and bottled water. Many dairy products are very high in cost, particularly double cream. Although chilled convenience meals are quite costly, they vary greatly, with corned beef hash being particularly bad. And washing your clothes frequently could make the carbon footprint of your clothing even greater than that of your food.

I wonder whether Tesco and other manufacturers are scared that if we know how carbon wasteful some of their products are, sales of the most wasteful ones such as double cream and chilled meals containing corned beef might collapse. And we might start washing our jeans every couple of weeks, rather than every three days. Will we ever be allowed to know the product carbon footprints of wind turbines, bottles of beer, or an iPhone?

So it looks like chilled carrot and coriander soup today, then…