If you walk to work you are entitled to feel pretty smug about how petite your carbon footprint is, aren't you? But what if someone told you it would be better for the environment if you drove to work instead?
Chris Goodall, Green Party candidate for Oxford West and Abingdon and author of How to Live a Low Carbon Life, caused a right old eco-kerfuffle on the blogosphere this week when it appeared that he was claiming exactly that - that driving is greener than walking.
This conjured up images of delighted SUV owners driving endless laps around town, happily belching out fumes and yelling "You're ruining the earth for all of us" at walkers along the way.
Goodall's claim seemed sure to make environmentalists and health officials incandescent with rage, while delighting car manufacturers and oil companies. And couldn't couch potatoes take it as a ringing endorsement of their lifestyles - all that stuff we've been told about taking the stairs instead of the lift was wrong all along. We knew it didn't feel right! Walking, that most arduous of pastimes, finally exposed for the ecologically unsound activity that it is. Hallelujah!
The science behind Goodall's argument is as follows. Driving a typical car for three miles (4.8km) from point A to point B adds about 0.9 kg of CO2 to the atmosphere. Which sounds like a lot - surely you would be kinder to the earth if you put on your runners and walked instead? Well, the catch is that while you walk, your body has the annoying habit of burning energy (in this case 180 calories), which is terribly bad for the environment because that energy has to be replaced using food. And the food which we eat has to be reared, grown, processed, sprayed, packaged, transported and refrigerated - all of which results in lots of emissions. For the purpose of the study Goodall uses beef for calorie replenishment and calculates that 100g of meat is required, resulting in 3.6kg of emissions - four times more than produced by driving. The net result is that the environment would prefer it if you didn't walk at all, thanks very much.
Before you go off and throw your tatty runners in the bin, there are a couple of points to bear in mind. First of all there is an obvious flaw in his methodology - most of us don't have a diet that consists exclusively of beef. A claim that driving is better for the environment than a group of perambulating Atkins Dieters might have some basis in fact. But the rest of us tend to eat other things as well.
Secondly, this is a classic case of taking the headline from a scientific study and completely missing the point of that study in the process. Goodall was not advocating driving as a greener mode of transport than walking; he was highlighting the fact that food production is now so energy-intensive that more carbon is emitted providing a person with the calories to walk to the shops than a car would emit over the same distance. So, in other words, if I am out walking and along the way consume a South American banana, the fact that the fruit has been flown half-way means my innocent walk has a carbon footprint the size of a small town.
Only a fool, however, would suggest that I should give up walking to fix the problem. Goodall's study, for those who bothered to read it fully before jumping out of their own skin in anger, makes a prescient point: the way we have chosen to set up our food chain is a major contributor to climate change.
All of this serves to highlight just how many complex decisions we face when it comes to food shopping. You would really want to be putting an entire day aside for trips to the supermarket these days to give you enough time for in-depth analysis of the environmental and health implications of every item that goes in the trolley.
I could stand in the aisles for hours riddled with climate change guilt, pondering the pros and cons of an organic mango, for example. On the one hand, it hasn't been sprayed with any nasties, which means it should be pretty good for me. But on the other hand it has been flown in from India so it's an air-miles disaster zone. Maybe I'm better with a bag of good old-fashioned Irish carrots; but hold on, they're not organic so they've probably been sprayed half to death and they're wrapped in plastic.
What to do? Should I be in a supermarket at all? Should I just stop eating altogether? Is my "five-a-day" health-kick responsible for turning the Irish summer into monsoon season? Perhaps the only realistic approach in a world gone mad is to try to be sensible, difficult as that may be.
In a supermarket a few weeks ago I had in my hand a garlic bulb from China priced at 99 cent. Ninety-nine cent! Garlic grows perfectly well here in Ireland, so how can it make sense to ship it over 5,000 miles and then sell it for less than €1? That's how bizarre our food chain has become. That's how unsustainable it is. That's why it's important than ever to try to grow some of your own, buy local when available, organic where possible, and try to keep away from processed and packaged food. Oh, and give up that walking. It's killing us all.