This magazine recently presented a rather touching portrayal of Ashton Hayes, a village in Cheshire with the aim of becoming ”carbon neutral” - that is, emitting no unnecessary carbon dioxide at all, and perhaps making up for all that troublesome breathing by planting a few trees. That will take some work because the villagers’ current emissions of carbon dioxide are about 25 per cent higher than the national average. In an effort to cut this to something more respectable, the villagers are urging each other to switch off unnecessary electrical items, insulate their lofts and trade in big cars for small ones.

This is all laudable stuff, so it feels a little mean to point out that the villagers could dramatically reduce their carbon footprints by bulldozing Ashton Hayes and moving to London. Yes, London: the ”big smoke”, the richest region in the European Union, is a city whose environmental statistics make it look dangerously like some hippie commune.

The Office for National Statistics reports that Londoners produce much less household waste than anywhere else in the UK. From the same source I learn that London’s households are the most likely to have no cars, and the least likely to have two or more cars. Even before the congestion charge came into force few Londoners commuted by car.

London’s Mayor’s office informs me that London emits 40 per cent less carbon dioxide a person than the national average - which would be less than half the rate of ”carbon neutral” Ashton Hayes. All this from a city that is hugely dynamic, innovative and, frankly, disgustingly rich.

It is true that these figures do not include the environmental cost of producing products elsewhere and shipping them to London. That would be a more telling omission if the rest of the country was growing its food in the back garden, but the truth is that most UK citizens fill their houses with products produced elsewhere. They just have bigger houses to fill.

London, like other big, dense cities, is good for the planet. That fact seems to surprise people. After all, cities are polluted places. But we need to make a fair comparison. There are 7,600 times more people in London than in Ashton Hayes, but if you took 7,600 villages like Ashton Hayes and tried to cram all of them inside the M25 you’d have a struggle. The first step, I suppose, would be to build a few thousand skyscrapers and fill them with gardens and garden sheds.

London’s environmental performance comes naturally. My in-laws live in the Lake District in a house that is twice as large as mine with half as many occupants; they drive into town to pick up the morning paper. We travel around by bicycle or walk pushing our kids in a baby buggy because a car is impractical. We are enormously greener than they, but not because we’re more virtuous nor because we’re poorer. We’d like a bigger house, but that costs too much in London. A fancy car would be a waste of money because we’d rarely use it. Economic necessity, rather than deeply held principles, compels us to be green.

None of this is to say that Ashton Hayes shouldn’t try to be greener, nor that Londoners couldn’t insulate a few lofts and switch off a few needless light bulbs themselves. But it is worth remembering that Londoners - like the citizens of New York, Tokyo and many other dense cities around the world - have found a way of life that combines green living with wealth and economic dynamism. It turns people into unconscious, even unwilling, environmentalists.

That is fortunate, because few people have the same passion for the environment as the good folk of Ashton Hayes.

Tim Harford’s book ”The Undercover Economist” (Little, Brown) is out now in paperback.