Last month, more people bought diesel cars (or, rather more accurately, more diesel cars were registered) than in any previous September.

Okay, they made up a slightly smaller proportion of new car sales than usual, but that’s not surprising. Firstly, because there’s a natural trend towards ever more efficient petrol engines and alternative fuels, and secondly, because some diesels have been getting a tough time of it recently. I don’t know. You might have heard.

Anyway, still encompassing nearly half of all new car sales, diesel is not really going anywhere. And in towns, lots of diesel cars not going anywhere is precisely the problem.

Diesels have been made popular because they will often emit less CO2 than a petrol equivalent – which is better for not warming the planet – but they’re worse for you if you live in an urban area, because they upset the local air quality by putting out more mono-nitrogen oxides and particulates. What’s good (or less bad) for Arctic tundra is bad for Mrs Miggins of Kilburn High Road, and vice versa.

As far as I can tell, there isn’t a solution that is good for both of those groups, except to stop driving completely (which is a luxury most of us cannot afford), or at least in towns, which is more compelling but often not particularly practical.

Towns and cities struggling to meet their own air quality targets can limit the number, or types, of cars that are allowed in – perhaps by charging those who want to. But although that’ll please the local air sensors, it won’t please local businesses. Nor does it actually help if it sends people to out-of-town retail parks instead. It results in precisely the same amount of emissions, just spread over a wider area and to the detriment of small businesses.

The mayor of London has suggested there should be another vehicle scrappage scheme to reduce tailpipe emissions. I’m not sure about that, either.

Manufacturers like scrappage schemes because they mean they shift new metal, which is what they’re in the primary business of doing.

But although what comes out of the tailpipe of a new car contains fewer harmful emissions than an old one, the benefit is paid for with the increased emissions and energy consumption of making the car in the first place: in mines where iron and aluminium are sourced, in steel plants where metal is recycled, in oil wells and in plastics and paint factories. In terms of minimising overall environmental impact, there must be an optimum time to replace a car, but finding it is not the aim of these schemes.

Yes, local air quality improves and, on paper, things seem tickety-boo. But all you’ve really done is cheated the system to made it look like the problem has gone away when it has actually just gone elsewhere. Which is a not unfamiliar scenario at the moment, no?