Are we creating too much carbon dioxide for the planet's temperature stability, or are we just beating ourselves up about it? If enough people say we are, then it becomes a truism. And a convenient one for those seeking control and the raising of revenue.
Well, it does no harm to be objective. Ultimately, though, two things are undeniable. Oil will run out, so the longer we can make it last, the better. And whether or not we are undergoing CO2-triggered global warming, the likelihood that we may be doing so poses too big a risk to ignore.
Here, then, is the car to salve our consciences. In the real world the new Volkswagen Polo BlueMotion is the most eco-friendly proper car you can buy in the UK today. It has four seats, it's properly safe, you can use it for all normal car duties, and – unlike an electric car – it converts fuel into energy at the point of use rather than simply moving the process elsewhere via an inefficient intermediate storage system.
The Polo BlueMotion's CO2 emissions are rated at 99g/km, which means you pay no road tax. That's less CO2 than is produced by the much-vaunted Toyota Prius, never mind that the Polo also takes much less energy to build and is easier to recycle. It has a diesel engine, true, but the diesel downside of particulate emissions is fixed by the fitment of a particulate filter.
The low CO2 figure is achieved even without the benefit of regenerative braking and a stop-start system, as fitted to the Mini Clubman tested this week. If the Polo had these as well, its frugality would be even more astonishing than the 74.3mpg it already achieves on the official "combined" test cycle.
In reality, few users would manage quite such impressive figures, but a regular 60mpg should be easy. And, unlike a Prius, the Polo runs very efficiently on a motorway. What's more, it does all this while being able to reach 109mph and sprint to 62mph – as if you would – in 12.8 seconds. It's an extraordinary achievement, and one that makes those of us who still love driving feel good about the future.
So, what's the secret? There's no single huge innovation, rather an accumulation of little clevernesses. The engine is Volkswagen's usual three-cylinder, 1.4-litre, 80bhp unit, but fitted here with variable geometry in its turbocharger to improve the engine's response from low speeds. Its maximum pulling power of 144lb ft arrives at only 1,800rpm, which means you seldom have to work the engine hard. There's an exhaust-gas recirculation system that softens the harshness of diesel combustion and makes the engine more responsive, and the gearbox has longer-legged gearing in all five gears. At 70mph the engine is spinning at just over 2,000rpm.
The other changes are to the body and the wheels. The nose is redesigned with the front grille reduced to a slot – the engine's increased efficiency means it needs less cooling air – and there's a drag-reducing lip above the rear window. The result is less aerodynamic drag, and the low rolling-resistance tyres on light, skeletal wheels help further to reduce the energy needed to pull the Polo along.
If a Polo can be this frugal, you wonder why other diesel superminis don't follow suit. Before long they will, but in the meantime, how much of the hair-shirt is there in the BlueMotion?
The Polo has fallen off my radar in recent years. Once praised for its Golf-in-miniature quality and rational appeal, the Polo breed has faded as more glamorous superminis have arrived. You just don't see as many as you did a decade ago. So it was interesting to meet the Polo again in its new guise. There was an added dimension to my encounter because my BlueMotion was a new addition to the Streetcar fleet of pay-as-you-drive hire cars.
Streetcar? It's a car club, a pool of cars any of which can be booked at short notice and returned after use. You book online or by telephone, maybe even just a few minutes before you need the car, choose the location for collection, then wait for a text message confirming which car you have and the time from which you have it. You can borrow it for as half an hour or for up to six months, although that would be expensive.
Once you've registered (for a fee), car hire costs £4.95 an hour with discounts once past 24 hours during the week, and fuel is free for the first 30 miles. Thereafter there's a charge of 19p per mile, and if you need to add fuel you use the fuel card that lives in the car. Streetcar then picks up the bill electronically.
Streetcar operates mainly in London but also in Brighton, Southampton, Cambridge, Maidstone and Guildford. One of its advantages is that the cars have their own parking bays, so once the car is returned you don't have to worry about it any more. You do have to return the car to where you picked it up, though.
Convenience is the main draw of Streetcar, but there are also environmental benefits in that Streetcar users typically won't need a car of their own and will tend, overall, to use a car less. The company has found that members treat the cars well and have a strong sense of community and responsibility. They often request particular cars – they all have names – because they have formed bonds with them. Even a car-club car is more than a consumer durable, it seems.
So I hold my new Streetcar membership card close to the reader in the Polo's windscreen. The car unlocks itself, I climb in and enter my Streetcar pin number. From now on the Polo behaves as normal, until I return it and sign off. And the BlueMotion is fascinating to drive, requiring you to use the low gears more than usual at slow speeds because otherwise the engine will turn shudderingly slowly. It's hardly a hot-rod, but it's a willing car with its deep three-cylinder engine note. You can feel the bumps through those eco-tyres, though.
Surprisingly, perhaps, the BlueMotion retains features such as electric windows and central locking. And if you buy a BlueMotion 2 (from £12,845 instead of £11,995), you get air-conditioning and a spare wheel, which add weight and raise the CO2 output to 109g/km. So you then pay road tax, albeit not much. How much do you need air-con? Well, we managed without before the world got so warm....