DAVID FINLAY COLUMN:

Air Quality Improving?

(14 February 2011)

Thanks to the wonders of social networking, I am sitting here looking at a claim that the financial crisis has persuaded people to save money by buying cars which are cheaper to tax because of their low CO2 emissions, and that air quality in the UK is improving as a direct result.

Well, now. Where do we start with this one? First of all, I think I'm right in saying that CO2 levels do not, within reasonable limits, have much to do with air quality, if by air quality we mean the tendency of what we breathe in to do direct damage to us. Other gaseous substances - the ones referred to in our long-ago series of articles on emissions - have a much bigger impact on that.

There have certainly been immense improvements over the years in the levels of carbon monoxide (poisonous), hydrocarbons (dirty) and oxides of nitrogen (which leads to acid rain) emitted by cars, but the claim does not refer to any of those. It's all about CO2, a lungful of which is not directly going to do you anything like as much harm as the same amount of carbon monoxide.

It also seems worth pointing out that we in the UK do not have our own air. The stuff surrounding you came from somewhere else, probably America, and will go somewhere else, probably Europe. And at some point it is going to be, and will again be, in Australia and Angola and Azerbaijan. If there were no mechanical CO2-emitting devices anywhere in this country, there would still be concern about whether the amount of CO2 being produced around the planet as a whole was affecting the climate. This is a global issue, not a national one.

All that being the case, I think that what the claimant really meant was that UK motorists are now contributing less carbon dioxide into the planet's atmosphere thanks to their tendency to buy cars with lower official CO2 figures.

But even that makes sense only if those official figures reflect real-life emissions, and a case built on that assumption is never going to be a strong one. The media, motoring or general, is never far away from yet another outraged splutter (there was one just last week) that official fuel economy figures established in the repeatable, EU-devised test have not been matched by a certain driver in an unrepeatable real-world journey.

Of course they haven't. It's very unlikely that they will. And, since fuel consumption are CO2 emissions are so closely related, the chances of matching a car's official CO2 figure (if you had a way of measuring it) are similarly low.

Manufacturers design their cars to achieve the best possible results on the EU test (see The Rise Of The Small Engine, which is even more relevant now than when it was first published). This is precisely what the politicians have asked them to do, so it's not the manufacturers' fault. Nobody has ever yet devised - and I doubt that anyone could devise - a test that was fundamentally better than the EU one, so it's not the politicians' fault either.

The problem is that fuel consumption, and therefore CO2 emissions, depend on factors that become apparent only when the car is being driven (see Fuel Economy Testing), some but by no means all of them the responsibility of the driver. You could make a Toyota Prius use more fuel than a Porsche Cayenne if you wanted to. Equally, if you drove one Prius gently, in the daytime, in good weather, with the air-conditioning off, it would emit CO2 at such low levels compared with another Prius - this one driven hard, at night, in the rain, with the air-conditioning blasting away at full power - that they might be two completely different kinds of vehicle.

And of course different people drive differently, in different conditions, on different routes, with different loads on board, so they will achieve different fuel economy and CO2 emissions (see Fuel Economy And Bad Science). And that is why there is no point in saying that actual CO2 emission levels have come down just because that is what has happened to the official ones.

Personally, I think they may well have done. If there were a way to compare past CO2 emissions from cars with current ones (there isn't), I would wager a moderate sum that they have gone down, even allowing for the fact that there are more cars on our roads than there used to be. But my bet would be based entirely on speculation, because that's all I've got. It's all anyone has got.

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