Twenty years ago, there were very few diesel cars in the road fleet in Ireland and in Europe generally. Buses, trucks and agricultural tractors were diesel but nearly all cars were petrol-driven. There has been a huge shift in the period since. In recent years, almost three-quarters of new cars sold have been diesel and about one-half of all road vehicles under license are diesel-driven.

Since the diesel vehicles (buses and trucks particularly) are high-mileage, the total sales of diesel now exceed petrol sales by a large margin. This is true around Europe: in the United Kingdom, for example, 63% by volume of road fuel consumption is now diesel.

There are two main reasons for the shift away from petrol in the car fleet. Fuel consumption is better with diesel, and it is also cheaper in most countries due to the somewhat more favourable tax treatment. As a result of the better fuel efficiency, emissions of carbon dioxide are moderated when the car fleet switches from petrol to diesel, so governments had been happy to see diesel usage rise and petrol fall. But it is beginning to look as if a mistake has been made through focusing exclusively on emissions of carbon dioxide, a greenhouse gas which contributes to global warming.

Diesel engines produce lower levels of carbon dioxide but also emit two other “nasties”. These are low-level pollutants in the form of particulate matter (think of soot) and nitrogen dioxide and these do not disappear into the upper atmosphere.

Policymakers have long been aware that high emissions in built-up areas of either particulates or nitrogen dioxide are damaging to human health, aggravating respiratory conditions including asthma. The replacement of petrol with diesel engines in the car fleet saves on greenhouse gas emissions but makes these low-level localised emissions worse. In the countryside, this does not matter too much, but in crowded city streets low-level emissions can reach critical levels.

The trade-off just got worse. Despite improvements in the technology of diesel engines designed to mitigate the low-level emissions, recent studies have been showing that air pollution in European city streets has worsened and the shift to diesel is being blamed.

According to the London Sunday Times, Oxford Street now has the highest levels of nitrogen dioxide in the world. In Paris, the mayor is considering restrictions on the use of diesel vehicles in the city. It is beginning to appear that emission standards agreed with the car manufacturers by the European Commission are not being delivered on the road. The laboratory emission testing has, it is alleged, not been stringent enough. The implication is that the official policy of encouraging the switch to diesel may need to be reversed.

Long run

In the long run, either the diesel vehicles will have to embody better emission reduction technology or their use in cities will have to be restricted. It appears that excess emissions are, however, so large that a technological fix may not be forthcoming.

The European Commission is considering a new testing regime but that does not address the issue – if there are excess diesel emissions in cities, there will have to be fewer diesel vehicles on city streets, whatever way they are tested.

Since there is now such a large fleet of diesel cars in place, this cannot be rectified overnight. Fortunately cars have a useful life, on average, of not much more than ten years or so, and an early shift back to petrol for new cars would work its way through the fleet reasonably quickly. To bring this about, governments would need to pare back the favourable tax treatment of diesel vehicles and of diesel fuel.

Ireland now has a rather overcomplicated system of purchase and annual taxes on road vehicles, including expensive subsidies for electric cars which appear to have had very little impact. Further fiddling with these is not a good idea. It would be best to focus on the motor fuel taxes. At my local filling station, petrol costs €1.43 a litre, diesel €1.33. The gap reflects the tax treatment – most of the retail price consists of excise duty and VAT. The 10 cent differential in favour of diesel cannot, it would now appear, be justified in terms of lower emissions as had been thought.

In October’s budget, the Government could commence closing the gap, say by cutting a couple of cents off petrol and adding a couple on diesel. Green diesel (currently costing about 90c) could be left alone – it is not used much in urban areas and the excess low-level emissions from diesel appear to be entirely an urban problem. Ultimately people will have to be given the correct price signals that reflect better the actual damage done by emissions from transport fuels.