Understanding Your Car's Carbon Footprint

The typical car travels around 14,500 kilometers per year and emits 181 grams of CO2 per each kilometer travelled. That adds up to an annual carbon footprint of around 2.6 tons - between a fifth and a seventh of the typical British citizen's total, depending on how you work it out. Of course, your own driving footprint could be very much bigger or very much smaller, depending on the efficiency of your car and the distance you drive each year.

To quickly get a rough sense of how green or ungreen your current car is, first consider its size. As basic physics dictates, the bigger and heavier a vehicle is, the more energy it will take to move it around. Next consider the engine. Roughly speaking, the larger the capacity and the greater the performance, the higher the fuel consumption. Finally, consider the age of the car. Older engines tend to be less efficient than newer ones.

Regardless of the vehicle that you drive and how far you drive it, it's easy to work out how much CO2 is released from its exhaust pipe. That's because there's a roughly fixed amount of carbon in each litre of fuel, and practically every gram of that carbon is converted into CO2 in the engine. According to govern­ment-approved figures, each litre of petrol becomes 2.315kg of CO2, and each liter of diesel become 2.630kg.

In reality, however, the total emissions caused by your car's fuel is greater than the emissions exiting the tailpipe. That's because each liter of petrol or diesel has been extracted, refined, processed and transported before being made available in a filling station. Each of these stages requires energy and causes emissions, but the official figures allocate these emissions to the countries and companies producing and delivering the fuel - not to the driver who ultimately burns it.

More significant than the produc­tion of the fuel is the production of the vehicles that consume it. Each car contains hundreds or even thousands of kilograms of metal and other materials, the production of which requires a huge amount of energy even before the components are produced and assembled. According to one expert at the Stockholm Environment Institute, producing a typical car results in a massive eight tons of CO2 - equivalent to more than three years of fuel consumption for the average driver.

Unfortunately, it's pretty well impossible to know the emissions involved in producing any specific car - either one you already own or one you're thinking of buying. You can get an approximate sense by considering the weight; the bigger and heavier the car, the more materials and energy it will have taken to build. But accurate figures are very hard to come by, and partly for this reason it's usually not possible to work out the precise carbon footprint of your driving.

According to the best available models the emissions of the "total" use of a car - including producing, fuelling and maintaining it - are typically around twice the direct emissions of the car itself. By this rule of thumb, the average car is responsible for around five tons of CO2 per year - enough to fill a medium-sized sports hall.

The high level of emissions involved in producing a new car can make it very tricky to work out whether it's environmentally beneficial to upgrade from your current car to a greener model.

Climate change may be the most pressing environmental problem associ­ated with our driving, but it's not the only one. Besides globe-warming CO2, car exhaust emissions also include a cocktail of local air pollutants, including poisonous gases such as carbon monoxide, and particulates - tiny solid hydro­carbon particles that can lead to lung and heart disease, among other health problems.

According to the government, vehicles are largely responsible for the airborne pollution that causes around 25,000 premature deaths and as many hospi­talizations each year in the UK alone: almost ten times as many deaths as are caused by road accidents. In countries with older, lower-quality vehicles, the local health impact of exhaust fumes is even greater.

Engines are much cleaner than they used to be but, with the exception of elec­tric and hydrogen vehicles, all cars still contribute some level of local air pollu­tion. Diesels and motorbikes - while being better on average in terms of climate change - are often worse in terms of local pollutants.