We Should Have An Air Cleaning Levy On Fuel


The Sunday Nation


18 December 2005


Trees “breath” in carbon dioxide and release oxygen into the air. A car engine on the other hand, burns petrol with oxygen and produces carbon dioxide (and steam). So the question arises, how many trees would be required to clean up the carbon dioxide released into the atmosphere by a car?

A well-tuned car will produce about 2.4kg of carbon dioxide from every litre of petrol consumed. The fuel consumption rate (litres per kilometre) depends on many factors but on average, a family saloon will do about 10km per litre in urban driving and about 15km/L on the highway.

The average motorist covers about 20,000km in a year – mainly urban driving during the week-days and highway trips only on weekends. Thus we can assume that five out of every seven kilometres are done in town and the remaining are on highways.

That is, about 14,000km are done on urban roads and 6,000km are on highways. 14,000km at 10km per litre works out to 1,400 litres of petrol. 6,000km at 15km/L gives 400 litres. Therefore, the average motorist consumes about 1,800L very year. Since each litre produces about 2.4kg of carbon dioxide, the total annual production of the gas from each car is 4,320kg.

Now, on average, one tree “breaths in” about three kilograms of carbon dioxide in one year. That may sound little about it is about 1,500 litres or four litres per day at normal atmospheric temperature and pressure. Therefore, it turns out that we need 1,440 trees to remove all the carbon dioxide produced by one car.

The next question is, how much would it cost to plant these trees? The cost of buying one seedling is about sh10. But the young tree has to be delivered from the nursery and planted in the right place.  This can raise the cost to, say, sh50.

Thus the total cost for one car comes to sh72,000. If this is spread over the life of the tree, say 20 years, then the cost is about sh3,600 per car per year. This is money that can be collected as part of the annual road license.

Alternatively, the money can be collected as an additional levy on fuel. Every year we consume 460 million litres of petrol in Kenya, from which we produce 1.1 million tonnes of carbon dioxide. To clean this up will require 368 million trees.

The total cost of planting these trees would be sh18.4 billion. Dividing this figure over the average life of 20 years gives about sh920 million per year. This total sum is then collected from the 460 million litres that we consume per year. The result is sh2 per litre.

But can’t our natural trees doing the job? Kenya has about 3.5 million hectares of forest. The average tropical forest has about 250 trees per hectare, therefore we have about 875 million trees. That is over 40 per cent of the national stock of natural forest!

Clearly, we must plant more trees. And a two-shilling per litre “air cleaning” levy on petrol can do the job.

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