Planting trees on roadside to clean CO₂ from cars

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

27 June 2021

The Kenya National Highways Authority (KeNHA) received a donation of 5,000 seedlings from Equity Bank Group PLC recently. This is refreshing as it comes after several public protests against the cutting of trees to make way for roads. The most recent one happened late last year and succeeded in saving the fig tree at Westlands in Nairobi. Presumably, the seedlings will be planted on roadsides. This brings up an interesting question: can they absorb all the carbon-dioxide emitted by the cars on that road?

I tackled that question in this column 16 years ago (2005) and it turned out that, yes, it is possible and can be done at a cost of about Sh2 per litre of fuel. That analysis was based on the national consumption of petrol, the total length of roads in the country in that year and the average tree density in a typical tropical forest.

This time, I want to look at the question from a slightly different angle: is it possible to plant enough trees on a road to absorb the carbon dioxide emitted by vehicles using that road? Take the Thika Super Highway, for example. It is used by almost 100,000 vehicle each day. How many trees would it need?

The first question is how much fuel they consume. Taking an average consumption rate of 10km per litre, we find that each vehicle consumes about 4L on the 40km highway. Thus, every day, about 400,000L of petroleum are consumed on this road alone, that is 146 million litres in a year.

Now, one litre of petrol emits about 2.3kg of carbon dioxide while the same volume of diesel gives off 2.7kg, therefore, we can work with an average of about 2.5kg/L. This works to a total of 365,000,000kg or 365,000 tonnes of the green house gas per year from Thika Superhighway.

The next question is: how many trees would be required to soak up all this carbon dioxide? Mature trees absorb about 25kg of the gas every year. So, to clean up the 365,000,000kg, we need about 14.6 million trees to be planted along the road; that is, 7.3 million on each side.

This highway is 40km or 40,000m long. If we maintained a separation of 2m, we would get 20,000 trees per line. So, we need about 365 lines on each side of the road. These will extend to a distance of about 730m. In short, there isn’t enough space along the road for all the trees. But that doesn’t mean we stop planting!