Can roadside trees offset the CO2 emissions from traffic?
We recently received this inquiry: “How many trees would have to be planted along German roadways in order to absorb all traffic emissions? In cities, for example, every homeowner could plant at least one tree in front of their house.”
Of course, car exhaust is not only harmful to the climate. Exhaust contains particulate matter and volatile organic compounds that penetrate deep into the lungs when inhaled, causing various respiratory diseases and increasing the risk of cardiovascular disease. We cannot compensate for these problems by planting trees. But this article focuses only on the climate. Our calculations show that even if all roads in Germany were completely planted with trees, unfortunately they would still not have sufficient power to absorb all the CO2 emissions currently caused by traffic in the country.
Here is our back-of-the-envelope calculation
The most recent estimate published by the Federal Ministry for the Environment of the total annual CO2 emissions from road traffic in Germany is 148.5 million tons.
ForTomorrow estimates that across all tree species, one tree stores an average of 25 kg of CO2 per year. If we divide 148.5 million tons by 25 kg, we estimate that about 6 billion trees that would be needed to absorb the CO2 emissions of German traffic in one year. This is only a rough estimate. The actual absorption capacity of a tree varies over the course of its life. A more precise calculation would require, among other things, an estimate of the age of the trees in question, which is too complex for this analysis.
How many trees can fit on German roads? According to the Federal Ministry for Digital and Transport Affairs, the total length of roads in Germany is estimated at around 830 000 km. If we multiply this figure by 2 to take both sides of the road into account, we arrive at 1 660 000 km, or 1 660 million meters.
For the sake of simplicity, we assume that trees need a distance of 3 meters from each other to grow. (This is actually too close for many tree species. Although when planting, it is common to leave only 70 centimeters between seedlings, as it is expected that not all of them will grow to their full size.)
This results in the following calculation: 1 660 million meters divided by 3 equals 533 333 333. That is over half a billion trees that could be planted along German roads.
Please note that this is a rough and relatively high estimate. There are many places on German roads where trees cannot be planted. These include driveways, intersections, and narrow areas that are completely paved with sidewalks.
Even if we could plant half a billion trees along the roads, that’s only about 9% of the 6 billion trees that would be needed to absorb all the CO2 emissions from transportation that we calculated above.
Planting trees is nevertheless an effective solution
The question of whether fully planting our roadsides with trees can solve the problem of traffic emissions is still a good one. It helps us to understand the scale of our emissions problem and the power of trees to solve it.
It also helps us to recognize the power of collective action: if all property owners in the country planted one tree for every three meters of their street property, together they could absorb 9% of Germany’s annual CO2 traffic emissions. Not insignificant! On average, every person in Germany produces around nine tons of CO2 emissions per year. In order to stay below 1.5 degrees global warming, this amount must be reduced to one to two tons.
Large-scale afforestation can be a good way to restore the balance between CO2 emissions and CO2 sequestration.
