How many trees to offset a car commute?

One of the measures proposed to mitigate global warming is reforestation – planting more trees. The World Wildlife Fund estimates that about one sixth of all greenhouse gas emissions come from deforestation. So, planting trees is both a commercial and ecological opportunity, as is reclamation of peat bogs and mangrove swamps. The Trillion Trees vision is described below:

The Trillion Trees vision for 2050: One trillion trees have been regrown, saved from loss, and better protected around the world. Deforestation has ended, significant numbers of trees have returned to areas where they were lost, and large areas of existing trees are better protected. The campaign to deliver the vision is a collaboration between three of the world’s largest conservation organisations – World Wildlife Fund, BirdLife International, and the Wildlife Conservation Society.

The scale of the task is illustrated by this simple calculation. How many trees does it take to offset a typical commute? I was not clear whether the CO2 capture from 100 trees, 10 trees, 1 tree or 0.1 of a tree could offset the CO2 emitted by a single car commuter.

The answer will obviously depend on the type of car and fuel used, the length of commute, and the type and size of tree. I was not thinking about a close link – this tree and this car – I was thinking more about a scientific equivalence. So I took an opportunity to work with an expert on scoping out an answer.

I was invited to give a lecture at the World Academy of Arts and Science Conference on “Approaching 20?? Year”, held in Montenegro in May 2019. One of the other speakers, on climate change, was Emeritus Professor Sebastien Balibar. He and I are both physicists by education so I thought he might have an answer. Instead we did a calculation. We articulated our assumptions as we went along: here is what we concluded.

For a person who commutes 20 km a day (10km each way) and work 250 days a year, this means the commuting distance is 5000 km a year. A typical car emits 130g CO2 per km. 5000 kms produces emissions of 650 kg CO2 per year. A tree needs about 9 m² space to grow, so 22 trees need 200 m² or half an acre. The average net absorption per tree is about 30 kg of CO2 per year, so it will take 22 trees to compensate for the emissions of one year’s commute of 20 km per day. That means that if you planted 22 trees, the CO2 absorption every year would offset your 5000 km of commuting every year.

The implication is that though each tree does indeed contribute to CO2 absorption, global warming cannot be stopped by tree planting alone. We must all also learn how to adapt to global warming and its effects.

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