1. The logistics
There are three trillion trees in the world. The timber industry currently cuts down 15 billion a year, so at current rates it would take at least 200 years to fell them all – probably much longer because a lot of virgin forest is hard to reach. If you gave everyone aged 15 to 65 a chainsaw, they would have to cut down 625 trees each, which might be manageable in a year. But collecting and processing that timber would take much longer and 99 per cent of the trees would just lie on the forest floor, rotting and releasing 35 billion tonnes of CO2.
2. Ecosystem collapse
Eighty per cent of land animals and plants live in forests and without the trees most of them will die. Trees also keep the ground wet and cool, and help to drive the water cycle. A large tree can push 150 tonnes of water into the atmosphere each year, which then falls back on the forest as rain. With no trees, the land will heat up and dry out and the dead wood will inevitably result in enormous wildfires. This will fill the sky with soot that blocks out the Sun, causing failed harvests for several years and leading to worldwide famine.
How many trees does it take to produce oxygen for one person?
Breathe in, you’re in for a big surprise when you find out how many trees we need to provide oxygen for our lungs.
Trees release oxygen when they use energy from sunlight to make glucose from carbon dioxide and water. Like all plants, trees also use oxygen when they split glucose back down to release energy to power their metabolisms. Averaged over a 24-hour period, they produce more oxygen than they use up; otherwise there would be no net gain in growth.
It takes six molecules of CO2 to produce one molecule of glucose by photosynthesis, and six molecules of oxygen are released as a by-product. A glucose molecule contains six carbon atoms, so that’s a net gain of one molecule of oxygen for every atom of carbon added to the tree. A mature sycamore tree might be around 12m tall and weigh two tonnes, including the roots and leaves. If it grows by five per cent each year, it will produce around 100kg of wood, of which 38kg will be carbon. Allowing for the relative molecular weights of oxygen and carbon, this equates to 100kg of oxygen per tree per year.
A human breathes about 9.5 tonnes of air in a year, but oxygen only makes up about 23 percent of that air, by mass, and we only extract a little over a third of the oxygen from each breath. That works out to a total of about 740kg of oxygen per year. Which is, very roughly, seven or eight trees’ worth.