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.