Even completely unchecked deforestation takes 200 years and won’t cause extinction.

There are approximately 3.04 trillion trees on planet Earth (Crowther et al 15), covering 31% of the world’s land surfacew1. Today, for Earth day, we’re taking a look at trees. Around 15 billion trees are cut down each year. So, hypothetically speaking, it would take just over 200 years for the world’s forests to completely disappear. While this scenario is unlikely, what would be the consequences of a tree-free planet? Let’s start with perhaps the most obvious difference – oxygen concentration. A lack of oxygen? Oxygen makes up roughly 21% of the Earth’s atmosphere, but you probably know that already. What you might be surprised to find out, however, is that only half of this oxygen is produced through photosynthesis in trees and other plants on land. The other half is produced in oceans, by microscopic marine organisms called phytoplankton. The environment would not be devoid of oxygen if all trees were lost but the oxygen level would be lower. Would it be sufficient for humans to survive? In one year, a mature leafy tree produces as much oxygen as ten people breathe. If phytoplankton provides us with half our required oxygen, at current population levels we could survive on Earth for at least 4000 years before the oxygen store ran empty. However, that’s not considering a number of other factors: increasing population size, for example, would reduce the amount of oxygen available, whilst phytoplankton blooms due to an abundance of carbon dioxide could increase oxygen levels. Suffocating smog Whilst there may be enough oxygen for humans to survive on Earth, at least to begin with, the air we breathe could still be responsible for our demise. Like giant filters, trees help to cut down on pollution levels. Leaves intercept airborne particles and ozone, carbon monoxide, sulfur dioxide and other greenhouse gases are absorbed through the leaves stomata. In 2012, outdoor air pollution was estimated to cause 3.7 million premature deaths worldwidew2. Imagine the impact removing these environmental sieves would have on humankind. Air-pollution masks would become a necessity and bottled ‘clean air’ could come at a premium. Full of hot air? Armed with pollution masks, would the climate and temperature still be suitable for us? One important consideration is carbon dioxide. In one year, an acre of mature trees soaks up the same amount of carbon dioxide that we produce by driving the average car 26 000 miles. Since human activities like this increase the normal level of carbon dioxide in the atmosphere, cutting down trees would tip the balance even further, not to mention the enormous amount of stored carbon that would be released from doing so. Deforestation is already responsible for up to 15% of global greenhouse gas emissions and you might think that an overwhelming increase in carbon dioxide would result in a much warmer planet. However, the relationship between trees and global temperature is much more complicated. Energy and water fluxes between trees and the atmosphere also play a role and a tree’s colour, for example, can affect the amount of the Sun’s energy that is absorbed or reflected. Studies have shown that Europe’s trees have actually caused a slight increase in regional temperatures since 1750w3, while transpiration from plants in tropical forests cools the surface temperature. Therefore, whether the temperature becomes too hot to handle could depend on many factors, although a recent study concluded that reducing forest size increases average air surface temperatures in all climate zones (Alkama & Cescatti 16).