Trees have this amazing ability of being almost entirely made up of carbon atoms. They need very few nutrients relative to their size so they don’t deplete the topsoil, get most of their water from deep underground, and their mass from the air they breathe. This means that the lifetime carbon value of a tree, the total amount of carbon it will absorb, is about the same as its mass.

So what’s the average mass of a tree? About 2 tonnes. Does that mean we need to plant 19 billion trees per year to offset our Carbon Dioxide emissions? Here’s the good news. Since photosynthesis takes in Carbon Dioxide (CO2) and releases all of the Oxygen (O2), we only need to take the weight of the Carbon atoms and subtract all of the Oxygen molecules away from the weight of each tree. So 2 tonnes of tree matter actually removes about 7 tonnes of Carbon Dioxide by converting the rest to Oxygen.

• The math:
  2 tonnes / (12 / 44) = 7.33, where 12 / 44 comes from the relative weight of the Carbon atom in each molecule of Carbon Dioxide.

Since each tree will remove 7 tons of Carbon Dioxide, we would need to plant about 5 billion trees per year to account for current emission levels.