Deforestation reduces the amount of carbon absorbed from the atmosphere and is the main contributor to climate change. If deforestation continues, along with the poisoning of our oceans then we are going to run out of oxygen and become a dead planet.
A video where this is explained in a way that I had not realised - https://www.youtube.com/watch?v=dRSPy3ZwpBk for information on why we need forests.