In short, because we are tearing up the oxygen factories to make way for carbon dioxide emitters. (Doesn't make a lot of sense, read the explanation)
Explanation:
So, 1000 years ago, we had a lot more trees, didn't have engines or cars or factories or anything, really that released carbon dioxide into the air and we had a lot more trees and since the invention of cars, engines, carbon dioxide-emitting tools and factories and all the other things that emit "Greenhouse gases" and in doing that, cleared more trees to make room for factories and roads and that has drastically changed the outlook of the carbon cycle.
