It was bound to happen; human disregard for our oxygen providers. Two years ago, the entire forest was destroyed in the Amazon, to make room for business conglomerates. And when they were done there, new land was required and more trees were massacred, along with the humans protecting it.
For every tree cut down, there was no replacement. The earth grew dense with carbon impurities in time.
Organisms fought to breathe.
Ecosystems crashed. There was barely enough air left before they realised their mistakes, and oxygen substitutions only lasted for a while.
Greed had triumphed over the value of life.