Question

What produces so much oxygen that it changes the atmosphere ?

Answer 1

Mainly we get oxygen from plants.

Answer 2

It's a belt that circles the globe, broken only by ocean, and containing a third of all trees on earth. During the short summer it produces so much oxygen that it changes the atmosphere. The forests may be vast but the animals are scarce and it's populated by wandering loners such as the lynx and wolverine.