I always heard that the rain forests were "lungs of the Earth" as they use sunlight to convert Carbon Dioxide in to Oxygen which we need to breath. The idea is that they create the Oxygen we all need to live.
And then, more recently, I heard that they are pretty much neutral. Indeed this article points out that the Carbon they extract ends up decomposing and releasing that Carbon Dioxide back in to the atmosphere (using Oxygen in the process). This makes perfect sense and made me wonder why I did not question the "lungs of the Earth" idea in the first place. There are other articles that point out that they will vary absorbing or releasing Carbon Dioxide over periods of years, so a sort of "capacitor" for Carbon Dioxide on the planet, but not the power plant for Oxygen.
So I pondered where does the Oxygen I need to live come from. Well, there is an incredibly simple answer!
The Oxygen I need is used to burn (oxidise) the Carbon from my food. That Carbon in my food ultimately comes from plants (directly, or via other animals). Those plants get the Carbon from Carbon Dioxide in the atmosphere, and using sunlight as an energy source they release Oxygen when they absorb that Carbon. Indeed, the sunlight's energy used to split the Carbon Dioxide, is the energy released when Carbon is burned in my body.
So there is a perfect balance. The exact amount of Oxygen I need in order to burn the Carbon in the food I eat is exactly the amount that was released by the plants that absorbed that Carbon from the atmosphere in order to end up in my food!
Of course, I also end up with some Carbon in my body as part of my body, or leaving my body by other means. That Carbon gets released as Carbon Dioxide when my poop decomposes, and finally when my body decomposes after I die - that decomposition needs exactly the amount of Oxygen that was released by plants that absorbed that Carbon.
So, there should always be exactly enough Oxygen for us to burn the food we eat simply because that food only exists because exactly that much Oxygen was released by photosynthesis in the first place.
Nice isn't it :-)