It did not rain for some 1600 years, the Flood was the first time it rained. Dew and/or water coming out of the ground kept vegetation growing.
The high oxygen content was very healing. I believe the atmospheric pressure was much higher too, maybe twice what it is now. The combination of higher oxygen content and higher atmospheric pressure is the basis for wound healing using a hyperbaric chamber.
The original creation was designed to provide an environment for Man to live forever. Sin changed that, but up to the time of the flood we had an environment conducive to long life.
It is all very amazing stuff. Sin is a destructive road. I have accepted that the world will get much worse; I don’t see how it will improve. On one hand traditionally Western countries are rejected Christ outright. On the other hand, Islam is on a rampage. Then the ones who reject all religion have increased in violence, sexual immorality, and deceit. We are labeled the enemy by our own government and by those who they are shipping in.
The end is near and God is in control.
Interesting, the same on how the atmosphere changed and caused the Flood, it would make sense on how final Judgement of the World would take place.