Re: Another dodgy headline
Who says there's any such thing as a "correct" level?
But it's the low levels of geologically-recent times that support high-metabolic-rate (warm-blooded) life forms. You'll suffocate in a stuffy room while there's still plenty of oxygen. Just like you'd suffocate on this planet's distant history, before most of the carbon was captured from the air and laid down as fossils over millions of years before we evolved.