>>16706251 (OP)Look up Lake Nyos for an explanation to your point, while simultaneously taking the facts these gentlemen have shared as information to why it doesn't happen in the same concentration globally.
Also, algae and plants release oxygen, winds mix air, mountains inhibit dense air flow.. CO2 doesn't settle instantly because of the size of the (medium {Earth}) it's being stirred in. Imagine a snowglobe the size of earth constantly shaking on its own... The mix doesn't settle quickly