[mempko]

Since we aren't able to export our pollution out into space, or mine food on mars, yes, we live in a closed system. We live on a round spaceship known as Spaceship Earth (https://en.wikipedia.org/wiki/Spaceship_Earth)

[fluoridation]

Strictly speaking, even if we were able to do that we'd still live in a closed system, just a much, much larger one. :)