G. Earth can be considered a closed system in most respects. Some elements used by humans are irreplaceable. Likewise, some pollutants created by humans will stay on earth for good. Human activity is very fast compared to a geologic timescale. The earth's population doubles every 30 years.
Humans consume more and more land and mineral and energy resources.
Some resources could be exhausted in our lifetimes. The earth is not growing at all. Ergo, we need to watch ourselves.