I really don't understand this disconnect between the message of urgency about the environment and the climate, and the widespread complacency about an actual root cause of the issue (as illustrated by your comment), which is population growth: "Oh it's slowing and will probably stabilise at 11 billions... that's fine. On the other hand, we must be emissions-neutral and stop using plastics by tomorrow." That's totally incoherent.
We just need to learn to be efficient.
The world is not empty. It's either exploited to cater for human needs or wilderness that should be protected at the point we've reached.
Math: 327 million US ppl * 274 UK ppl per sq km / 35 US ppl per sq km.
Granted, a few pockets of wild forests left, Amazon, parts of Central Africa, New Guinea, Siberia. Under intense development pressure. Hard to call that "most of the world".
Southern California is an inhospitable desert, but tens of millions of people live there nonetheless.
PS. Mars colonization is folly.