Forcing lower numbers of humans is not unheard of; usually this is considered cruel. Mass killings aside, take a look at policies enforced by India and China in 1970-80s.
Choosing not to have kids is myopic: if your genes make you more conscious and thus caring about the planet as a whole, removing them from the gene pool lowers the chances that next generations would share these traits of yours.
The best medicine found so far is exactly what the ultra-green do not like: urbanization, education, higher standards of living, minimal numbers of people doing the traditional countryside agriculture. This puts reproduction rates safely below the replacement rate all over the world, no matter what culture and religion.
You can put every living human on Sicily and still have space. The problem is that 99% would have died of thirst after a few days.
I think today is earth overshoot day, so we used resources the earth could recoup within a year. That problem isn't solved with everyone moving into cities, it is externalities and our resource consumption. Maybe cities are a few percent more efficient, but it doesn't really matter that much. Cities are also relatively unhealthy in most cases. Living expectation are countered by the proximity of medical professionals perhaps.
edit: It is indeed today: