Environmental friendly matters only because it is human friendly. Removing all the humans is not environmental friendly since climate change and extinctions happen a lot without humans too. Reducing number of people to half the current population is not environmental friendly since we were cutting down huge forests and driving species to extinction even before the industrial age.
The most environmental friendly thing you can do is have more than 2 children, and make sure they are well educated.
There are still lots of deserts on earth that they can make into forests, we just need the science and manpower for that.
Warmer arctic region is not a problem, it gives more livable space. Higher sea level is not a problem, it gives more water to create lakes and fill aquifers in places like Sahara, Iran, and central Asia. 50 bln people on earth is not a problem, it gives more creativity and more workforce to go to Mars.
> Higher sea level is not a problem, it gives more water to create lakes and fill aquifers in places like Sahara, Iran, and central Asia.
Higher sea level is absolutely a problem, as it disproportionately takes away the most valuable, economically productive, and inhabited land, and causes huge refugee crises. There's no shortage of seawater. Everything you mentioned could be done now (if we had enough power for desalination anyway). Higher sea levels don't make it appreciably easier, but they sure make everything else way worse.