In 1804 the human race reached 1 billion people for the very first time, and humans entered a pre-extinction zone as human beings

 Going over 1 billion people put humans (and all life on earth) on a road towards potential extinction.

This century we are living the fact that human extinction now is possible.

It isn't wars  or diseases or starvation that is driving  humans toward extinction. 

No. Actually what drives human extinction is having over 1 billion humans on earth and everything they do here. But, most people didn't realize this fully until maybe until the 1960s and 1970s when scientists first began to fully realize we had too many people on the surface of the earth for the human race not to go extinct eventually.

At this point now most educated people who have actually studied this at all all realize there is presently no way for over 1 billion people to survive on the surface of the earth given all present variables in place.

Could more people stay alive if the atmosphere and oceans were technically altered along with the weather?

Possibly.

But, likely all other animal and fish and bird life and bees that pollinate vegetables,  fruit and nut trees could not likely survive this transition so it would be ONLY people and their pets who survived this change.

And it is very debatable whether humans would stay functional and sane during these transitions or whether humans in some ways would not longer really be humans with all animals and most plant life extinct on earth.

I for one am not sure I would want to still be alive without wilderness and trees and flowers growing around me (at least part of the year) when it's not winter.