The threat however, in the sequence of events, is the extinction of human civilization, not the human species.
Civilization can go down with a billion or so human survivors still remaining, yet only being taken back into the dark ages by a "time machine" built by the civilization that erased itself from the face of the earth.
One of the global warming factors which may make that easier to see is the potable water problem:
All this talk about global warming raising the depth of the ocean yet there being less and less drinking water, makes one think of the "water water everywhere, nor any drop to drink" saying of old.(Global Warming & Potable Water). There will not be enough of this or enough of that for civilization, however, there will be plenty of water to support groups of human beings and other species around the globe:
World civilization means the nations of the world interconnected by trade, travel, treaties, and international commerce.(What Do You Mean - World Civilization?). That is not ultimately comforting, however, it is better than what would result if we got into an all-out nuclear war.
So, when climate change scientists talk about dangers to the existence of civilization they do not mean that the population of human beings as a species is going to become extinct.
In other words, the human species would live on even if civilization ended.
That scenario could bring the wrath of heat that will destroy the earth environment way, way before its time.
If some live on through the global warming catastrophes, they will have the wisdom which world civilization ignored, and we can hope they will develop a new and wiser civilization.
One that will have a better go at finding a new home world before the real heat is put on this planet.