On its own, global warming shouldn't cause the extinction of humanity. It will be disruptive and deadly, sure, but we ought to be able to survive it, just as other humans have survived the collapse of their civilizations.

The problem that I see is in our reaction to the global warming crisis. The current reaction is to deny there's a problem and aggressively reassert our "right" to continue living as we did in the 20th century, and to even ramp up the destruction of our environment and habitat. Denying the realities of global warming and the consequences of our ignorance of natural factors is likely to ensure that many more lives are lost in the coming troubles than is necessary.

Species extinction? Not quite sure about that. But Lovelock's hypothesis seems plausible, at least in terms of overall numbers.

And the world will live as one