There is no reason to believe this is so. A less advantageous climate will almost certainly in any reasonable projections kill/impoverish only some and in all p

There is no reason to believe this is so. A less advantageous climate will almost certainly in any reasonable projections kill/impoverish only some and in all probability a small minority of the human race unless we under stress decide to kill the rest. No reasonable scientists are projecting human extinction and by positing it as such you are erecting a trivially demolished straw man for the opposition to mock.