I doubt many in America thought either world war was the end of the world.
But the difference is that all of those required affirmative action to cause a global catastrophe, someone had to reach out and press a metaphorical button. With global warming the status quo will get us all killed.
Global warming is not lethal in itself. The biggest threat in it is destabilisation of societies and resulting wars - but for a war to start, somebody has to press that metaphorical button. It is not inevitable.
But the difference is that all of those required affirmative action to cause a global catastrophe, someone had to reach out and press a metaphorical button. With global warming the status quo will get us all killed.