Question

HOW IS GLOBAL WARMING LARGEST THREAT TO HUMANITY ?

Answer 1

The first scenario assumes global warming is real and humanity tries to prevent or mitigate it. The best possible outcome for this scenario is that civilization is saved. The worse possible outcome for this scenario is that civilization collapses. 
The second scenario assumes global warming is not real and humanity still tries to prevent it. Therefore civilization stays intact, but lots of money is spent to prevent something that doesn’t happen. 

Answer 2

as it is the main reason of the 2 global consequences,
1)it is rising the temp of the earth
2)it is melting the ice caps which are causing floods