What is Putin's Last Card

Someone said to me this week that nuclear weapons were almost as bad as global warming.   You have to be kidding me ... maybe they needed to be sitting under the study desks in grade school when we had nuclear bomb drills.  Yes, the drills were stupid, about as stupid as thinking global warming and nuclear weapons are about the same.   Have we learned nothing about Putin over the last few decades?  What makes us think we will accept defeat without using all of the cards in his deck?  Why does that not terrify us?