You are so naive my friend, if you think you are smarter than all those scientists who made models of global nuclear war. Let me repeat: Google is your friend - use it sometimes
Start with this:
'Nuclear winter' may kill more than a nuclear war - environment - 01 March 2007 - New Scientist
Some more articles for you, if Goggle is not reachable for you:
What would nuclear winter be like? - HowStuffWorks
Climatic Consequences of Nuclear Conflict
https://www.uow.edu.au/~bmartin/pubs/88spp.html
etc..
