If it were only that simple but it is not. Do a little research and you will learn, for example,

A nuclear winter would not reverse the effects of global warming. The changes in surface temperature, solar radiation and precipitation, would exacerbate some effects, including ocean acidification and damage to the ozone layer. Recent studies modeling the potential climate effects of nuclear war using crop yields, marine fishery and livestock production as indicators, found that production levels across the board would drop catastrophically, leading to global famine. https://www.icanw.org/what_would_nuclear_war_do_to_our_natural_environment