Why ? Even if USA/Europe/Russia and China nuke themselves to oblivion that's all northern hemisphere. I'm sure the atmosphere would be fucked up for a while but

Why ? Even if USA/Europe/Russia and China nuke themselves to oblivion that's all northern hemisphere. I'm sure the atmosphere would be fucked up for a while but after that ? Would Australia/NZ/South Africa/Argentina etc. be livable ? After how long ?