A 'small' local war -- 50 Hiroshimas -- could damage the ozone layer in mid-latitudes -- where you and I and other rich people live -- for years.
http://news.yahoo.com/s/ap/20080408/ap_ on_sc/nuclear_ozone_3
http://news.yahoo.com/s/ap/20080408/ap_

Comments
Soot might cool things off temporarily, by blocking sunlight. "nuclear winter", Pinatubo, Krakatoa, and all that.
Huh, Firefox's spellchecker knows about Pinatubo and Krakatoa, but not about "spellchecker".
A bigger war would, of course, be much worse.