If sufficient ash from burning cities and forests ascended into the sky, it could effectively work as an umbrella, shielding large portions of the Earth from the sun. If you diminish the amount of sunlight that makes its way to the surface, then you diminish the resulting atmospheric temperature -- as well as potentially interfere with photosynthesis.

s for, inter alia, the control of atomic energy to the extent necessary to ensure its use only for peaceful purposes. The resolution also decided that the Commission should make proposals for "the elimination from national armaments of atomic weapons and of all other major weapons adaptab