It has been estimated that a 10,000-megaton war with half the weapons
exploding at ground level would tear up some 25 billion cubic meters of
rock and soil, injecting a substantial amount of fine dust and
particles into the stratosphere. This is roughly twice the volume of
material blasted loose by the Indonesian volcano, Krakatoa, whose
explosion in 1883 was the most powerful terrestrial event ever
recorded. Sunsets around the world were noticeably reddened for
several years after the Krakatoa eruption, indicating that large
amounts of volcanic dust had entered the stratosphere.
Subsequent studies of large volcanic explosions, such as Mt. Agung on
Bali in 1963, have raised the possibility that large-scale injection of
dust into the stratosphere would reduce sunlight intensities and
temperatures at the surface, while increasing the absorption of heat in
the upper atmosphere.
The resultant minor changes in temperature and sunlight could affect
crop production. However, no catastrophic worldwide changes have
resulted from volcanic explosions, so it is doubtful that the gross
injection of particulates into the stratosphere by a 10,000-megaton
conflict would, by itself, lead to major global climate changes.