Nuclear winter is a hypothetical global climate condition that was predicted to be a possible outcome of a large-scale nuclear war. It was thought that severely cold weather would be caused by detonating large numbers of nuclear weapons, especially over flammable targets such as cities, large amounts of smoke and soot would be injected into the Earth's stratosphere.
This layer of particles would significantly reduce the amount of sunlight that reached the surface, and could potentially remain in the stratosphere for weeks or even years (smoke and soot arising from the burning petroleum fuels and plastics absorbs sunlight much more effectively than smoke from burning wood). The smoke and soot would be shepherded by strong west-to-east winds, forming a uniform belt of particles encircling the northern hemisphere from 30° to 60° latitude. These thick black clouds could block out much Sun's light for a period as long as several weeks, causing surface temperatures to drop by as much as 20°C for several weeks.
The combination of darkness and killing frosts, combined with high doses of radiation from nuclear fallout, would severely damage plant life in the region. The extreme cold, high radiation levels, and the widespread destruction of industrial, medical, and transportation infrastructures along with food supplies and crops would trigger a massive death toll from starvation, exposure, and disease. It was also thought that nitrogen oxides generated by the blasts would degrade the ozone layer; this phenomenon was observed in the first thermonuclear blasts, which had unanticipated degrading effects on the ozone. These effects have since been mitigated by ozone regeneration, but the effect of a full-scale war would undoubtedly be much greater. Secondary effects from ozone depletion (and concomittant increases in ultraviolet radiation) would be significant, with impacts on the viability of most human staple agricultural crops as well as disruption of ocean food chains by killing off phytoplankton.
One effort to predict the metereological effects of a large-scale nuclear war was the 1983 "TTAPS" study (from the initials of the last names of its authors, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan). The authors were inspired to write the paper by cooling effects due to dust storms on Mars and to carry out a calculation of the effect they used a simplified two dimensional model of the earth's atmosphere that assumed that conditions at a given latitude were constant. The consensus with more sophisticated calculations is that the atmospherical model used in TTAPS probably overestimates the degree of cooling although the amount of this overestimation remains unclear. Although such nuclear war would undoubtedly be devastating, the degree of damage to life on Earth as a whole remains controversial.