Nuclear winter is a theoretical concept which has been put forward by some members of the scientific community. It originated in 1982 with John Birks and Paul Crutzen. Put briefly, the detonation of large numbers of nuclear weapons could trigger a dramatic change in the global climate, causing extreme cold and potentially resulting in serious trouble for the living organisms which call Earth home. Numerous studies on the possibility of nuclear winter have been conducted, and it is difficult to prove that it would happen in the event of a nuclear war, but it is one of the many arguments used against nuclear attacks on other nations.
According to the theories, nuclear winter would be caused by a huge cloud of dust, smoke, and particulate matter resulting from large-scale detonations over cities around the world. As the cities and surrounding areas burned, they could inject huge amounts of material into the atmosphere, slowly blocking sunlight. Because sunlight would not be able to reach Earth, global temperatures would drop dramatically, and our ability to produce food would be greatly reduced.
Scientists have also suggested that a large-scale detonation of nuclear weapons could damage the ozone layer. Because harmful UV radiation can penetrate layers of particulate matter, people would still be at risk of UV exposure despite the fact that it would be dark and cold. People would be at risk from fallout. This could combine with low food production to threaten many organisms on Earth, from humans to birds.
Most of the studies on nuclear winter point out that a massive number of nuclear weapons would need to be detonated to trigger climate change on this scale; something along the lines of half of the known nuclear devices on Earth. It has also been suggested that the detonations would need to be fairly close together, creating a steady stream of material which would work its way up into the atmosphere. Nuclear winter is probably also more likely to impact the Northern Hemisphere, given that this is where the bulk of potential nuclear targets are located.
Critics of the nuclear winter theory have suggested that while we might see some climate change, it wouldn't be as dramatic as the nuclear winter theory proposes. These critics suggest that the particulate matter would be scrubbed from the atmosphere by rain and wind. However, proponents of the theory have pointed out that massive volcanic eruptions have historically caused climate change by shooting streams of smoke and ash into the atmosphere, and the climactic effects of the oil fires in Kuwait during the First Gulf War also lend credence to the theory.