Ragnarök Question Series: if a nuclear catastrophe occurs, will it reduce the human population by 95% or more?

You can now see an excellent visualization of global catastrophic risks estimates produced in the Ragnarök series here.

A nuclear exchange could cause a nuclear winter – a release of black carbon into the atmosphere which would according to some studies, result in the blocking the Sun’s thermal energy. This would lower temperatures regionally and globally for several years, and open up new holes in the ozone layer protecting the Earth from harmful radiation, reduce global precipitation by about 10%, trigger crop failures, and result in widespread food shortages.

According to some models, the smoke would rapidly engulf the Earth and form a dense stratospheric smoke layer. The smoke from a war fought with strategic nuclear weapons would quickly prevent up to 70% of sunlight from reaching the surface of the Northern Hemisphere and 35% of sunlight from reaching the surface of the Southern Hemisphere. Such an enormous loss of warming sunlight would produce Ice Age weather conditions on Earth in a matter of weeks. For a period of 1-3 years following the war, temperatures would fall below freezing every day in the central agricultural zones of North America and Eurasia.

This could leave some survivors in parts of Australia and New Zealand, but they would be in a very precarious situation and the threat of extinction from other sources would be great. Whether a nuclear winter could cause extinction is currently unclear. Some models consider total extinction very unlikely, and suggest parts of the world would remain habitable.

These studies consider what is currently the worst-case nuclear exchange, namely, that between the US and Russia. However, It is possible that a future nuclear arms race someday leads to larger stockpiles or more dangerous nuclear weapons than existed at the height of the Cold War. In this case, chances of a nuclear extinction might be higher than they currently are.