The term ‘nuclear winter’ is usually related to a massive global nuclear war between the United States and Russia, lobbing hundreds or thousands of ICBMs across the ocean. Such an exchange would spew millions of tons of fine particles into the atmosphere, eventually encircling the planet in a permanent dark haze for years or decades. This global sunscreen would drop temperatures (in some cases by as much as 35 degrees Celsius) and block sunlight. Photosynthesis would no longer occur, plants would die and the entire food chain would collapse.
It is virtually assured that if the US attacked Russia (or vice versa) with nuclear weapons the subsequent retaliation would ensure self-destruction, so such an exchange is unlikely (despite the several near-nuclear exchanges over the past several decades).
In contrast, a regional exchange – perhaps on the Korean Peninsula or Indian Subcontinent – is far more likely. Not only are the players involved making decisions independent of US and Russian foreign policy and nuclear strategic positioning, there is also a general view that a regional nuclear conflict would not spell global disaster. More specifically, typical Western thinking is that stuff that happens in far-off lands won’t affect us.
This is wrong.
Several recent studies have argued that even a small regional nuclear conflict would cause a planetary catastrophe.
Study 1: Rutgers, the State University of New Jersey. "Regional Nuclear War Could Devastate Global Climate." ScienceDaily. ScienceDaily, 11 December 2006.
"We examined the climatic effects of the smoke produced in a regional conflict in the subtropics between two opposing nations, each using 50 Hiroshima-size nuclear weapons to attack the other's most populated urban areas," Robock said. The researchers carried out their simulations using a modern climate model coupled with estimates of smoke emissions provided by Toon and his colleagues, which amounted to as much as five million metric tons of "soot" particles.
"A cooling of several degrees would occur over large areas of North America and Eurasia, including most of the grain-growing regions," Robock said. "As in the case with earlier nuclear winter calculations, large climatic effects would occur in regions far removed from the target areas or the countries involved in the conflict."
Study 2: Mills, Michaels, et al. “Massive global ozone loss predicted following regional nuclear conflict.” Proceedings of the National Academy of Sciences, 2008.
“…firestorms resulting from a limited, regional nuclear war between India and Pakistan could produce global ozone losses of 20%-50% over populated areas, levels unprecedented in human history, accompanied by the coldest average surface temperatures in the last 1000 years, persisting for decades.”
For this reason, an irrational state cannot be allowed to obtain nuclear weapons. A nuclear North Korea would likely force Japan and South Korea to re-start their nuclear weapons research if they felt the US nuclear security guarantee was not rock solid. An arms race and escalation of conflict on the Korean Peninsula would have devastating consequences to people around the world, resulting in a worldwide nuclear winter and mass starvation.