Ragnarök Question Series: If a global catastrophe occurs, will it be due to nuclear war?

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

The invention of nuclear weapons gave humanity the technical capacity to cause devastation on a hitherto unseen scale. Although there have been no nuclear attacks since the Second World War, we have come close to inadvertent and intentional nuclear war on a number of occasions.

The Cuban Missile Crisis of October 1962 was a direct and dangerous confrontation between the United States and the Soviet Union during the Cold War and was the moment when the two nuclear superpowers came closest to nuclear conflict. U.S. president John F. Kennedy estimated the odds of nuclear war at "somewhere between one out of three and even".

Twenty events that might be considered ‘near-miss’ incidents – incidents that could potentially have resulted in unintended nuclear detonation or explosion – have been reported in declassified documents. There are potentially more ‘near-misses’ that have remained classified and concealed. Moreover, most of these incidents on our timeline were reported by US sources, and there is no reason to believe that the opposing superpower had fewer incidents, or that there have been zero incidents in China, the UK, France, Israel, India, Pakistan or North Korea.

Currently, there are an estimated total of 14,185 nuclear weapons in existence, of which russia and the USA possess 13400. The remained is divided between (in descending order of number of weapons possessed) France, China, UK, Pakistan, India, Israel, and North Korea.

Although tensions between the US and Russia have eased somewhat since the Cold War the geopolitical situation could become more unstable over the next few decades. Another possible intentional nuclear war is between India and Pakistan. The two countries have gone to war four times since then, in 1947, 1965, 1974 and 1999, and have been on the brink of war as recently as 2008. Pakistan has pledged to meet any Indian attack on its territory with a retaliatory nuclear strike. Most recently, the world has witnessed displays of brinkmanship by North Korea and the US with explicit threats of nuclear warfare.

Although initial effects from such a nuclear exchange would be horrible, the after-effects could be worse. A nuclear exchange could cause a nuclear winter – a release of black carbon into the atmosphere. According to some studies, this would result in the blocking the Sun’s thermal energy, and lowering temperatures regionally and globally for several years, opening up new holes in the ozone layer protecting the Earth from harmful radiation, reducing global precipitation by about 10%, triggering crop failures, and resulting in widespread food shortages

Recent calculations of the dust, particulates and smoke thrust into the atmosphere by as few as 100 nuclear weapons indicate that even a regional war could have major impact on the planet’s atmosphere and climate.

The fact that we’ve never had a mass-casualty accidental nuclear detonation or explosion might suggest that some estimates of the odds of nuclear war –such as John F. Kennedy’s– are too gloomy. Others have pointed out that the strength of this evidence is perhaps surprisingly weak: if nuclear war removes many observers, then realizations of world history we see are unlikely to have included nuclear war. Hence observers in surviving worlds will see the world to be much safer than it actually is.