THE eruption of Tambora 200 years ago was the largest volcanic blast of modern history, killing something like 100,000 people in what is today Indonesia, producing an ash cloud that covered a million square kilometres and temporarily changing the global climate. The veil of sulphate particles which Tambora created in the stratosphere cooled the planet’s continents by an average of about 2ºC in the following year; local changes were in some places much more severe, and there was agricultural chaos in North America, Europe and China. Volcanoes are nasty; other natural disasters—such as storms, floods and earthquakes—only have global effects because of the interconnectedness of human societies and economies, while large eruptions have direct geophysical effects worldwide. Yet volcanoes are not the only existential threats humanity faces. Which catastrophes could actually end human civilisation?

A repeat of Tambora would have similar climatic effects, though it is possible that in today’s world their human impact would not be so severe. But the next large eruption could be a lot bigger than that. In terms of the volume of rock the eruption of the volcanic caldera at Taupo, in New Zealand, 26,500 years ago, was well over ten times the size of Tambora; that of Toba, in Indonesia, 75,000 years ago was almost 100 times larger than the 1815 blast. The area covered by ash from Toba to a depth of a centimetre or more has a population of about 2 billion people today. The climatic effects of an eruption do not increase linearly with its size, but if it arrived without years of warning (time to allow massive precautionary stockpiling) a repeat of the Toba eruption today would be a devastating blow to food security on a global scale and could be expected to cause tens of millions or hundreds of millions of casualties. Some researchers believe that the Toba eruption was sufficiently devastating to Homo sapiens, which may well then have been found at that time only in east Africa, as to almost wipe the species out. That could explain why modern humans have a very low level of genetic diversity compared with other apes.
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The only other natural disasters of similar consequence are impacts by asteroids and comets, which can massively disrupt the climate in similar ways. The impact of an asteroid 1km-2km across is expected roughly once every million years or so, and some estimate that its effects could be severe enough to bring death to hundreds of millions or billions. There are also larger impacts; the one that occurred in what is now the Yucatan at the end of the Cretaceous period, 66m years ago, was probably 10km or more across. That impact is widely thought to have brought with it a mass extinction which killed off well over half the species on the planet. The good thing about asteroid impacts is that they are both rare and avoidable. Although humans can change the climate themselves—and the changes they are currently bringing about look likely to have have much larger long-term effects on the climate and other aspects of the Earth, such as its ocean chemistry, than even an eruption as large as Toba’s did—there is no currently imaginable way for people to stop a major volcanic eruption. But in the highly unlikely event that a 1km asteroid discovered tomorrow was shown to have a high probability of hitting the Earth on some day towards the middle or end of this century then it is quite plausible to assume that its orbit could and would be altered in the intervening decades (decades of warning are much more likely, in this case, than just a few years). Blowing the offending object apart with nuclear weapons, as favoured by Bruce Willis and other heroes of the silver screen, would probably be a bad idea. But using such blasts to nudge it onto an Earth-missing trajectory could make a lot of sense if there was not enough time to develop more precise and easily controlled approaches to the problem.

This is not a reason to rejoice in the availability of nuclear weapons, though. It is hard not to think that they represent a far more likely cause of global disruption on a civilisation-ending scale than either supervolcanoes or asteroids. The direct casualties in an all-out nuclear war would in all likelihood greatly outnumber those of a supervolcano eruption, and such a war might easily have climate effects that were just as bad or even worse. And if the chances of a nuclear war in any given year are as little as 10,000 to one, that would still mean a better than even chance of seeing one in the next 7,000 years—far sooner than the time you would expect to wait for a Toba-scale eruption, let alone a world-altering asteroid impact.