In 1942, it occurred to Edward Teller, one of the Manhattan scientists, that a nuclear explosion would create a temperature unprecedented in Earth’s history, producing conditions similar to those in the center of the sun, and that this could conceivably trigger a self-sustaining thermonuclear reaction in the surrounding air or water (Rhodes, 1986). The importance of Teller’s concern was immediately recognized by Robert Oppenheimer, the head of the Los Alamos lab. Oppenheimer notified his superior and ordered further calculations to investigate the possibility. These calculations indicated that atmospheric ignition would not occur.

This prediction was confirmed in 1945 by the Trinity test, which involved the detonation of the world’s first nuclear explosive.

In 1954, the US carried out another nuclear test, the Castle Bravo test, which was planned as a secret experiment with an early lithium-based thermonuclear bomb design. Lithium, like uranium, has two important isotopes: lithium-6 and lithium-7. Ahead of the test, the nuclear scientists calculated the yield to be 6 megatons (with an uncertainty range of 4–8 megatons). They assumed that only the lithium-6 would contribute to the reaction, but they were wrong. The lithium-7 contributed more energy than the lithium-6, and the bomb detonated with a yield of 15 megaton – more than double of what they had calculated (and equivalent to about 1,000 Hiroshimas).

The unexpectedly powerful blast destroyed much of the test equipment. Radioactive fallout poisoned the inhabitants of downwind islands and the crew of a Japanese fishing boat, causing an international incident. We may regard it at as lucky that it was the Castle Bravo calculation that was incorrect, and not the calculation of whether the Trinity test would ignite the atmosphere.

Counterfactually, if the atmosphere had been susceptible to ignition by a nuclear detonation, and if this fact had been relatively easy to overlook – let us say as easy as it was to overlook the contribution of the lithium-7 in the Castle Bravo test – then the human story (and that of all terrestrial life) would have come to an end in 1945.

We can call this scenario ‘Castle Bravissimo’.