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Nuclear Winter

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Nuclear Winter

Nuclear winter is the theorised global climate collapse that would follow a large-scale nuclear war: soot from firestorms lofted into the upper atmosphere blocks enough sunlight to freeze surface temperatures, collapse agriculture, and kill far more people through starvation than the initial blasts themselves. The theory was first published in the early 1980s by five authors including Carl Sagan (giving rise to its common shorthand, the ‘TTAPS’ study), and remains an active area of climate modelling today.

As reported by Annie Jacobsen via physicist Brian Toon — one of the theory’s five original authors — the mechanism begins where the immediate destruction of a nuclear war ends. Each detonation over a populated area is expected to ignite a megafire burning across 100 to 300 square miles; scaled across a full nuclear exchange, Toon’s modelling estimates roughly 300 billion pounds of soot lofted into the stratosphere. That soot blocks sunlight for years, driving surface temperatures down sharply enough to freeze inland bodies of water for as long as a decade and to collapse agriculture worldwide, long after the shooting itself has stopped.

Mechanism: from megafire to famine

The theory’s causal chain runs firestorm → soot injection into the stratosphere → blocked sunlight → collapsed photosynthesis and crop failure → mass starvation. This is distinct from, and additional to, the direct casualties of blast, fire, and radiation: Jacobsen’s scenario has the acute phase of a nuclear war ending within about 72 minutes of the first launch, with nuclear winter’s famine deaths — numbering in the billions, on her account — unfolding over the years that follow. Toon also notes a secondary hazard once the soot eventually clears: a severely depleted ozone layer would leave surviving populations exposed to intensified ultraviolet radiation just as agriculture is attempting to recover.

Toon’s own framing, reported by Jacobsen, draws a direct comparison to the Chicxulub asteroid impact 66 million years ago, which killed the dinosaurs and wiped out roughly 70% of species on Earth — with the pointed distinction that an asteroid strike is not a choice, while a nuclear war is.

Where mainstream views differ

Nuclear winter has been scientifically contested since its first publication. The US Defense Department initially dismissed the original 1980s modelling as Soviet propaganda, a characterisation the theory’s authors reject; a body of subsequent research, sometimes summarised as ‘nuclear autumn’, argued the original models overstated the severity and duration of cooling by using unrealistically large and uniform firestorms. The theory’s original authors, including Toon, have consistently maintained that their 1980s modelling was the best available given the computing power of the time, and argue — based on improved climate models run since — that the actual risk is at least as severe as first estimated, not less. The scientific debate is genuinely live: it concerns the scale and duration of the effect, not whether large-scale soot injection into the stratosphere would have some cooling effect, which is broadly accepted physics.

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