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A · DocumentedNuclear
Nuclear war and nuclear winter
The immediate effects of nuclear weapons (blast, fire, fallout) are well understood. The larger uncertainty is the climate effect: burning cities could loft soot into the stratosphere, dim sunlight and shorten growing seasons worldwide. Climate-model studies agree the effect scales with the number of weapons used, and food production is the main way a regional war could become a global one.
The tier rates the quality of the evidence, not how likely the event is.
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- 1Use of nuclear weapons against cities and industrial targets● strong evidence
- 2Firestorms inject soot into the stratosphere (how much depends on targets, fuel loads and fire behaviour)◐ moderate evidence
- 3Sunlight drops and the planet cools for years◐ moderate evidence
- 4Shorter growing seasons and disrupted fisheries cut global calories◐ moderate evidence
- 5Famine, trade collapse and political instability◐ moderate evidence
- No analog for the climate effect. Volcanic eruptions are used as a test of how models handle stratospheric dimming.
- Hiroshima and Nagasaki (1945) are the only wartime uses; modern arsenals and cities are very different.
| Source | Statement | Kind |
|---|
| Xia et al. 2022 (Nature Food) ↗ | A regional war between India and Pakistan injecting 5 Tg of soot: global average calorie production falls about 7%. Larger wars: far larger losses; the authors' summary states that most of the world's population would face starvation after a US–Russia war. | PROJECTION |
|---|
| Toby Ord ↗ | 1 in 1,000 chance, per century, of an existential catastrophe from nuclear war (his own estimate) | EXPERT ESTIMATE |
|---|
Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.
Caveat
The climate effect is modelled, not observed, and the soot estimates are debated. The Doomsday Clock is a symbolic setting, not a probability, and the tension index on this site describes media coverage, not intent.
- Official guidance if a detonation or radiological release occurs: get inside, stay inside, stay tuned, for at least 24 hours (Ready.gov).
- Know your shelter room and keep a battery radio. Potassium iodide only on official advice.
Open the preparedness checklist for this scenario →
- Xia, L. et al. (2022). Global food insecurity and famine from reduced crop, marine fishery and livestock production due to climate disruption from nuclear war soot injection. Nature Food 3:586-596. link ↗
- Coupe, J. et al. (2019). Nuclear winter responses to nuclear war between the United States and Russia in WACCM4 and ModelE. J. Geophys. Res. Atmos. 124:8522-8543. link ↗
- Robock, A. et al. (2007). Nuclear winter revisited with a modern climate model and current nuclear arsenals. J. Geophys. Res. 112:D13107. link ↗
- SIPRI Yearbook 2026, world nuclear forces (January 2026). link ↗
- Bulletin of the Atomic Scientists, Doomsday Clock. link ↗
- Ord, T. (2020). The Precipice: Existential Risk and the Future of Humanity, Table 6.1. Hachette. link ↗