In 1983, Turco, Toon, Ackerman, Pollack and Sagan published "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions" in Science, known as the TTAPS study after their initials. [1] It argued that even a relatively small nuclear war could trigger global climate cooling so severe that crops would fail worldwide. Its central claim — that soot from burning cities could darken the planet for years — was controversial at the time, but recent modeling has confirmed and in some ways strengthened the original conclusions.
The TTAPS hypothesis
The TTAPS authors observed that nuclear weapons detonated over cities would ignite enormous fires. Modern cities contain large amounts of fuel — wood, plastics, asphalt, vehicle fuel. A single 300-kiloton airburst over a city could start a mass fire covering roughly 100 to 170 square kilometers within tens of minutes. [2] The resulting plume of soot, lofted by the heat into the upper troposphere, would drift on winds and could be transported globally by stratospheric circulation.
The 1983 TTAPS paper estimated that a nuclear war would likely exceed 5,000 megatons and that as little as 100 megatons could plunge the Northern Hemisphere summer into subfreezing temperatures lasting months. [1] Soot would remain in the stratosphere for years, because at those altitudes there is no rain to wash it out. Crops would fail across most of the Northern Hemisphere.
Critique and refinement
The TTAPS paper drew immediate criticism. Defense-aligned analysts argued that the soot loading was overestimated, that smoke would not reach the stratosphere, that smoke would rain out faster than predicted, and that warming caused by reduced cloud cover might compensate. By the late 1980s, a more cautious "nuclear autumn" framing had partly displaced the original "nuclear winter" claim.
Subsequent modeling — particularly the work of Alan Robock and Owen Brian Toon and collaborators in the 2000s and 2010s — addressed these critiques using modern coupled climate models. [3] The results were closer to TTAPS than to the more cautious revisions: soot from city firestorms does reach the stratosphere, does persist for years, and does cause significant cooling. The original hypothesis has held up remarkably well.
The mechanism, in more detail
The mechanism is straightforward but not intuitive. A nuclear weapon detonated over a city ignites multiple fires that merge into a firestorm — a self-sustaining fire so intense it generates its own weather. A firestorm can carry smoke into the upper troposphere, roughly 9-14 km up, where sunlight heats it and lifts it into the stratosphere. [4]
In the stratosphere, soot absorbs sunlight and re-radiates it as heat. This warms the stratosphere and cools the surface — the opposite of greenhouse gases, which cool the stratosphere and warm the surface. The cooler surface produces less convection, weaker monsoons, and shorter growing seasons. The stratospheric soot is removed only by gradual gravitational settling (months to years) since stratospheric circulation does not include rain.
The total effect depends on how much soot reaches the stratosphere. Modeling by Robock and colleagues (2007) found that fires from 100 Hiroshima-size weapons could inject about 5 million tonnes of soot and cool the global average surface by about 1.25 °C for several years. [4]
Even a regional war causes global cooling
The most striking result of recent modeling is that even a regional nuclear war — say, between India and Pakistan — could cause significant global climate disruption. A 2007 study by Robock, Toon and colleagues modeled a regional war using 100 Hiroshima-size (15 kt) weapons. About 5 million tonnes of soot rose into the stratosphere, cooling the global average surface by 1.25 °C for several years, with 0.5 °C of cooling left after a decade. [4]
A 2014 follow-up by Mills, Toon, Lee-Taylor and Robock in Earth's Future found global ozone losses of 20-50% over populated areas, growing seasons shortened by 10-40 days a year for five years, and surface cooling lasting more than 25 years. [5] The conclusion is that a "regional" nuclear war is not really regional — its climate effects affect the entire planet.
Crop failure and famine
Research by Jägermeyr, Robock, Xia and colleagues found that a regional India-Pakistan war could cut global maize, wheat and soybean production by 13%, 11% and 17% over five years, sufficient to trigger food shortages, price spikes, and famine in regions that depend on food imports. A 2022 study led by Xia found a full US-Russia war could cut global calorie production by about 90% — and the resulting global famine would likely kill more people than the bombs themselves. [6]
The 2022 paper by Xia et al. in Nature Food estimated that a US-Russia nuclear war scenario could lead to over 5 billion deaths from famine in the years following the exchange — far more than the few hundred million who would die from the immediate effects of the bombs. [7]
Why the implications matter
Nuclear winter changes the strategic calculus of nuclear weapons in important ways. A "limited" nuclear exchange may not be limited in its consequences. A nuclear war between two countries on the other side of the world could cause harvest failures in countries that have nothing to do with the conflict. The traditional concept of nuclear sovereignty — that a nuclear-armed state's decisions are its own — breaks down when the climate effects are global.
The simulator does not model nuclear-winter effects directly. Its calculations are limited to the immediate physical effects of single detonations. But for context: the global consequences of even a regional nuclear war would, on current modeling, dwarf the local effects shown in any individual scenario page on this site. See the resources page for nuclear-disarmament organizations and the sources page for primary references.