Carl Sagan warned the world about nuclear winter

Person: Carl Sagan
Lived: 1934–1996
Field: Astronomy, planetary science
From: American

The story

At the height of the Cold War, Carl Sagan believed scientists had a duty to explain the full consequences of nuclear war. After his marriage to Ann Druyan in 1981 he became increasingly active against the arms race. His thinking was also shaped by a wider question: he suspected that the absence of contact with other civilisations might mean that technological societies tend to destroy themselves, and he wanted humanity to avoid that fate.

In 1982 Paul Crutzen and his colleague John Birks published “Twilight at Noon,” which suggested that smoke from a large nuclear exchange could darken the sky and cool the surface. Sagan joined four colleagues, Richard Turco, Owen Toon, Thomas Ackerman and James Pollack, to model the effect. Their study, published in Science in December 1983 and known by their initials as TTAPS, contained the first use of the term “nuclear winter,” which Turco had coined. The team's earlier work on dust storms on Mars and on asteroid impacts helped them think about how soot high in the atmosphere could block sunlight.

The study argued that soot from burning cities could chill the Earth's surface, threatening crops and life far beyond the battlefields. Sagan brought the idea to the public through articles and the books The Cold and the Dark: The World after Nuclear War (1984) and A Path Where No Man Thought: Nuclear Winter and the End of the Arms Race (1990), written with Turco, which argued for deep cuts in nuclear arsenals. He also spoke out against the Strategic Defense Initiative and joined protests at the Nevada Test Site.

The hypothesis was fiercely debated. Critics, including the physicist Edward Teller, accused Sagan of overstating an uncertain result, and later three-dimensional models generally predicted less severe cooling than the first estimates. Sagan acknowledged in The Demon-Haunted World that one of his related predictions, about the Kuwaiti oil fires of 1991, had not come true. Yet most later models continued to find that a large nuclear war would cause harmful global cooling, and the TTAPS team refined its work in a 1990 Science paper.

In 2022 Sagan was posthumously awarded the Future of Life Award for reducing the risk of nuclear war by developing and popularising the science of nuclear winter. He shared the honour with seven other researchers in the field, and Druyan accepted it on his behalf.

Why it matters

Nuclear-winter research showed that a nuclear war would endanger people far from any battlefield, adding a powerful scientific argument to the case for arms reduction.

About Carl Sagan

Brooklyn-born astronomer who explained the Venus greenhouse, helped send humanity’s messages on Pioneer and Voyager, and taught Cosmos viewers worldwide to look at the universe with wonder and doubt.

Read the full profile

Carl Sagan

1934–1996 · Astronomy, planetary science

View profile →