When Carl Sagan Warned the World About Nuclear Winter

November 16, 2017

Before the official report came out, the popular scientist took to the presses to paint a dire picture of what nuclear war might look like.

If you were one of the more than 10 million Americans receiving Parade magazine on October 30, 1983, you would have been confronted with a harrowing scenario. The Sunday news supplement’s front cover featured an image of the world half-covered in gray shadows, dotted with white snow. Alongside this scene of devastation were the words: “Would nuclear war be the end of the world?”

This article marked the public’s introduction to a concept that would drastically change the debate over nuclear war: “nuclear winter.” The story detailed the previously unexpected consequences of nuclear war: prolonged dust and smoke, a precipitous drop in Earth’s temperatures and widespread failure of crops, leading to deadly famine. “In a nuclear ‘exchange,’ more than a billion people would instantly be killed,” read the cover. “But the long-term consequences could be much worse…”

According to the article, it wouldn’t take both major nuclear powers firing all their weapons to create a nuclear winter. Even a smaller-scale war could destroy humanity as we know it. “We have placed our civilization and our species in jeopardy,” the author concluded. “Fortunately, it is not yet too late. We can safeguard the planetary civilization and the human family if we so choose. There is no more important or more urgent issue.”

The article was frightening enough. But it was the author who brought authority and seriousness to the doomsday scenario: Carl Sagan.

By 1983, Sagan was already popular and publicly visible in ways most scientists weren’t. He was a charismatic spokesperson for science, particularly the exploration of the solar system by robotic probes. He hosted and co-wrote the PBS television series “Cosmos,” which became the most-watched science program in history and made him a household name. His 1977 book, The Dragons of Eden, won the Pulitzer Prize. He was well-known enough to be parodied by Johnny Carson on “The Tonight Show” and Berkeley Breathed in the “Bloom County” comic strip.

But with his Parade article, he risked puncturing that hard-won popularity and credibility. In the fallout from the article, he faced a barrage of criticism—not just from pro-nuclear conservatives, but also from scientists who resented him for leveraging his personal fame for advocacy. Sagan later called discussion surrounding nuclear winter following the article “perhaps the most controversial scientific debate I’ve been involved in.” That might be an understatement.

So the question is: What was a scientist doing getting involved in politics and writing about nuclear war in the popular presses in the first place?

The nuclear winter chapter of history began in the late 1970s, when a group of scientists—including Sagan—entered the nuclear arms fray. These weren’t nuclear physicists or weapons experts: they studied the atmospheres of Earth and other planets, including dust storms on Mars and clouds on Venus.

In 1980, paleontologist Luis Alvarez and his physicist father Walter presented evidence that an asteroid had hit Earth at the end of the Cretaceous Period. They argued that the impact had thrown so much dust and debris into the air that Earth was blanketed in shadow for an extended period, long enough to wipe out the last of the non-bird dinosaurs. If true, this hypothesis showed a way that a catastrophe in one location could have long-term effects on the entire planet.

Sagan and his former students James Pollack and Brian Toon realized this work applied to climate change on Earth—as well as nuclear war. Along with meteorologists Tom Ackerman and Rich Turco, they used computer models and data collected by satellites and space probes to conclude that it wouldn’t take a full-scale thermonuclear war to cause Earth’s temperature to plummet. They found average global temperatures could drop between 15º and 25º Celsius, enough to plunge the planet into what they called “nuclear winter”—a deadly period of darkness, famine, toxic gases and subzero cold.

The authors acknowledged the limitations of their model, including poor predictions for short-term effects on small geographical scales and the inability to predict changes in weather as opposed to climate. Nevertheless, their conclusion was chilling.

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