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?

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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. If the United States managed to disable the Soviet arsenal and launch its own preemptive nuclear strike (or vice versa), they wrote, the whole world would suffer the consequences:

When combined with the prompt destruction from nuclear blast, fires, and fallout and the later enhancement of solar ultraviolet radiation due to ozone depletion, long-term exposure to cold, dark, and radioactivity could pose a serious threat to human survivors and to other species … The possibility of the extinction of Homo sapiens cannot be excluded.

The nuclear winter paper was accepted for publication in the journal Science, where it was destined to reach millions of scientists and influence decades of future research. Known colloquially by the acronym "TTAPS" after its authors' last names, the academic article would be published on December 23, 1983. But in October, Sagan made the decision to announce his warning to the world using what amounted to a very unorthodox medium: the popular media.

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Sagan, like many at the time, believed nuclear war was the single greatest threat facing humanity. Others—including policymakers in the Reagan administration—believed a nuclear war was winnable, or at least survivable. Making the danger of nuclear winter real to them, Sagan believed, would take more than science. He would have to draw on both his public fame, media savvy and scientific authority to bring the what he saw as the true risk before the eyes of the public.

That meant a rearranging of personal priorities. According to his biographer, Keay Davidson, at a meeting in the early 1980s to plan the Galileo space probe, Sagan told his colleagues: “I have to tell you I’m not likely to do much of anything on Galileo for the next year or so, because I am concentrating most of my energies on saving the world from nuclear holocaust.”

According to Grinspoon, whose father, Lester, was a close friend of Sagan's and who knew all the authors (Pollack was his postdoctoral advisor), Sagan wasn’t a major scientific contributor to the TTAPS paper, though he was intimately familiar with the research it contained. However, the collaboration needed his high public profile to navigate the inevitable public controversy to come, in part because NASA was worried about political retaliation that might rebound on funding, Grinspoon writes in his book Earth in Human Hands.

Toon, Ackerman and Pollack all worked at the NASA Ames Research Center. As Davidson notes, “Ames director Clarence A. Syvertson … was also evidently terrified of doing anything to antagonize the Reagan Administration.” So Pollack called up Sagan, who intervened and got Syvertson to drop his objections.

Though his role in TTAPS was largely greasing the wheels, Sagan’s prominence and Parade piece meant the public tended to associate nuclear winter with him alone. As Davidson’s biography notes, Sagan was the one invited to debate nuclear winter before Congress in 1984. He was later invited by Pope John Paul II to discuss nuclear winter. And in 1988, he was mentioned by Soviet Premier Mikhail Gorbachev in his meeting with Reagan as a major influence on ending proliferation.

That meant people's personal feelings about Sagan colored their assessment of TTAPS. Unfortunately, it wasn't hard to attack such an outspoken messenger. As historian of science Lawrence Badash writes in A Nuclear Winter's Tale: "The columnist William F. Buckley Jr. said Sagan was 'so arrogant he might have been confused with, well, me.' He was faulted for strutting around on the TV screen, conveying an uncomfortable image for most scientists, one to which they had difficulty relating.”

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Of course, Sagan was hardly the first or last scientist to use his public fame for advocacy, nor to face criticism for it. Scientists who have stepped into the public eye include Marie Curie, Linus Pauling and Freeman Dyson; celebrity physicist Albert Einstein used his platform to decry American racism.

These figures are often seen alternatively as either noble, fearless explorers bound to discover the truth, no matter how challenging—or stooges of the establishment, easily bought off with government and industrial money, compromising their research. The reason for the contradictions is straightforward: scientists are people, and as such hold a variety of political opinions.

But the Cold War in particular threw those differences into stark contrast. Though his research credentials were impeccable, Carl Sagan was in many ways a Cold War warrior's stereotype of a hippie scientist. He wore his hair long by conservative academic standards, dressed modishly and casually, and was an outspoken critic of nuclear proliferation. (He also smoked marijuana, which likely would have made his more straight-laced critics flip out if that fact had been widely known.)

He even helped write the nuclear arms-control section of President Carter's farewell address, using phrases familiar from Cosmos and his other writings. "Nuclear weapons are an expression of one side of our human character,” Sagan wrote. “But there's another side. The same rocket technology that delivers nuclear warheads has also taken us peacefully into space. From that perspective, we see our Earth as it really is—a small and fragile and beautiful blue globe, the only home we have. We see no barriers of race or religion or country. We see the essential unity of our species and our planet. And with faith and common sense, that bright vision will ultimately prevail."

On the other side of the spectrum were scientists like physicist Edward Teller, whose anti-Communist zeal was particularly notable. He pushed for the U.S. to increase weapons research, and believed the U.S.S.R. was a more powerful adversary than American intelligence agencies were reporting. Teller often took existing threat analyses and extrapolated them into worst-case scenarios in the interests of spurring the government toward more aggressive action. He strongly opposed nuclear test bans and believed the Soviets were close to beginning a full-scale nuclear war.

Teller supported the Strategic Defense Initiative (SDI), a system of anti-nuclear satellites colloquially known as "Star Wars." Many analysts opposed SDI because it would potentially escalate the arms race; in 1986, 6,500 scientists pledged their opposition to SDI in part because they doubted it would work at all.

Nuclear winter pitted Sagan against Teller, culminating in both men giving testimony before the U.S. Congress. Teller took personal offense at the conclusions of TTAPS: if the nuclear winter hypothesis was right, SDI and other strategies Teller promoted were doomed from the start. It didn't hurt that their tactics were similar: in public statements, Sagan focused on the most extreme predictions for nuclear winter, just as Teller cherry-picked data to exaggerate the Soviet threat.

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Sagan's actions drew a personal backlash that reverberates into the present—most notably, in the realm of climate change.

At the time, many of Sagan’s opponents were strong supporters of SDI, which has been unsuccessfully re-proposed multiple times since. “Carl Sagan and his colleagues threw a [wrench] in the works, arguing that any exchange of nuclear weapons—even a modest one—could plunge the Earth into a deep freeze,” write Naomi Oreskes and Erik M. Conway in their book Merchants of Doubt. “The SDI lobby decided to attack the messenger, first attacking Sagan himself, and then attacking science generally.”

Similar tactics were used against environmental scientist Rachel Carson, Oreskes and Conway point out. Long after her death, anti-environmentalists and pro-DDT activists continue to focus on Carson the person rather than the research done by many scientists across disciplines, as though she alone ended the indiscriminate use of that insecticide.

In the case of nuclear winter, the consequences of this backlash would be profound. In 1984, a small group of hawkish physicists and astronomers formed the George C. Marshall Institute, a conservative think-tank that supported SDI.

Their leader was Robert Jastrow, a bestselling author and occasional TV personality whose politics were nearly opposite Sagan's. The Marshall Institute's tactics largely involved pressuring media outlets into "balancing" pieces critical of SDI with pro-"Star Wars" opinions. The Marshall Institute—and its successor the CO2 Coalition—later applied those same tactics to the issue of climate change. A former director of the institute, physicist William Happer, is a prominent climate-change denier who has consulted with President Trump.

Climate scientists have been hurt by these tactics, to the point where they often emphasize the best-case scenarios of climate change, as climate scientist Michael E. Mann writes in his book The Hockey Stick and the Climate Wars. Others, however, are concerned that downplaying the crisis makes it sound like we don't have to worry as much. Like Sagan, many researchers want to issue a direct call to action, even at the risk of being labeled a scientific Cassandra.

Comparing 1983 with 2017, the best word Grinspoon can think of is “denial”: “People didn't want to change the way they were thinking of [nuclear] weapons,” he says. “I see an echo of that now. What nuclear winter shows is that they're not really weapons in the sense that other things are weapons: that you can use them to harm your adversary without harming yourself. People are not really considering that if there really were to be a nuclear conflagration, in addition to how unthinkably horrible it would be in the direct theater of the use of those weapons—say in the Korean peninsula and surrounding areas—there would also be global effects.”

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Today we live in a vastly different world. Global nuclear weapons number around one-fourth of what they were in the '80s, according to The New York Times. And the threat of global thermonuclear war has mostly faded: Few believe that North Korea’s potential arsenal is capable of wiping out American cities and nuclear silos the way the former Soviet Union could.

But that doesn’t mean the legacy of TTAPS and Sagan is dead. The nuclear winter hypothesis could mean even a smaller nuclear war such as one fought between the U.S. and North Korea would damage the world for years to come. Thus, nuclear winter is still an important area of research, forming much of TTAPS author Brian Toon’s subsequent research. Lately he and collaborators have focused on the consequences of hypothetical smaller-theater wars, such one between India and Pakistan, or between North Korea and the U.S.

The debate over climate change isn't going away anytime soon, either. And the way Sagan and his scientific colleagues handled publicizing and debating the nuclear winter question seems very similar to those tracking climate change. In both instances, the potential impact of the science is huge, with implications beyond the scope of the research, and valid concerns about either understating or overstating the risks.

“Both nuclear winter and global climate change are fairly abstract phenomena that occur on a scale beyond our immediate sensory experience,” says Grinspoon. “We're asking people to accept a result and imagine a change that is just beyond the realm of any of us, what we've experienced in our lives. That's something human beings aren't great at!”

That means that the debates will continue. And whenever there are scientific issues that spill over into human affairs, similar issues will crop up. After all, scientists are humans, who care about politics and all the other messy matters of life. In his 1994 book Pale Blue Dot, Sagan wrote upon seeing an image of Earth from Voyager 1, "To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known."