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When Carl Sagan Warned theWorld About Nuclear Winter

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If you were one of the more than 10 million Americans receiving Parade magazineon 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 ofdevastation were the words: “Would nuclear war be the end of the world?”

This article marked the public’s introduction to a concept that would drasticallychange the debate over nuclear war: “nuclear winter.” The story detailed thepreviously unexpected consequences of nuclear war: prolonged dust and smoke, aprecipitous drop in Earth’s temperatures and widespread failure of crops, leading todeadly famine. “In a nuclear ‘exchange,’ more than a billion people would instantly bekilled,” 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 theirweapons to create a nuclear winter. Even a smaller-scale war could destroyhumanity as we know it. “We have placed our civilization and our species injeopardy,” the author concluded. “Fortunately, it is not yet too late. We cansafeguard the planetary civilization and the human family if we so choose. There isno more important or more urgent issue.”

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

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

But with his Parade article, he risked puncturing that hard-won popularity andcredibility. In the fallout from the article, he faced a barrage of criticism—not just

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from pro-nuclear conservatives, but also from scientists who resented him forleveraging his personal fame for advocacy. Sagan later called discussion surroundingnuclear winter following the article “perhaps the most controversial scientificdebate I’ve been involved in.” That might be an understatement.

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

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Scientist and science spokesperson Carl Sagan poses in front of the solar system (NASA Photo / Alamy)

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

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

Sagan and his former students James Pollack and Brian Toon realized this workapplied to climate change on Earth—as well as nuclear war. Along withmeteorologists Tom Ackerman and Rich Turco, they used computer models anddata 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 foundaverage global temperatures could drop between 15º and 25º Celsius, enough toplunge the planet into what they called “nuclear winter”—a deadly period ofdarkness, famine, toxic gases and subzero cold.

The authors acknowledged the limitations of their model, including poor predictionsfor short-term effects on small geographical scales and the inability to predictchanges in weather as opposed to climate. Nevertheless, their conclusionwas chilling. If the United States managed to disable the Soviet arsenal and launch itsown preemptive nuclear strike (or vice versa), they wrote, the whole world wouldsuffer the consequences:

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

The nuclear winter paper was accepted for publication in the journal Science, whereit was destined to reach millions of scientists and influence decades of futureresearch. Known colloquially by the acronym “TTAPS” after its authors’ lastnames, the academic article would be published on December 23, 1983. But inOctober, Sagan made the decision to announce his warning to the world using whatamounted to a very unorthodox medium: the popular media.

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(Parade Magazine)

Sagan, like many at the time, believed nuclear war was the single greatest threatfacing humanity. Others—including policymakers in the Reagan administration—believed a nuclear war was winnable, or at least survivable. Making the danger ofnuclear winter real to them, Sagan believed, would take more than science. Hewould have to draw on both his public fame, media savvy and scientific authority tobring 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, KeayDavidson, at a meeting in the early 1980s to plan the Galileo space probe, Sagan toldhis colleagues: “I have to tell you I’m not likely to do much of anything on Galileo forthe next year or so, because I am concentrating most of my energies on saving theworld from nuclear holocaust.”

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

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

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

That meant people’s personal feelings about Sagan colored their assessment ofTTAPS. Unfortunately, it wasn’t hard to attack such an outspoken messenger. Ashistorian of science Lawrence Badash writes in A Nuclear Winter’s Tale: “Thecolumnist William F. Buckley Jr. said Sagan was ‘so arrogant he might have beenconfused with, well, me.’ He was faulted for strutting around on the TV screen,conveying an uncomfortable image for most scientists, one to which they haddifficulty relating.”

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

These figures are often seen alternatively as either noble, fearless explorers boundto discover the truth, no matter how challenging—or stooges of the establishment,easily bought off with government and industrial money, compromising theirresearch. 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. Thoughhis research credentials were impeccable, Carl Sagan was in many ways a Cold Warwarrior’s stereotype of a hippie scientist. He wore his hair long by conservativeacademic standards, dressed modishly and casually, and was an outspoken critic ofnuclear proliferation. (He also smoked marijuana, which likely would have made hismore 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 farewelladdress, using phrases familiar from Cosmos and his other writings. “Nuclearweapons are an expression of one side of our human character,” Sagan wrote. “Butthere’s another side. The same rocket technology that delivers nuclear warheads hasalso taken us peacefully into space. From that perspective, we see our Earth as it

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really is—a small and fragile and beautiful blue globe, the only home we have. Wesee no barriers of race or religion or country. We see the essential unity of ourspecies and our planet. And with faith and common sense, that bright vision willultimately prevail.”

On the other side of the spectrum were scientists like physicist Edward Teller, whoseanti-Communist zeal was particularly notable. He pushed for the U.S. to increaseweapons research, and believed the U.S.S.R. was a more powerful adversary thanAmerican intelligence agencies were reporting. Teller often took existing threatanalyses and extrapolated them into worst-case scenarios in the interests ofspurring the government toward more aggressive action. He strongly opposednuclear test bans and believed the Soviets were close to beginning a full-scalenuclear war.

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

Nuclear winter pitted Sagan against Teller, culminating in both men giving testimonybefore 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 promotedwere doomed from the start. It didn’t hurt that their tactics were similar: in publicstatements, Sagan focused on the most extreme predictions for nuclear winter, justas 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—mostnotably, in the realm of climate change.

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

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Similar tactics were used against environmental scientist Rachel Carson, Oreskesand Conway point out. Long after her death, anti-environmentalists and pro-DDTactivists continue to focus on Carson the person rather than the research done bymany scientists across disciplines, as though she alone ended the indiscriminate useof 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 personalitywhose politics were nearly opposite Sagan’s. The Marshall Institute’s tactics largelyinvolved pressuring media outlets into “balancing” pieces critical of SDI withpro-“Star Wars” opinions. The Marshall Institute—and its successor the CO2Coalition—later applied those same tactics to the issue of climate change. A formerdirector of the institute, physicist William Happer, is a prominent climate-changedenier who has consulted with President Trump.

Climate scientists have been hurt by these tactics, to the point where theyoften emphasize the best-case scenarios of climate change, as climate scientistMichael 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’thave to worry as much. Like Sagan, many researchers want to issue a direct callto action, even at the risk of being labeled a scientific Cassandra.

Comparing 1983 with 2017, the best word Grinspoon can think of is “denial”: “Peopledidn’t want to change the way they were thinking of [nuclear] weapons,” he says. “Isee an echo of that now. What nuclear winter shows is that they’re not reallyweapons in the sense that other things are weapons: that you can use them to harmyour adversary without harming yourself. People are not really considering that ifthere really were to be a nuclear conflagration, in addition to how unthinkablyhorrible it would be in the direct theater of the use of those weapons—say in theKorean 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 aroundone-fourth of what they were in the ’80s, according to The New York Times. And thethreat of global thermonuclear war has mostly faded: Few believe that North Korea’s

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potential arsenal is capable of wiping out American cities and nuclear silos the waythe former Soviet Union could.

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

The debate over climate change isn’t going away anytime soon, either. And the waySagan and his scientific colleagues handled publicizing and debating the nuclearwinter question seems very similar to those tracking climate change. In bothinstances, the potential impact of the science is huge, with implications beyond thescope of the research, and valid concerns about either understating or overstatingthe risks.

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

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

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