ARCHIVAL (ABC, 10-20-81):SAM DONALDSON: Administration officials from the President on down have been using hardline rhetoric against the Soviets all year long.

ARCHIVAL (NBC, 2-5-81):ANCHOR: The administrations message to Moscow has been that its not going to be business as usual any more.

ARCHIVAL (ADDRESS TO THE NATION ON STRATEGIC ARMS REDUCTION AND NUCLEAR DETERRENCE, 11-22-82):PRESIDENT RONALD REAGAN: Today, in virtually every measure of military power, the Soviet Union enjoys a decided advantage. The Soviet military build up must not be ignored.

ARCHIVAL (CBS, 11-25-83):RICHARD OVINNIKOV (DEPUTY SOVIET AMBASSADOR TO THE UNITED NATIONS): If the United States are going to continue their course, then I am afraid that the world is doomed to be on the brink of nuclear war.

DR. FRANK VON HIPPEL (FORMER CHAIRMAN, FEDERATION OF AMERICAN SCIENTISTS): It was a real fear in the early 80s that we were in a more dangerous period than we had been, perhaps, since the missile crisis in 1962.

NARRATION: In the 1980s tens of thousands of nuclear warheads already faced off, but Cold War calculations pushed the superpowers to build even more.

MICHAEL MACCRACKEN (FORMER DEPARTMENT OF ENERGY PHYSICIST): It was a balance with tremendous destructive power on both sides. So, we were in this very, very tenuous situation right at the edge of a cliff.

ARCHIVAL (ABC VIEWPOINT, THE DAY AFTER, 11-20-83):ROBERT MCNAMARA: There are 40,000 nuclear warheads in the inventories of the U.S. and the Soviet Union today. We must ensure it not be used.

ARCHIVAL (ABC, VIEWPOINT, THE DAY AFTER, 11-20-83):FRANK REYNOLDS: It started quietly, but it is picking up steam and maybe gathering strength, the movement if thats the right term to somehow bring pressure on leaders of both the United States and the Soviet Union to stop just stop the nuclear arms race.

NARRATION: That movement was called Nuclear Freeze, and as its message spread across the nation, it brought together a wide swath of Americans.

ARCHIVAL (ABC, NUCLEAR FREEZE PROPOSAL, 1982):PAUL NEWMAN: In order to stop this arms race youve first got to freeze it.

NARRATION: One of those was the astronomer Carl Sagan.

ARCHIVAL (ABC, VIEWPOINT, THE DAY AFTER, 11-20-83):CARL SAGAN: Imagine a room awash in gasoline. And there are two implacable enemies in that room. One of them has 9000 matches. The other has 7000 matches. Each of them is concerned about whos ahead. Whos stronger. Well, thats the kind of situation we are actually in.

MICHAEL MACCRACKEN: Sagan was a very effective communicator. I mean he was a voice for the scientific community in some sense.

NARRATION: In 1983, Sagan used that popularity to draw attention to a troubling new scientific finding about nuclear war.

ARCHIVAL (ABC, 10-31-83):CAROLE SIMPSON: Dr. Carl Sagan and more than 100 other scientists have concluded that the long-term effects of nuclear war would be much worse than anyone has predicted so far.

ARCHIVAL (WORLD AFTER NUCLEAR WAR CONFERENCE, SEPTEMBER, 1983):CARL SAGAN: We studied a range of consequences of various nuclear war scenariosIf I may have the first slide. High yield nuclear weapons explosions

NARRATION: Climate scientist Alan Robock was in the conference audience.

ALAN ROBOCK (PROFESSOR OF CLIMATE SCIENCE, RUTGERS UNIVERSITY): It was a very new idea. That smoke from fires started by nuclear weapons would go up in the atmosphere, block out the sun and make it cold and dark and dry at the Earths surface, having impacts on agricultural production.

NARRATION: As Russian counterparts weighed in via satellite link, this view of nuclear wars destructive power took hold.

ARCHIVAL (WORLD AFTER NUCLEAR WAR CONFERENCE, SEPTEMBER, 1983):CARL SAGAN: For the first time we see that the consequences of a nuclear war might be absolutely devastating for nations far removed from the conflict.

ANDREW C. REVKIN (THE EARTH INSTITUTE, COLUMBIA UNIVERSITY): The initial splash on this story was profound. It was kind of self-assured, even existential destruction. Nuclear winter even the verbiage is portentous.

NARRATION: To illustrate the point, Sagan helped produce a short film showing just how devastating nuclear winter might become.

ARCHIVAL (THE WORLD AFTER NUCLEAR WAR, TBS PRODUCTIONS)):CARL SAGAN: Beneath the clouds, virtually all domesticated and wild sources of food would be destroyed. Most of the human survivors would starve to death. The extinction of the human species would be a real possibility.

ARCHIVAL (ABC, 12-11-84):PETER JENNINGS: It is known as nuclear winter.

ARCHIVAL (NBC, 8-7-84):JOHN CHANCELLOR: This is not some peacenik nightmare. It is a theory supported by at least 40 American scientists of high repute.

ARCHIVAL (NBC, 1211-84):NEWS REPORT: Today, a panel appointed by the prestigious National Academy of Sciences agreed with Sagan.

ARCHIVAL (NBC, 3285):CONNIE CHUNG: The Pentagon has accepted as valid the theory of a nuclear winter.

ARCHIVAL (NBC, 3-2-85):REPRESENTATIVE TIMOTHY WIRTH: The implications of nuclear winter are that we shouldnt build more, but we should build less.

ALAN ROBOCK: It was a combination of everybodys work that kept making a stronger and stronger case that this theory was true.

NARRATION: The fear of nuclear winter soon became another of the many issues impacting Cold War strategy.

ARCHIVAL (REAGAN PRESIDENTIAL LIBRARY, 2-11-85):PRESIDENT RONALD REAGAN: A great many reputable scientists are telling us that such a war would just end up in no victory for anyone because we would wipe out the Earth as we know it. What are we talking about with a whole nuclear exchange the nuclear winter?

DR. FRANK VON HIPPEL: Gorbachev certainly has testified to the fact that this increased his concern about the consequences of nuclear war and the arms race.

ARCHIVAL (AP ARCHIVE, REAGAN/GORBACHEV SIGNING TREATY, 12-8-87):PRESIDENT RONALD REAGAN: Today I, for the United States, and the General Secretary, for the Soviet Union, have signed the first agreement ever to eliminate an entire class of U.S. and Soviet nuclear weapons. We have made history.

NARRATION: But even before that treaty was signed, some of the gravest predictions made by nuclear winter theorists had begun to thaw.

MICHAEL MACCRACKEN: Those of us who were doing more global models we didnt get anything like the result that Sagan was getting. We got a climatic effect if you put that much smoke on there, but we didnt get the kind of effect they were talking about.

ARCHIVAL (CBS, 12-12-86):JERRY BOWEN: Nuclear winter, argues one group of scientists, is what will surely follow nuclear war. Other scientists with their own computer calculations of the doomsday scenario, say life after nuclear war will not be so much nuclear winter as nuclear fall, severe, but survivable.STARLEY THOMPSON (ATMOSPHERIC SCIENTIST): Nothing weve seen in our simulations or in the work going on new leads me to believe that the extinction of the human race is a real possibility.

NARRATION: Over time better modeling caused many of the original nuclear winter theorists to agree that nuclear winters effects were likely more moderate than they had initially supposed.

ANDREW C. REVKIN: Theres a pattern to how some phenomena and stories play out. The first idea is very stark. And then the scientific process is like pirhaas that nibble away at the soft stuff, and whatevers left is the hard skeleton of the ideas, the enduring part.

MICHAEL MACCRACKEN: Even when you get back to the nuclear autumn thing youre still having huge environmental effects that would have agricultural effects. So it was more a matter of nuance and intensity rather than a matter of is it real or not. It didnt have to be quite hyped so much, but that there were climatic effects was important.

NARRATION: But as the predicted effects of nuclear winter became more subtle, the headlines faded away.

DR. FRANK VON HIPPEL: I think a lot of the public went away with just the message that this was an exaggerated concern and they didnt have to worry about it.

NARRATION: Over the years, another global issue began to focus the publics concern.

ALAN ROBOCK: When I show my results of the climate response to smoke from nuclear war inevitably, I get a question. So, is that a solution to global warming?

NARRATION: Nobodys talking about exploding a nuclear bomb. But the idea of harnessing a nuclear-winter-like effect to reduce global temperature has intrigued policymakers and scientists for some time and it is gaining traction.

ARCHIVAL (FAREED ZAKARIA, CNN, 11-20-17):DAVID KEITH: The scientific evidence that these technologies could reduce risk is very strong.

NARRATION: One technique under consideration involves dispersing a cloud of sulfate particles into the stratosphere to partially obscure the sun and reflect sunlight away from the Earth.

ALAN ROBOCK: The main benefit is it would cool the climate, and so it would reduce all the impact of global warming. Thered be fewer severe storms. There would be less sea level rise. There would be less temperature change, which might affect agriculture. So, all the negative aspects of of global warming would be reduced, if you could do it.

NARRATION: The concept is not without controversy.

ANDREW C. REVKIN: This idea of responding to global warming essentially with nuclear winter light, I guess you could call it.. the science leads to some very worrisome questions.

ARCHIVAL (FAREED ZAKARIA, CNN, 11-20-17):FAREED ZAKARIA: The problem is its unclear what else solar geoengineering would do.

ALAN ROBOCK: I should mention that I have written a paper, with 26 Reasons Why Geoengineering Will be a Bad Idea. Anything built by humans and operated by humans can fail. So, would you trust our only planet to this?

NARRATION: Meanwhile, the Cold War danger that pushed scientists like Robock to study these climate effects remains. In fact, the nuclear landscape is more complex than ever before.

MICHAEL MACCRACKEN: Therere a lotta weapons out there.

ARCHIVAL (CBS, EVENING NEWS, 8-16-19):NEWS REPORT: Rarely has North Korea tested missiles at this pace.

ARCHIVAL (FOX NEWS, 4-2-15):PRESIDENT DONALD TRUMP: I want this, our nuclear arsenal to be the biggest and the finest in the world.

MICHAEL MACCRACKEN: Its important to for countries and leaders to be keeping in mind what nuclear war would do. Even small exchanges could be absolutely devastating. Youre gonna have first the destruction effects. Youll have fallout. You may have some climatic effects that spread.

ANDREW C. REVKIN: Heaven help us if there were a small nuclear exchange. You could end up with it being on the worse end. Its like with global warming. You know, the worst-case scenarios can happen. I guess the good news about nuclear winter is that it remained a theory.

ARCHIVAL (THE WORLD AFTER NUCLEAR WAR, TBS PRODUCTIONS):CARL SAGAN: The global consequences of nuclear war is not a subject amenable to experimental verification or, at least not more than once. Maybe weve all made some serious mistake in the calculations, but I wouldnt want to bet my life on it.