- Amid mounting evidence today that global temperatures are on the rise, some scientists have suggested cooling things down artificially.

They propose trying to manipulate the earth's climate through a process known as geo-engineering.

- It's a radical idea that comes from a very unlikely place.

Scientific research developed during the 1980s to study the environmental impact of a nuclear war.

The fear was that nuclear war, most likely between the United States and the Soviet Union, could plunge the globe into a civilization ending ice age.

Scientists called it nuclear winter.

- Administration officials from the president on down have been using hard line rhetoric against the Soviets all year long.

- The administration's message to Moscow has been that it's not going to be business as usual anymore.

- Today in virtually every measure of military power, the Soviet Union enjoys a decided advantage.

The Soviet military buildup must not be ignored.

- 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.

- 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.

- [Narrator] In the 1980s, tens of thousands of nuclear warheads already faced off.

But Cold War calculations pushed the superpowers to build even more.

- 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.

- There are 40,000 nuclear warheads in the inventories of the US and the Soviet Union today.

We must insure it not be used.

- It started quietly, but it is picking up steam, and maybe gathering strength.

The movement, if that's 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.

- [Narrator] That movement was called nuclear freeze.

And as its message spread across the nation, it brought together a wide swath of Americans.

- In order to stop this arms race, you first gotta freeze it.

- [Narrator] One of those was the astronomer Carl Sagan.

- Imagine a room awash in gasoline.

And there are two implacable enemies in that room.

One of them has 9,000 matches.

The other has 7,000 matches.

Each of them is concerned about who's ahead.

Who's stronger.

Well, that's the kind of situation we are actually in.

- Sagan was a very effective communicator.

I mean, he was a voice for the scientific community in some sense.

(somber music) - [Narrator] In 1983, Sagan used that popularity to draw attention to a troubling new scientific finding about nuclear war.

- [Journalist] Dr. Carl Sagan and more than 100 other scientists have concluded that the longterm effects of nuclear war would be much worse than anyone has predicted so far.

- We studied a range of consequences of various nuclear war scenarios.

If I may have the first slide.

High yield nuclear weapons explosions.

- [Narrator] Climate scientist Alan Robock was in the conference audience.

- 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 out the earth's surface, having impacts on agricultural production.

- [Narrator] As Russian counterparts weighed in via satellite link, this view of nuclear war's destructive power took hold.

- For the first time we see that the consequences of nuclear war might be absolutely devastating for nations far removed from the conflict.

- The initial splash on this story was profound.

It was kind of selfish or even existential destruction.

Nuclear winter, even the verbiage is portentous.

- [Narrator] To illustrate the point, Sagan helped produce a short film showing just how devastating nuclear winter might become.

- [Announcer] 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.

- It is known as nuclear winter.

- This is not some peacenik nightmare.

It is a theory supported by at least 40 American scientists of high repute.

- [Reporter] Today, a panel appointed by the prestigious National Academy of Sciences agreed with Sagan.

- The Pentagon has accepted as valid the theory of a nuclear winter.

- The implications of nuclear winter are that we shouldn't build more, but we should build less.

- It was a combination of everybody's work that kept making a stronger and stronger case that this theory was true.

- [Narrator] The fear of nuclear winter soon became another of the many issues impacting Cold War strategy.

- A great many reputable scientists are telling us that such a war could just start to 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, a nuclear winter.

- Gorbachev certainly has testified to the fact that this increased his concern about the consequences of nuclear war and the arms race.

- 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 US and Soviet nuclear weapons.

We have made history.

- [Narrator] But even before that treaty was signed, some of the gravest predictions made by nuclear winter theorists had begun to thaw.

- Those of us who were doing more global models, we didn't get anything like the result that Sagan was getting, we got a climatic effect if you put that much smoke on there.

But we didn't get the kind of effect they were talking about.

- [Announcer] 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.

- Nothing we've seen in our simulations or in the work going on now leads me to believe that the extinction of the human race is a real possibility.

- [Narrator] Over time, better modeling caused many of the original nuclear winter theorists to agree that nuclear winter's effects were likely more moderate than they had initially supposed.

- There's a pattern to how some phenomena and stories play out.

The first idea is very stark.

And then the scientific process is like piranhas that nibble away at the soft stuff, and whatever's left is the hard skeleton of the ideas, the enduring part.

- Even when you get back to the nuclear autumn thing, you're 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 didn't have to be quite hyped so much, but that there were climatic effects was important.

- [Narrator] But as the predicted effects of nuclear winter became more subtle, the headlines faded away.

- I think a lot of the public went away with just the message that this was an exaggerated concern, they didn't have to worry about it.

- [Narrator] Over the years, another global issue began to focus the public's concern.

- 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.

- [Narrator] Nobody's talking about exploding a nuclear bomb.

But the idea of harnessing a nuclear winter-like effect to reduce the global temperature has intrigued policymakers and scientists for some time.

And it is gaining traction.

- The scientific evidence that these technologies could reduce risk is very strong.

- [Narrator] 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.

- The main benefit is it would cool the climate, and so it would reduce all the impacts of global warming.

There'd 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 global warming would be reduced, if you could do it.

- [Narrator] The concept is not without controversy.

- This idea of responding to global warming essentially with nuclear winter-lite, I guess you could call it, the science leads to some very worrisome questions.

- The problem is, it's unclear what else solar geoengineering would do.

- I should mention that I have written a paper with 26 reasons why geoengineering would be a bad idea.

Anything built by humans and operated by humans can fail.

So would you trust our only planet to this?

- [Narrator] 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.

- There are a lot of weapons out there.

- [Reporter] Rarely has North Korea tested missiles at this pace.

- I want this nuclear arsenal to be the biggest and the finest in the world.

- It's important for countries and leaders to be keeping in mind what nuclear war would do.

Even small exchanges could be absolutely devastating.

You're gonna have first the destruction effects.

You'll have fallout, you may have some climatic effects that spread.

- Heaven help us if there were a small nuclear exchange.

You could end up with it being on the worse end.

It's like with global warming, you know, the worst case scenarios can happen.

I guess the good news about nuclear winter is it remained a theory.

- The global consequences of nuclear war is not a subject amenable to experimental verification.

Or at least not more than once.

Maybe we've all made some serious mistake in the calculations, but I wouldn't want to bet my life on it.

(bright music)