Narrator: There is a theory that scientists once dismissed as absurd:

that long ago the earth became so cold, every inch of it was entombed in ice.

The whole planet became one vast snowball,

and it lasted for 10 million years.

[Theme Music]

Earth has no more terrible force than an ice age.

Only 5 times in its 4 billion year existance

has our planet suffered this kind of catastrophe.

No one is certain what started them, or why they happened,

but there is one thing about ice ages on which most scientists agree:

the conventional view is that no matter how ferocious the advance of the cold in any ice age,

there will always be parts of the world that are safe and warm.

The freeze spreads out slowly from the poles,

but never reaches the hottest part of the planet, the tropics.

It has long been assumed that this is a basic law of nature.

The tropics can never freeze.

But then, this basic law of nature can't explain what happened 600 million years ago in Namibia.

[Jazz Music]

Paul Hoffman is one of those defiant geologists who disputes the conventional view.

He has spent years trying to convince his colleagues about the snowball earth.

As a scientist, I was completely convinced it was right.

But I knew I was going to have a struggle on my hands.

Because any idea as radical as this is going to be difficult to get across,

and there are going to be skeptics for sure.

Narrator: The place Hoffman and his colleague, Dan Schrag,

come back to study again and again is Namibia.

It is not an obvious place in which to seek evidence of a catastrophic ice age,

it is a land of relentless heat.

One of those locations that has been in the tropics for hundreds of millions of years.

The conventional laws of nature say an ice age could never have penetrated here.

But lurking among these sun-baked rocks is something that the conventional laws of nature

cannot explain away.

600 million years ago, these hills were at the bottom of the ocean,

and lying on the old sea bed are mysterious boulders.

[Eerie Music]

Geologists call them drop stones,

and drop stones could only have been brought here by one thing: a glacier.

To recreate drop stones in the lab, first you need ice.

This is going to take a while.

Narrator: The ice needs to float on shallow water above a sandy sea floor.

This is to simulate a glacier as it flows off the land, onto the surface of the sea.

Because it is carved through the landscape, a glacier always collects a huge amount of rock debris.

The ice would then get filled up with all sorts of boulders and cobbles,

and in this case, we're just using some gravel,

which we can pour on top of this layer of ice in our tank here.

Narrator: Having recreated a boulder-filled glacier, you need to melt the ice.

[Crashing sound]

As the ice melts, the boulders are released.

While most of the ice has melted at this point,

you can see that all the rocks that the glaciers carried out to the ocean

are now dropped to the sea floor.

In addition to all of this--in the real world--boulders,

there would also be a lot of sand, and a lot of much finer gained sediment.

And so these rocks would get filled in with very fine-grained sediment as well.

Narrator: In a few thousand years, the fine-grained sediment will cement itself together to form rock.

With the boulders trapped inside.

And that's how drop stones are made.

All you need is a glacier.

The message from the drop stones suggests just one thing:

Namibia, in the tropics, a place that everyone says has been hot for hundreds of millions of years,

must once have been covered in ice.

The most compelling evidence here is the juxtaposition of a large boulder like this

enclosed within very fine and delicately layered sediment.

And so it's difficult, if not impossible to imagine any way to account for this

other than a very extensive glaciation.

But for 50 years, since the idea that an ice age must have smothered the tropics was first proposed,

it has been deemed so radical that most scientists have dismissed it as absurd.

The theory has its roots in the 1940s, when a young geologist called Brian Harland,

made a series of intrepid field trips deep into the arctic.

Conditions were very different then.

Man hauling in the old Scotch tradition.

It was cold, but, um, you get used to it.

You can get used to anything, really.

Narrator: In the arctic, Harland had ample opportunity to learn all there was to learn

about that tell-tale sign of glaciation, drop stones.

Glacier deposits were a very dramatic occurrence, very distinctive, easy to identify.

And when I saw them, I knew from my own experience that this is glacial.

I didn't have to think very hard about it.

Narrator: But it was when he returned from the arctic, armed with his new-found knowledge,

that Harland made his scientific breakthrough.

With his now expert eye, he analyzed rocks from all over the world.

What he discovered amazed him.

Almost everywhere he looked, in rocks formed about 600 million years ago, he found drop stones.

Classic evidence that there had once been thick ice, even on the hottest continents.

The drop stones were so wide-spread that Harland began to contemplate the unthinkable.

The fact is that we've got evidence of a global glaciation.

Narrator: He knew it made no sense.

He knew that at the peak of the last ice age, which ended just 10,000 years ago,

ice spread down to where New York is today, but no further.

But the evidence was staring Harland in the face.

Evidence which seemed to flout the laws of nature.

600 million years ago, the drop stones said, ice had spread over every continent.

Right across the tropics, to the equator itself.

It meant there must have been an ice age of unimaginable ferocity.

In fact, too unimaginable.

When Harland presented his theory of global glaciation, colleagues dismissed it as laughable.

The skeptics had a whole host of reasons why Harland had to be wrong.

Reasons that were utterly convincing,

as they were based on the most basic rules about how the world works.

For a start, it's a given that the tropics, that wide belt around the earth,

has always received the full glare of the sun.

As they have always basked in sunlight, so they must always have remained warm.

Even during the last ice age, geological evidence suggests the tropics were pleasantly hot.

And they had another reason why the snowball theory must be wrong.

There was a far simpler explanation for drop stones in the tropics,

one that was absolutely consistent with scientific knowledge,

and which didn't involve a ludicrous idea like the tropics freezing over.

Continents drift.

They scrape across the surface of the earth at about the same pace that your fingernails grow.

This means that in about 400 million years, continents can shift from one end of the globe to the other.

Obviously, the doubters said, 600 million years ago all the continents had drifted into icy polar regions.

Once there, glaciers would have formed, and drop stones would be common in all rocks of this area.

And after those drop stones had been formed at the poles,

the land and the drop stones would have been carried back down to the tropics by continental drift.

For years, this remained the accepted explanation for why drop stones appeared everywhere.

The only way to make the theory of a global glaciation credible

was for someone to make the impossible possible.

They would need to find a way for the tropics to freeze over.

And this is just what happened next.

[Counting in German]

[Explosion]

It was the 1960s, the height of the cold war.

The world was obsessed with calculating the odds of surviving a nuclear holocaust.

It was known that a series of massive nuclear explosions

would create clouds of dust, smoke and soot.

Sunlight would be blocked out.

Hypothetically, the earth would enter a nuclear winter.

A man-made ice age.

In the Soviet Union, finding out how severe this man-made ice age could be

became the task for one climatologist.

Mikhail Budyko was that man.

Long ago, probably 25 or 30 years,

I compiled a number of studies which could be used to describe origin of ice ages.

Narrator: What Budyko was to uncover would fly in the face of conventional wisdom.

He would show how those predictions, that the tropics couldn't freeze over,

were complacent and unfounded.

Budyko knew that because the land and oceans are dark,

they absorb most of the heat coming from our sun's rays,

and that is how our planet is warmed up.

But sheets of ice are white.

They reflect sunlight like a mirror.

So an ice-covered earth absorbs far less solar heat.

During an ice age, as the freeze spreads, the earth grows whiter.

More heat is reflected away, so less and less heat is absorbed.

And so the earth grows ever colder.

It means that potentially, the Earth could be

caught in an vicious circle of unstoppable freezing.

Budyko converted this hypothetical scenario into a mathematical formula,

and that formula produced a terrifying prediction.

The Earth's climate has a theoretical breaking point.

[Dramatic Music]

Narrator: As long as the ice sheets remain close to the poles,

the Earth is safe but if the freeze continues,

such as might happen in an nuclear winter,

they could advance down to about where Texas is today.

Once the freeze had reached that point,

so much of the Earth would be covered in white ice

but over half the solar heat that normally warms the planet

would be reflected back into space.

At that point, there wouldn't be enough heat left to warm up the Earth.

Once this happens, there could in theory, be a runaway freeze.

A freeze that nothing could stop.

Temperatures plummet, ice sheets spread across all the continents,

the oceans and eventually, even the tropics.

If this was ever to happen, the entire planet would be trapped in ice.

There would be a snowball Earth.

What was most disturbing about Budyko's calculations

was that an Earth encased in ice would reflect so much solar heat,

it could never warm up enough to thaw, ever.

It was one billion and twenty years ago that the system

will be stable for a very long time and possibly,

all life will disappear.

Narrator: This was Budyko's paradox.

He had shown that a runaway freeze could ice over the tropics in theory,

but he had also shown that it could never have happened in practice

because if it had, the world would still be a snowball today

and that would mean we would never have existed.

For 15 years, nobody could solve the paradox of the runaway freeze.

The snowball Earth theory remained a logical impossibility.

To resurrect the theory, believers would have to do two things.

First, they had to prove that ice really had reached

the hottest parts of the planet, the tropics and that the

dropstones hadn't arrived there merely through continental drift.

Second, they had to come up with a theory that offered

an escape from the runaway freeze.

[Music]

That was when a new scientist had entered the picture.

In the late 70s, Joe Kirschvink was one of geology's rising stars

and back then, he thought the idea of a snowball Earth was mad.

The willingness to accept a nutty idea like freezing the whole planet

really needs to be supported by incredibly strong evidence

and overall, if you had asked me to place a wager in the late 70s,

I would've been on the conservative side on that one.

Narrator: Kirschvink's conservatism came about because he believed

the simple conventionalist explanation of why there were dropstones at the equator today.

600 million years ago, he thought, all the continents must've been

up near the icy poles when the dropstones were formed.

There was however only one technique to test this theory,

and it just so happened that Kirschvink had dedicated

his entire professional career to that technique.

It involved magnets and Kirschvink is a man in love with magnets.

I have wiped out my credit cards so many times by playing with strong magnets,

I don't like to remember it.

Narrator: Few people realize it but every rock is a weak magnet.

Each one contain magnetic minerals and the direction

these minerals pull in is fixed forever the moment the rock is first formed and

what gives these minerals their magnetic direction is the magnetic field around the Earth's core.

This means that rocks formed at the poles will always have a

magnetic direction that points up and down.

Rocks formed at the equator will always have a

magnetic direction that points from side to side.

The rocks have the ability to preserve magnetic direction

because of magnetic minerals in them.

They act like little compass needles and will line themselves up.

Narrator: By finding a rock's magnetic direction, Kirschvink is able to

identify where in the world it was originally formed,

no matter where continental drift might've taken it in the millions of years that followed.

In the 1980s, Kirschvink heard of a team of Australians who had run

tests which they claimed showed evidence of glaciers in the tropics.

Ever the skeptic, Kirschvink decided to duplicate their tests on his own, more sophisticated equipment.

It meant examining dropstones from some of the hottest parts of Earth.

The question was were they already at the equator when they were

formed by the ice or had they been created at the poles

and then carried to the tropics by continental drift?

To answer the question, Kirschvink turned to his pride and joy:

a custom built, hyper-sensitive magnetometer.

It's an incredible instrument.

You lower the sample into the actual magnetometer chamber.

The weak magnetic field of the sample generates an electric current

and so it allows us to measure the magnetic moment of an extraordinarily weak magnet.

One billionth say the level of a ordinary hand magnet type thing.

Narrator: Attached to the magnetometer was a computer which plotted

each rock's origin on an electronic outline of Earth as seen from above.

The center of the circle represented the poles, the circumference, the equator.

If the dropstones had been formed at the poles, as Kirschvink and everyone else thought,

they would show up at the center of the circle on the computer.

If they had been formed at the equator, they would show up at the circle's edge.

The machine gave its answer.

The dropstones had been formed in the tropics.

600 million years ago, there must have been a snowball Earth.

Here's a fundamental theorem in science.

If a theory is in conflict with data, modify the theory.

There's so many wonderful, beautiful theories that have been slaughtered by a nasty little fact.

Narrator: But Kirschvink was racked by the insurmountable paradox of the runaway freeze.

That if a snowball Earth had ever happened,

then science said that we should still be entombed in ice today.

How do you get out of it?

Obviously, the climate modelers had assumed that that was

a irreversible step and that you would never get out of it

and yet, we're out of it now and if we'd been in it before,

at some point, we must've gotten out of it.

Narrator: To get out of the deep freeze, what Kirschvink needed was

a power that would stay hot, even when the whole planet had frozen over.

Something that Budyko hadn't thought of, something that could burn forever, something like hell.

Looking at an active volcano, realize that magma tens or hundreds of kilometers

below the surface couldn't care less whether there was a thin layer of ice over the oceans.

It would still emerge.

Narrator: Volcanos survive ice ages because they have a direct channel

to the molten rock deep within the Earth,

rock that reaches temperatures of over a thousand degrees but that would

only melt ice in their immediate area.

Kirschvink has spotted something else about volcanos.

They also produce gas 10 billion times a year.

One gas volcanos emit in huge quantities is carbon dioxide,

a gas that causes the greenhouse effect and global warming.

Today, carbon dioxide is being pumped into the atmosphere by both volcanos and industrial activities,

but what stops the Earth from overheating is that we have a natural way

of removing the excess carbon dioxide from the atmosphere.

Rain is the Earth's natural cleaning agent.

As it falls through the atmosphere, each droplet of rain

absorbs carbon dioxide and cleans the air but Kirschvink realized

that on a snowball Earth, there could've been no rain.

The snowball was so cold, all the water on the planet's surface was frozen solid.

Without liquid water, nothing could've evaporated into the air

so there would've been no clouds and without clouds,

there can be no rain.

There would've been nothing to stop the carbon dioxide from the volcanos

from building up over millions of years.

It would've caused global warming on an inconceivable scale.

Kirschvink came across calculations showing that after 10 million years without rain,

the atmosphere would've been 10% carbon dioxide.

Today, it's far less than 1%.

This extra carbon dioxide would've created a greenhouse effect

that raised the temperature to an average of 50 degrees Centigrade,

hotter than the Earth has ever been.

Hot enough to melt the ice.

Kirschvink: And that seemed to be a natural and plausible escape.

Certainly enough to break the snowball or the ice condition.

Narrator: Joe Kirschvink had cracked it.

He had found the way out of the runaway freeze,

a way that made perfect scientific sense.

A way that was consistent with the laws of nature.

Kirschvink: The realization that we may have found

the way out of the snowball, it was wonderful.

Narrator: By 1990, Kirschvink had evidence that the tropics

had frozen over for 10 million years and he had

come up with a theoretical escape route from the runaway freeze,

but the problem was it was just a theory.

He had no physical evidence to prove the ice had melted

because of an extreme greenhouse effect and without that evidence,

the snowball Earth theory continued to be ignored.

[Music]

In 1992, Paul Hoffman entered the story.

One of geology's most respected but open-minded figures.

He was to turn into one of the snowball Earth

theory's most fervent disciples.

I loved the snowball Earth theory.

I mean it's almost a religious ferocity with which

I want to get across this saga of Earth history with

so many twists and turns and events that are

so far outside our own experience.

Narrator: Hoffman's mission was to find that

hard evidence that had eluded Kirschvink.

The final piece of the puzzle that would

prove the snowball had ended because of the greenhouse effect

and he knew just where to start looking.

Namibia.

Hoffman was drawn to Namibia first by the dropstones

but even more by what sat immediately on top of them.

Towering directly above were huge formations called cap carbonates.

Calcium carbonate crystalized into rock.

These bizarre structures seemed to appear right out of the blue.

Above this line right here, ??, no more stones whatsoever

so the glaciation must've come to an abrupt end

and sitting directly on top is a thick pile of carbonate

and it's puzzled geologists for generations.

[Dramatic Music]

Narrator: What really puzzled geologists was that caps made

from calcium carbonate are usually formed in warm water,

but in Namibia, they had suddenly appeared on top

of an ice cold glacier.

What this indicates is that you go from glacial conditions

to an ocean that is warm in a flash.

Narrator: To most geologists, this instant change from

cold to hot defied their understanding

but Hoffman suspected it might be a huge clue.

That the sudden appearance of mountains of

calcium carbonate must somehow be connected to

the death of the snowball in a colossal greenhouse effect.

But how?

Hoffman needed someone to explain why

calcium carbonate would be formed not just because

of warm conditions in general but specifically in the

conditions created by a melting snowball Earth.

So he turned to a geochemist, Dan Schrag.

Painstakingly, they analyze the snowball's theoretical

final moments, stage by stage,

working out what chemical and climatic processes were at work.

[Music]

First, they realized that as the ice melted,

it would produce liquid water.

The water would then evaporate, and create clouds

and those clouds would cause a change in the weather,

the likes of which has never been seen by human eyes.

Daniel Schrag: When the ice is retreating,

this is probably the most spectacular climate change

in Earth's history.

In some ways, we're going from the coldest climate

the Earth's ever experienced to the warmest climate

the Earth's ever experienced.

We immediately say that this was the mother of all climate changes.

Narrator: The most elemental powers of nature

would be unleashed upon the Earth.

One would predict hurricanes of such intensity that are unimaginable

so it's quite possible you could've had waves of

100 meters in amplitude coming in and crashing

in on the shore.

It would be unbelievably violent.

Narrator: And above all, there would be rain.

The first rainstorms for millions of years.

Rainstorms that would last at least a century

and it wouldn't be just any rain.

The rainwater would react with the vast quantites

of carbon dioxide in the atmosphere and form carbonic acid.

This acid rain would then deluge the Earth.

The acid rain would pound the exposed rocks,

producing a violent chemical reaction.

It would break the rocks down into their constituent parts,

one of which was calcium.

This would then fuse with the carbon in the acid rain.

The result, mountains of calcium carbonate,

exactly as they could see right above the dropstones in Namibia.

Suddenly, it became clear that the natural expectation

of a prolonged global glaciation ending in extremely

high levels of carbon dioxide was that you would expect

these very unusual thick carbonate rocks should

immediately follow the glacial deposits.

[Music]

Narrator: Hoffman and Schrag had found the final evidence.

Schrag: The beauty of the snowball Earth theory is exactly that

it links the chemistry, the geology, the planetary science

and bam, bam, bam, all of the facts

are consistent with the predictions of the theory.

[Jazz Music]

Narrator: In 1998, Hoffman and Schrag made an

triumphal tour of the world's universities,

promoting the snowball theory,

and then someone threw a spanner in the works.

An entire scientific discipline, the biologists took one look

at the 10 million year long snowball Earth and said

it was quite simply impossible.

One scientist who had problems with the snowball was Guy Narbonne.

He like many other biologists could see there was a

fundamental problem reconciling the snowball Earth theory

with what he knew.

The geological evidence speaks very strongly

of ice sheets marching on the equator.

The biological evidence speaks very strongly of open water

being available at this whole time.

Narrator: Narbonne's problem was another set of nature's law.

Nothing less than the laws that governed life itself.

600 million years ago, when the snowball was set to have struck,

there was nothing living on the land.

It all existed in the sea and it was not life as we know it.

The most common organism on the planet was this.

Cyanobacteria.

This mass of cyanobacteria is so ancient,

it predates even the snowball.

The oldest fossils that we have in the world

are stromatolites and these are colonies like you see here.

They're made out of literally billions of cyanobacteria.

Narrator: What made cyanobacteria and more sophistocated

ancient organisms like green algae essential to the snowball

debate was that they photosynthesize.

Just like plants today under the water, these lifeforms

absorb sunlight and convert it into energy.

So these species' chances of surviving the snowball

would've depended on whether there was enough

sunlight for photosynthesis in the snowball seas

at a time when every inch of the planet was

allegedly covered in thick ice.

Light penetration through ice is not great if the ice

is more than a couple tens of meters thick.

Narrator: And that was the problem biologists pointed

to in Hoffman and Schrag's work.

Their calculations had reckoned on the snowball

producing ice sheets up to a kilometer thick,

and even in the warmest places around the equator,

they would've been tens of meters thick.

More than enough, biologists believe, to block out

any sunlight from the oceans and that meant death

to most things that depended on photosynthesis for life.

Narbonne: You cut off photosynthesis and within a few years,

let alone a few million years, mass extinction.

Narrator: It meant that many of the photosynthesizing

species should've become extinct 600 million years ago

and the trouble was they hadn't.

Cyanobacteria grow in rock-like colonies all over the tropics today

as do other photosynthetic plants, dating from before

the snowball like green algae.

For many biologists, there seemed to be only one possible conclusion:

there may have been a severe ice age but not a fully

fledged snowball that covered all the oceans

because lifeforms that should've died out are still with us.

Unless someone could find a way for marine life

to survive under a thick sheet of ice,

the whole theory would be back in the bin.

[Music]

Then along came the man from NASA, Chris McKay.

Chris McKay: I'm planetary scientist with an interest

in life in extreme environments.

In particular, environments that are cold and dry.

Several years ago, I saw the papers about the snowball Earth,

indicating the geological evidence that the Earth was ice covered

and had been ice covered for millions of years.

Plus the evidence that there are organisms (bacteria and green algae)

that survived this ice covered state and that's the puzzle.

How could these photosynthetic organisms survive?

Narrator: McKay was intrigued because it just so happened

he had visited a place on Earth today that was so cold,

it could actually be compared with the snowball of 600 million years ago.

Antarctica, the coldest place on Earth.

Here, temperatures can fall to -30.

Almost as severe as the snowball at its warmest point, the equator

and deep in Antarctica are the dry valleys where there are lakes

smothered by an ice sheet many meters thick.

McKay: We think these little lakes represent a good

model for looking at the snowball Earth.

Could there be life underneath the ice cover on a snowball Earth?

Narrator: Scientists were eager to discover if there was

enough light for life to survive in the water below the thick ice.

They were about to be astonished.

[Music]

There was light, lots of it

even though over head,

the ice was five meters thick.

There was so much light,

the water was filled with plant life and bacteria,

including two organisms dating from the time

of the snowball that would've needed sunlight to live.

McKay: When we're diving in the lakes

and swimming around, there's life in the water.

There's bacteria, cyanobacteria

but there's also green algae.

Narrator: And McKay had a hunch as to why this

photosynthetic life could survive.

It was something about the nature

of the ice itself.

McKay: The transparency of this ice is very high

and it's due to the fact that it's freezing very slowly

underneath this thick ice cover.

Now let's imagine this snowball Earth where

there's also a thick ice cover,

the ice on the bottom is freezing very slowly as well,

it should have the same optimal properties

as this clear, clean ice.

Narrator: By freezing slowly, water rejects impurities

such as salt or dirt which make it cloudy.

So the ice becomes clean and transparent.

Because of this transparency,

life giving sunlight would've been reaching down

into the snowball seas.

McKay realized that even at its height,

there would've been havens around the snowball's

equator where the ice was thin enough

for photosynthetic life to cling on below but there was more.

Although disciples of the theory accept

the snowball would've been death to most things,

they argue whatever life did survive

would've emerged into a new world almost

devoid of competition.

The perfect conditions for an explosion of evolutionary change.

[Music]

These fossils show the new species which appeared

soon after the time of the snowball.

All display a massive increase in size and complexity

from the simple organisms that lived before.

These are the first multi-cellular lifeforms.

Even those with some questions about the snowball theory

recognize a new climate after a mass extinction

could be a spur for evolution.

[Music]

But if the snowball can change the course of life on the planet,

there remains one final question.

No one knows why the snowball happened

or if it could happen again.

The sun is hotter than it was 600 million years ago

and is getting hotter all the time.

Climate trends would have to change dramatically.

If it ever did happen again,

it almost certainly wouldn't be for millions of years

but if it ever did, that would mean death to almost everything

including us.

[Music]