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This is a fantastic way to view the Swiss Alps.

But I haven't just come here to admire the view.

I've come to Switzerland
because the whole country

is littered with features which were
a great puzzle to geologists for many years.

These features are the first hints
of a dramatic series of changes

which have repeatedly transformed
the planet throughout its history.

Stranded in the middle of the Swiss countryside
is this enormous boulder.

It's been made a national monument.

A local geologist, Christian Schluchter,

explained to me why it's so important.

What we see here,
it's really a strange piece of rock.

It certainly stands out from the landscape.

SCHLUCHTER: Yeah, this big boulder

is a piece of rock which actually
doesn't belong here.

If you look at its composition,

it's completely different from the bedrock here.

It's what we call an erratic boulder.

It has travelled from far away to the place
where it is today.

So where does it come from?

Well, it comes from the mountains
of the Canton of Valais

and has been travelling for 200 to 250 kilometres.

What brought it here, then?

Well, originally people thought
that this was the big flood.

The big flood waters would carry
these rocks down from the mountains,

into the midlands where they are now.

Well, you'd certainly need a flood
of biblical proportions.

Of course. That was about the idea.

MANNING: Erratic boulders like this were found
all over northern Europe and America.

To 19th century geologists,
this seemed like good evidence

that the Earth had once been covered
by flood waters, just as the Bible said.

But Swiss scientists could see that water
was not the only thing that could move rocks.

What caused this colossal jumble
and barrier of rocks here?

We call it terminal moraine
and the cause is very simple.

All these boulders around here were carried down

by the actively advancing glacier.

And some of them,
I mean, this must weigh 500 tons,

I mean, the colossal force of the ice
pushing all this stuff down.

Yes, of course.

Picking it up, high in the mountains,

carrying it down here

and depositing at its terminal moraine.

MANNING: This jumbled wall of boulders

marks the position the glacier reached
in the last century.

Glaciers all over the Alps stretched further down
the mountains than they do today.

It was around this time that a Swiss naturalist,
Louis Agassiz,

began to wonder how far these great rivers
of ice had once reached.

The answer, as is so often the case,
was in the rocks.

The rock here is looking like glass,

I mean, there's an amazing reflection off it.

This is one of the places Agassiz was visiting

in the early part of the last century.

This is amazing. Satiny, satiny rock.

And how does that...
How does it get such a fine surface as that?

The actual polishing takes place

by the rock flour incorporated
at the base of the glacier, the moving ice.

So this is really finely ground,
like jeweller's rouge almost.

- Of course. That's what it is, yes.
- Very fine rock.

MANNING: Today this polished rock
is 12 kilometres from the nearest glacier.

The smooth surface stretching far up
the side of the mountain

shows that the glacier was once
one kilometre thick.

But this massive ice carried more
than just powdered rock.

Look here.
This is really something I want to show you.

These scratches, they are proof

of the moving boulders at the base of the glacier.

MANNING: Following the scratches
across the landscape,

Agassiz realised that it wasn't biblical floods
that had left erratic boulders stranded.

It was ice that once had filled this entire valley.

Agassiz went all over the Alps

looking for evidence of rock
that had been shattered, pulverised,

polished by the ice.

He found it everywhere that he looked.

Geologists have now been over
the whole of Switzerland,

plotting out terminal moraines, erratic blocks,

to get the measure of the extent of the ice.

And here's what they found.
This is the map they've drawn.

This is Switzerland as it would've looked
18,000 years ago.

Here's that erratic block,

carried there by the ice sheet
and standing there today.

Here's the site of the present Geneva

which then would've been under a kilometre of ice.

Here's Mont Blanc and the Matterhorn,

which would just have appeared
as pinnacles of rocks

sticking up above this sea of ice.

It soon became clear that the ice
had spread beyond Switzerland.

The erratic boulders found in other countries

marked the outlines of vast ice sheets

that had also covered much of Europe
and northern America.

The ice was up to four kilometres thick in places,

transforming the landscape into bleak,
featureless plateaus.

The discovery that the Earth's climate
had once been completely different

had a profound impact.

Geologists started on a journey through time

to trace the tangled history
of climate change on our planet.

A crucial step in understanding
the history of the massive ice sheets

came from looking at a place

thousands of kilometres
from the furthest reaches of ice cover:

The tropical island of Barbados.

Maureen Raymo is a geochemist

visiting Barbados to track
the precise course of the Ice Age.

Strange as it may seem this far south,

the ice has left its mark on the rocks
that cover the island.

Here on Barbados,
the island is made almost entirely

of ancient coral reefs.

And that's, I mean, most of the other islands,
St Lucia and St Kitts,

they're all volcanic in that chain.

That's right.
This is very unusual for this area of the Caribbean.

And all... everything we're seeing here

is an ancient fossilised coral reef.

It's beautiful.
I mean, you can see beautiful fossils.

Here's some cervicornis or staghorn.

That's staghorn, yeah.
You can see where the little polyps lived.

RAYMO: Yeah.

RAYMO: Look at this. Beautiful conch shell.
MANNING: Oh, yes, that's a conch shell.

RAYMO: Trapped in the reef.
MANNING: Yeah.

MANNING: Extraordinary, the detail.

I mean you can see this reef
must've just grown itself.

I mean, you've got them at all levels
and this presumably was buried

- by new growth coming on top.
- That's right.

MANNING: This fossilised reef
can only have been stranded

by a massive drop in sea level.

The cause of this drop was surprisingly obvious.

As the ice sheet grew,
it locked up millions of cubic kilometres of water.

At their maximum extent,
ice sheets caused worldwide sea levels

to drop by 120 metres.

This would have had a global effect,

changing the shape of the continents.

In Barbados,
the coral reef growing around the island

would have emerged and become dry land.

But there's not just one coral terrace on Barbados.

There are several
stepping up the side of the island.

Repeated terraces must mean
repeated changes in sea level.

And repeated changes in sea level mean not one
but many ice sheets waxing and waning.

Barbados is the only island
to have these multiple terraces.

And they exist because it's doing
something rather strange.

RAYMO: Barbados is very interesting because
it's slowly being uplifted out of the sea

by tectonic forces.

So as Barbados comes up, the reefs get stranded,

I mean, they're just left to die?

They're left high and dry. Exactly.

So contrary to usual practice, as we go higher,

we're getting into older areas

- rather than the opposite?
- Exactly.

- Sedimentary rocks.
- Right.

Because Barbados is being lifted up,

it's producing a unique record
of changing sea levels and climate.

Every time sea levels rise and fall,

a new coral terrace emerges.

So here we are, we're walking along the top
of the ancient reef crest.

- A nice exposed bit here.
- Yeah, yeah, these are nice.

MANNING: This is stagshorn coral.
RAYMO: That's right.

RAYMO: These would've grown
within a few metres of sea level.

So, you're really seeing firsthand evidence
that this used to be at sea level,

it's been lifted up.

And this terrace is telling us
about a time in the past

when it was warmer, when sea levels were high.

If we look down over the horizon here,
we see another large, flat terrace

that's a younger reef system
that's been lifted up out of the sea.

And just over the horizon is another reef terrace.

So that marks three high points of sea?

Right, three times when it was warm,
when sea levels were very high.

In the late '60s, early '70s,

we developed techniques that allowed us
to date these corals.

Now, we know, for instance,
that this terrace right here is 125,000 years old.

And what about these down here?

This one's 105,000 years old

and the one just over the horizon
is 82,000 years old.

So you've got very nice time markers

- for periods of high sea level?
- That's right.

- That's right.
- Very beautiful system.

MANNING: As scientists found out more
about the ice sheets,

a pattern began to appear.

In the last million years,
the ice sheets have waxed and waned ten times.

They've never entirely disappeared.

But a million years is just a brief moment
in Earth's history.

To see whether this pattern
continued back indefinitely

meant looking in a surprising place.

This ship is pushing back
the boundaries of climate history.

It's the only ship in the world
that drills deep into the ocean sediment.

It drills 24 hours a day, seven days a week.

The running costs are $45 million a year.

And all this to bring up kilometre
after kilometre of mud.

The notion of glaciations and inter-glaciations,

the waxing and waning of continental ice sheets,
was exactly that.

It was first recognised and studied on continents.

The trouble with continental records
is that they're fragmented.

You're studying outcrops, erosion is there,

you're missing part of the record.

AUSTIN: That is not the case nearly as much
in the oceans.

MANNING: The cores of mud are reduced
to this dusting of microscopic shells.

The shells belong to animals called forams.

Imprinted in the shells is information
about what the climate was like

when the forams were alive.

When they died, they sank to the sea floor

and they were incorporated in layers.

Those layers stack up over time

and we core those layers continuously

to get a continuous record of climate change.

And this particular place is a good place
because we have a very thick section,

so that we can tell that story in great detail.

MANNING: This section of mud built up
over 20,000 years.

At this site in the North Atlantic,

there's 20 million years of mud
stacked up on the ocean floor.

A constant supply of forams
is produced by the drilling programme.

Different species thrive
in different water temperatures.

Counting the various types of forams

and analysing the chemistry of their shells

gives an indication of the climate
at the time the forams were alive.

The analysis is agonisingly slow work.

But it has revealed the pattern of climate change

since the age of the dinosaurs.

We've been able to reconstruct
how global temperatures have changed

over the last 70 million years.

Here's today and this is 70 million years ago,
the time of the dinosaurs.

And this is warm, hot climates

and this is cold.

What we've seen is from about 70
to about 40 million years ago,

it was extremely hot, much warmer than today,

about 15 degrees warmer than today, on average.

And that since that time,
temperatures have been gradually falling.

At 35 million years, the very rapid cooling

associated with the glaciation of the Antarctica.

Then the last few million years,

we've been growing large ice sheets
on North America and Scandinavia.

So we really see a large-scale pattern

of global cooling that's characterised
the last 70 million years.

MANNING: Now we can see the pattern
of fluctuating ice sheets in a new light.

They're a comparatively recent event.

It may seem a bizarre thing to say
standing here on a tropical island,

but in geological terms,
we're in an ice age right now.

35 million years ago the ice began to cover
the continents

and although it's retreated,
we still have ice caps over Antarctica

and Greenland and glaciers on the mountains.

But during the time of the dinosaurs

and for millions of years after,

it was too warm to allow any ice
on the planet at all.

So what about before then?

Had the climate always been warm?

I find my mind struggling just to come to terms
with the enormity of the timescales involved.

Maarten de Wit is a geologist
well-used to travelling back in time,

back through hundreds of millions of years.

Here in South Africa he was taking me
to see a mosaic of rock fragments called tillite.

The discovery of the tillite turned geologists' view
of the African climate on its head.

Look at these rock fragments everywhere.

Big ones and small ones, round ones.

MANNING: Just scattered everywhere.

DE WIT: Look at these angular ones,
they're very angular.

This is the unusual thing,

you don't usually get angular
and round ones together

and big ones and small ones together.
This is just chaotic.

MANNING: What did geologists
make of this, then?

Well, it was a geologist that had been
to the Northern Hemisphere

that interpreted this rock as being deposited
by an ice sheet.

And the way he envisaged this,

this grey mass here, the ground mass,

the grey-greeny ground mass we have
is a rock flour

that would have been picked up by a glacier,
as a glacier goes over rock,

just grinding it down to...

- To a fine dust.
- That's right.

Now, at the same time it would have plucked up
a whole lot of rock fragments

and as that ice sheet moved over a body of water,

it would've sat there until it started melting.

And as it melted, all the stuff
would've dropped out at different times,

a big one would cluster, a small cluster.

That accounts for this chaotic sort of deposition.

And the rocks above it and below
contain fossils which we can date

and that places this rock round about
300 million years ago.

So 300 million years ago

there was an ice age in this part of Africa?

That's amazing, isn't it?

To think that here we're walking
in this hot climate here in South Africa,

that 300 million years ago

there must have been a phenomenally large
ice sheet depositing this.

- Well, and I think that's an incredible story.
- Quite extraordinary.

MANNING: The ancient tillite
found in this part of Africa

has also been found scattered in other continents,

like India and Australia.

It's all about the same age,
300 million years or so.

But as Maarten explained,

this widespread distribution
all makes perfect sense.

DE WIT: You've got to remember
that 300 million years ago

the world was very different.

All the continents in the Southern Hemisphere
were together as one big supercontinent.

MANNING: I mean,
here's India and Madagascar there.

- Australia?
- Yes, that's correct.

Madagascar right up against this part of Africa,

here you can see the whole of Africa here.

South America here in this position.

So in this framework, bunched together,

the deposits here in Madagascar
and India and Australia

make sense in terms of a huge ice sheet

that was covering part of, or the largest part
of this supercontinent called Gondwana.

And was this Gondwana positioned
roughly over the South Pole at that time?

Well, it must have been.
This ice sheet covered this area here

right into South America,

so that whole supercontinent was centralised

on the South Pole.

MANNING: The tillites have revealed
that the Earth has experienced an ice age before.

On that occasion, ice gripped the supercontinent

for more than 60 million years.

What about before then?

Well, the geological record is rich
in these kind of deposits.

For example, near Cape Town we have a tillite
that is 420, 450 million years.

In Namibia we have very good evidence now

of something around 720, 730 million years.

And elsewhere in the world we can go back

as far as almost two-and-a-half billion years ago,

a set of tillites in North America.

So it looks like that
as far back as we can go in this record

that the Earth has been in and out of ice ages.

Combining evidence from the bottom of the sea

and from studying rocks like this,

it's been possible to build up a picture

of the changing climate of the Earth
throughout its history.

It's usually been warmer than this,

sometimes much warmer,
as during the great age of dinosaurs.

But every now and then,
Earth has plunged into vicious cold,

so that huge areas are covered by ice
for millions of years.

Then finally the ice disappears.

And the obvious question is why?

The first hint of the answer came from looking
at the ice remaining from the current ice age.

This is one of the most isolated laboratories
on the face of the planet.

Apart from the occasional wayward bird,

the nearest sign of life is 500 kilometres away.

This is the North GRIP campsite
in the centre of Greenland.

For three months of the year it's the home
to a team of 30 scientists.

THORSTEINSSON: Until about 12,000 years ago,
the whole of Scandinavia

was covered by a large ice sheet.

This ice sheet also stretched into parts
of Germany and Great Britain.

Iceland had a separate ice sheet
covering it completely.

And then we had a huge ice sheet
over all of Canada,

reaching into the present United States.

Then this dramatic change comes about
in only a few thousand years.

All these big ice sheets, they disappear completely

except for the Greenland ice sheets,

on which we are sitting right now.

It's very good that we have this ice sheet,

it's a remnant of the Ice Age,
because it really makes it possible for us

to understand what was going on
at the time, really.

This is what we are aiming at with our work here.

MANNING: The scientists here
have one aim in mind,

to produce one of the longest
continuous cores of ice ever drilled.

So far they're about halfway.

There's one and a half kilometres to go.

Each metre of ice brought to the surface

is a step further back into the Ice Age.

THORSTEINSSON: We see structural differences
between fine-grained winter snow

and coarse-grained summer snow,

and these differences are preserved in the ice

so we're able to count annual layers,
all the way down through it,

just like tree rings.

If you measure the lead content in the ice cores,

down through time you can see
that this increased during Roman times

because the Romans, they had a lot of lead mines

all over their empire.

Then you see that this decreases
after the decline of the Empire.

You see it increases in our century

and when everybody stops using leaded gasoline

then the curve is falling again, you know.

There's much less lead in the atmosphere.

So everything that has to do with the atmosphere,

history of it and history of human beings
is really preserved in those ice cores

and it's really fantastic.

MANNING: The ancient history of the atmosphere
can also be found in the ice.

Tiny pockets of air become trapped
as snow falls and compacts.

These bubbles of atmosphere are time capsules.

When researchers began sampling them
to test for carbon dioxide content,

they found something that stopped them
in their tracks.

During the last cold period,

carbon dioxide levels were much lower than today.

This is very interesting, it makes sense

because of the role that carbon dioxide plays
in controlling Earth's climate.

It's a greenhouse gas and it's always present
in our atmosphere

and it acts as a thermal blanket,

keeping the Earth's surface temperature warmer
than it would otherwise be

if there were no CO2,
if there were no greenhouse gases.

So the fact that during the cold periods,

that these greenhouse carbon dioxide levels
went down

and the climate got colder

makes sense from our understanding
of the physics

and chemistry of the greenhouse effect.

MANNING: In recent years, we've heard a lot
about the possible links

between carbon dioxide levels today
and global warming.

But could it be that carbon dioxide
is behind the swings in temperature

which have taken place over hundreds
of millions of years?

Bob Berner, a geochemist, believes it is.

Before humans were adding carbon dioxide
to the atmosphere,

there were other natural processes

that were adding and subtracting
carbon dioxide from the atmosphere.

And these were all part of what is known
as the long-term carbon cycle,

which operated over many millions of years.

The whole process starts with the introduction
of CO2 to the atmosphere

via CO2 coming out of volcanoes.

MANNING: Every time a volcano erupts,

carbon dioxide spews out into the atmosphere.

Left unchecked it would build up,

but rain gradually washes it out
and plants draw it into the soil.

And this carbon dioxide in the soil

reacts with rainwater that's percolated
into the soil to form an acid.

MANNING: The acid eats away at the rock
in a process called weathering,

which converts carbon dioxide into carbonate.

This dissolved carbonate
eventually makes its way into rivers

and eventually, by flow of rivers, to the sea.

MANNING: The dissolved carbon is removed
from sea water by coral and other organisms.

When those organisms die,
their remains build up on the seabed,

eventually turning into limestone.

Now, this limestone we have down here
completes the cycle eventually

because it becomes buried to such a depth
that it's heated,

broken down and the carbon dioxide
comes off of the...

is removed from the limestone
and it finds its way back to the surface

in the form of seepage out of the ground
or eruption from volcanoes.

And this is the long-term carbon cycle.

MANNING: All things being equal,
levels of carbon dioxide, CO2, would stay constant

as the gas is continuously pumped
in and out of the atmosphere.

But the planet is in a constant state of flux.

Continents drift across the face of the Earth,

volcanoes burst into life and then become extinct,

vegetation comes and goes.

All these factors can affect the carbon cycle.

So using geologists' knowledge of Earth history,

Bob Berner set out to estimate
how much CO2 levels

have actually changed over time.

Bob has now calculated CO2 levels
over half a billion years.

BERNER: As you can see most of the time,

carbon dioxide has been considerably higher
than it is at present.

And you can see high values in the earliest period

dropping to lower values in intermediate times,

then to intermediately high values
between that time and the present.

But the lowest levels you can see are the present

and a time about 300 million years ago.

MANNING: This drop here is caused
by one particular factor in Bob's equations:

Plants.

Plants helped to remove CO2 from the atmosphere

by speeding up the weathering of rocks.

So when plants first evolved and flourished,

CO2 levels began to fall.

Those levels reached an all-time low
300 million years ago,

precisely the time when a massive ice age
left its imprint

in the rocks of South Africa.

This is very gratifying,

it indicates that, and this drop I believe is due
to the rise of the plants

and their acceleration of weathering.

This acceleration of weathering I believe
caused the CO2 drop

which led to the glaciation
at 300 million years ago

and it was, I think,
the major cause of this glaciation.

MANNING: It is amazing to think
that plants could've triggered the ice age

300 million years ago.

But carbon dioxide levels also appear
to have been falling

over the last 65 million years,

a decline that seems to have led
to the ice age we're in today.

An explanation for this drop
is also in the equations:

Mountain building.

We know that CO2 gets into the atmosphere
through volcanoes

but it comes out through rock weathering.

And rock weathering goes on primarily
in the mountainous regions of the world.

We know that the last 40 to 50 million years

have been unusually active with respect
to mountain building.

There's the Himalayas, the Andes, the Rockies.

And this tectonic activity has probably
been in large part responsible

for the reduction in atmospheric
carbon dioxide levels.

Now, that's very interesting,

it's obviously been cooling over that time period

and so we probably are seeing this causal link

between the Earth's tectonic activity
and the Earth's climate.

MANNING: Now we can begin to understand
the pattern of climate change.

It's a pattern which changes
to the rhythms of evolution

and the movement of the Earth's tectonic plates.

The climate wanders from one ice age to another

as the continents drift around the planet.

But the movements of the continents
can't explain everything.

Over the last million years,
the ice sheets have been pulsating

to a rhythm faster than continental drift.

Something else must be driving these cycles.

Ice will never accumulate

if all the snow of winter
melts in the following summer.

And about 80 years ago,
a Serbian scientist called Milankovitch

realised that the crucial factor about ice sheets,
the accumulation of ice sheets,

was not the cold of winter,
it was the temperature of summer.

A cool summer will not melt all the snow,
snow will accumulate.

A succession of cool summers
will cause ice to advance,

just as a succession of warmer summers
will cause it to recede again.

Now, we all know that some summers
are hotter than others

but what Milankovitch went on to explain

was how summers can warm and cool

in a regular cycle over thousands of years

in a way that mirrors the waxing and waning
of the ice sheets.

And it's all down to the way
that the Earth orbits the sun.

The Earth circles the sun with its axis tilted.

That tilt gives us our seasons.

When the North Pole points away from the sun,
we experience our winter,

the Southern Hemisphere its summer.

On the other side of the orbit
the situation is reversed.

This is now the northern summer.

Over thousands of years, the precise orbit

and the tilt of the Earth varies slightly.

It wobbles and dips, making our summers
hotter or colder

as the Northern Hemisphere
moves toward or away from the sun.

Using this information, Milankovitch calculated

when the summers would become cool enough
to cause the ice to return.

Despite the logic of his idea,

it was impossible to prove at the time.

Nobody knew precisely when summers
had warmed and cooled.

Then, 10 years after Milankovitch died,

the coral terraces of Barbados were dated
for the first time.

RAYMO: This is a curve of how hot
Northern Hemisphere summers have been

over the past 250,000 years.

This curve was first calculated,
predicted by Milankovitch.

So these are his predictions?
We don't have any measurements.

I mean, he's just predicting this?

Right, you can calculate,
you can easily make the calculations, actually...

Right.

...about how much sunlight
we would be receiving back in time.

Yes.

And so, you know,
this curve would obviously predict

that these periods would be periods
that were very cool in the summer,

and that these would be periods
that would have very hot summers

in the Northern Hemisphere.

MANNING: Ice ages here.
RAYMO: Exactly.

Now, if we take the Barbados data and overlay it,

where the warm sea level high stands are,

they fall very close, if not exactly on,

when the Northern Hemisphere summers
are very warm.

- That's very beautiful, I think.
- Yeah.

It's... I mean I wish Milankovitch were alive
to have seen this.

It really had vindicated his theory
once we were able to date the corals

- and demonstrate this relationship...
- That's very beautiful.

...between sea level high stands
and the predicted warm summers.

MANNING: So with this complex pattern
of climate change behind us,

what can history tell us about the future?

Today the fear is that man-made global warming

could bring about a sudden collapse
of the ice sheets.

And the fact is it looks like such breakdowns

have happened several times in the past

with dramatic and unexpected effects.

Once more, the story's in the mud
of the ocean floor.

In his lab near New York,

geologist Gerard Bond was looking
at a core from the North Atlantic

when he spotted some subtle variations.

We noticed, as you can see in this core,

it was 150 centimetres,
there are a number of changes in colour.

There's a light layer here,
a little darker layer here,

an intermediate-coloured layer here,

it becomes light again down to about here,

this one's a little lighter,

this one's lighter yet and then it goes back
to a darker layer here.

And we knew from radiocarbon dating of this core

that the top is about 9,000 years,

this level is 14,000 years
and about 20,000 years here.

These bands were coming very fast.

And what that meant was that
these changes in colour

were coming about every 1,000 to 2,000 years.

MANNING: Having identified these strange bands
of colour,

Gerard set to work to find out what caused them.

It was a gruelling process.

Sample after sample of the cores
were reduced to sand grains.

Each grain was counted, classified and recounted.

After about a million grains of sand,

Gerard took stock of his curious findings.

BOND: We found six layers that had
a marked increase

in limestone and dolomite fragments.

The origin of these appears
to have been eastern Canada.

In addition, when we looked more carefully
at the sediment,

in-between the six events,
there were additional layers.

These showed increases in volcanic ash
that came from Iceland

and increases in grains,
mostly quartz and feldspar,

that were stained with an iron oxide
that's called hematite.

And you can see some of that here and here,

and we believe that these particular grains

are coming from the Gulf of St Lawrence.

MANNING: There's only one way so many grains

could have been lifted from the bedrock
of Canada and Iceland

and transported to the ocean.

They must have hitched a ride.

During the last glaciation,
ice sheets scraped over the continents,

picking up rock fragments on their way to the sea.

From there, icebergs carried the grains
across the Atlantic.

As the ice melted, the grains were released
and drifted to the bottom of the ocean.

The sheer number of grains
found in the layers of the sea floor

suggest that enormous armadas
of icebergs swept across the ocean

every few thousand years.

Whatever caused this,
work on the Greenland ice cores

suggests the invasion of the ocean by icebergs

could've tipped the climate into turmoil.

Each of these slices of ice represents six weeks
of climate history.

When the researchers looked at the ice
from the last glaciation in this kind of detail,

they found a completely unexpected series
of temperature jumps.

These jumps coincided with the surges
of the icebergs.

By looking at the ice core in such detail,

the researchers in Greenland discovered
the most astonishing thing.

Within the glacial period, there are climate cycles
that are even more rapid.

They last a few thousand years,

so if I describe a single cycle

you could imagine a long, slow cooling

and then there's a very rapid warming,

10 to 12 degrees Celsius.

And that warming happens within
the span of a single human lifetime.

It's quite astonishing how quickly

the regional climate of the North Atlantic changed.

MANNING: Ever since these frenetic changes
in temperature came to light,

researchers have struggled
to understand the cause.

One thing they do know is
that climate in the North Atlantic

is strongly influenced by the warm water
flowing up from the tropics.

RAYMO: The ocean plays a very important role
in climate.

Most of the solar radiation received by the Earth

comes in at the equatorial regions.

And to get that heat to the poles,
to keep the poles warm,

you have to use the ocean currents,

so currents such as the Gulf Stream
are carrying heat to the high latitudes.

And it's this heat that's an important source
of energy and warmth

to places like western Europe and Great Britain,
for instance.

About 25% of their yearly heat budget
is from the ocean currents.

'Cause we're on, I mean,
if you compare us in northern Europe

with Labrador or with the north of the Pacific,

I mean we're much, much warmer in climate

- than they are.
- That's right.

MANNING: This warm water is drawn up
from the tropics

because of sinking cold water
in the North Atlantic.

In the tropics,
evaporation caused by the sun's heat

creates surface waters
that are particularly salty, as well as warm.

As the Gulf Stream carries
this warm water northwards,

it gives up its heat, becoming cold and dense.

Near Greenland, it's dense enough
to plummet to the ocean floor,

drawing up more tropical waters behind.

It's the sinking of this cold, salty water

which is the engine behind the conveyor belt

that's bringing warmth to the northern latitudes.

Stop this water from sinking

and you stop this heat supply to the north.

And that's exactly what scientists believe
the armadas of icebergs may have done.

You can imagine if all these icebergs
were choking the North Atlantic,

they start to melt and a huge amount
of freshwater

is then delivered to the surface waters.

That freshwater lowers the density
of the surface waters

inhibiting its ability to convect.

So essentially what these huge iceberg
melting events do

is shut down the conveyor belt

and so the entire region is plunged
into a more frigid state.

MANNING: So perhaps here's the explanation

of the rapid flips in climate
during the last glaciation.

Enough icebergs surging out into the Atlantic

would have switched off the conveyor.

After the ice had melted,

the conveyor would suddenly switch back on,

leading to rapid warming in the north.

These sudden swings happened
at least 20 times in 60,000 years.

But eventually the icebergs stopped coming
and the climate calmed down.

RAYMO: Over the last 8,000 years
the climate system has been very stable.

And during this time agriculture's developed,

human civilisations have flourished,

and yet we know from the geological record
that this is quite unusual.

For most of Earth's history,
climate has been much more variable

and changing much more rapidly.

MANNING: Perhaps human civilisation
only emerged

because this pattern of rapid change
came to an end.

Today we're reaping the benefit
of a few thousand years of stable temperatures.

But no one knows how long
this benign lull will last.

It remains to be seen whether human influence

will prematurely tip the balance back
into more turbulent times.

As they travel back in time,

geologists have uncovered a history
of temperature change far more profound

than anything those early pioneers
in Switzerland could've suspected.

It's 150 years since Louis Agassiz
first presented evidence

that the Earth's climate
had once been brutally cold.

But it's only in the last few years
that scientists have come to recognise

that climate change is just
an inevitable consequence

of the way the Earth works.

Climate's been changing one way or another
throughout the history of the planet.

And frankly, there's every evidence to believe

that it will continue to do so.

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