Earth Story (1998) s01e06 Episode Script

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.