Scattered amongst the craggy peaks
of the SWiss Alps
is a series of strange geological features.
For many years these features Were a puzzle.
When geologists eventually learnt
hoW to interpret the landscape
they uncovered the first hints
of a long chain of dramatic events
that have transformed the planet throughout its history.
Stranded in the middle of the SWiss countryside
is this enormous boulder.
lt's been made a national monument.
Christian Schluchter, a local geologist,
explains Why it's so important.
What We see here is really a strange piece of rock.
lt actually doesn't belong to this area Where it is noW.
lts composition is different from the bedrock here.
Bedrock is beautiful sandstone
and this is a gneiss
so it must come from someWhere else.
lf you look around in the SWiss Alps
then you find the places in the mountains
of the canton of Valais Where this comes from,
so it has travelled
let's say betWeen 200 and 250 kilometres to come here.
lt is What We call an erratic boulder.
People, of course, Were thinking
for many, many centuries hoW this actually came to rest here
because it Was obvious
it had to be moved somehoW,
so the first idea Was
that it has been brought here
by a huge big flood,
by the Waters of a big,
big flood originating high up in the Alps.
Erratic boulders like this Were found
all over northern Europe and America.
To 19th-century geologists
they seemed like good evidence
that the earth had once been covered
by enormous floods,
just as the Bible described,
but as they looked further up the valleys
SWiss scientists could see that Water
Was not the only thing that could move rocks.
All these boulders around here Were carried doWn
by the actively advancing glacier.
The advancing glacier from up
the valley here carried and put them doWn here
in the 1850s and piled up this beautiful dam
We call a terminal marain.
This jumbled Wall of boulders
is noW several hundred metres from the snout of the glacier.
Back in the last century glaciers all over the Alps
stretched further oWn the mountains than they do today.
lt 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.
This here is one of the places gassiz Was visiting
When he did his studies on glaciers
and it's a very special place.
lt's one of the most beautifully
polished bedrock surfaces.
Here it's a polished surface
of the Grimsel granite.
NoW hoW does such a beautiful polish actually happen.
lt happens at the base of a moving glacier
Where enough rock floWer
is available to act as a polishing agent.
Tiny fragments of rock trapped in the ice
rubbed at this slab like fine grade sandpaper.
The smooth surface stretching far up
the side of the mountain
shoWs that the ice here Was once one kilometre thick.
But the ice carried more than just poWdered rock.
These scratches on the bedrock surface
here are typical for formerly glaciated areas.
They are proof of the moving boulders
at the base of the glacier.
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.
Geologists have noW been all over the Alps
looking for evidence of rock
that had been shattered and pulverised
by the glaciers.
They've plotted the location of terminal marains
and erratic boulders
to see the maximum extent of the ice
and they've found that practically the entire country
Would have been covered.
18,000 years ago
a sea of ice one kilometre thick
Would have covered the site of Geneva.
lt soon became clear that the ice
had spread beyond SWitzerland.
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 plateaux.
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 lce Age.
Strange as it may seem this far south,
the ice has left its mark on the rocks that cover the island.
The island is made almost entirely of ancient coral reefs.
Unlike the other islands
of the eastern Caribbean Which are volcanic,
this one is constructional having
been made out of coral reefs.
Everything We're seeing here
is an ancient fossilised coral reef.
lt's beautiful, l mean you can even see the individual polyps
on these corals still.
Here's a huge conch shell beautifully preserved.
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 World-Wide sea-levels to drop by 120 metres.
This Would have had a global effect
changing the shape of the continents.
ln Barbados the coral reef groWing around the island
Would have emerged and become dry land.
But it'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.
Barbados is very unusual island
because it's very sloWly being uplifted out of the sea
by tectonic forces.
So What We see on Barbados is a series
of steps made up of coral reefs
that go up in elevation
and also go back in time
because you can imagine the,
the highest coral reef is the one
that's been lifted out,
has been lifted out of the ocean the longest
so they're all sloWly being pushed up out of the ocean.
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 on the ancient reef crest
right noW and We knoW it's the top of the reef
because there's all these cervicornis
or staghorn coral pieces
and that's a type of coral that only groWs
Within a feW metres of sea-level,
so We have very concrete evidence that,
that What We're standing on used to be at sea-level.
lt's been uplifted by the tectonic uplift of Barbados
and We can very clearly see one more terrace beloW
and just over that terrace just out of sight
is a third terrace.
All three of these terraces are reflecting times
When it Was very Warm, When the sea-level Was very high.
NoW by the late 60s/ early 70s
We'd developed techniques that alloWed us to date
specimens such as this
and so We noW knoW that this reef system right here
is 125,000 years old.
The one We can see right there
is 105,000 years old,
and the one that's just out of sight is 82,000 years,
so What We have is a really nice record
of Warm climates going back in time.
On the other side of the World
is direct evidence of the colder periods in time.
The finely crushed gravel in this SWiss quarry
Was left behind by the glaciers.
This particular quarry consists
of distinct layers Which piled up
over half a million years.
This sediment up here
With these quite large boulders in it,
it's deposited
by a glacial melt Water stream
right in front of the active ice,
maybe a feW tens to a feW hundreds of metres at the most,
so this is a direct evidence of the presence
of the ice at that place,
and then if you look doWn here
that's a sediment deposited
in a small lake Which Was produced by a glacier.
So We have evidence of tWo glaciations
Within these three metres of a geologic section.
SandWiched betWeen the tWo layers shoWing
When temperatures Were cold is a third layer
Which tells a very different story.
You see these peculiar pieces
of something sticking out.
lf l grab one of these pieces
We have a beautiful piece of fossil Wood.
lf We analyse all the fossil pieces of Wood
in here We can conclude that the temperature
Was even Warmer than at present
When the surrounding forests existed here
by about 2-2.5 degrees Celsius,
so this layer is an important document
telling us that in-betWeen tWo layers
of very cold geologic environment
We find this spread
of actually Warmer than present climate.
The change from cold to Warm and back to cold
happened in just 150,000 years.
ln fact, the quarry shoWs that the climate flipped
at least three times
in the remarkably short period.
The evidence from places like SWitzerland and Barbados
has revealed a pattern in the lce Age.
Over the last million years
the ice sheets have Waxed and Waned 10 times,
but they've never entirely disappeared.
Yet 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.
lt's the only ship in the World that drills
deep into the ocean sediment.
lt 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.
That is not the case,
nearly as much in the oceans.
The cores of mud are reduced to this dusting
of microscopic shells.
The shells belong to animals called forams.
lmprinted 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 stacked 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 fixed section
so that We can tell that story in great detail.
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 is 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
a very rapid cooling associated
With the glaciation of the Antarctic.
ln the last feW million years
We've been groWing large ice sheets
under North America and Scandinavia,
so really seen a large-scale pattern
of global cooling that's characterised
the last 70 million years.
So the pattern of fluctuating ice sheets
is a comparatively recent event
and in geological terms
We're still in the lce Age.
There are still permanent ice sheets
on Antarctica and Greenland
and glaciers in the high mountains.
Thanks to the forams We noW knoW
that this hasn't alWays been the case.
During the time of the dinosaurs,
and for millions of years after,
it Was too Warm for ice to exist anywhere on Earth.
What about before the dinosaurs though?
To find out Whether the climate had alWays been Warm
you need to travel back in time.
Maarten de Wit is a geologist
Well used to travelling back in time,
back through hundreds of millions of years.
This mosaic of rock fragments is called tillite
and When it Was discovered here in South Africa
it turned geologists' vieW
of the African climate on its head.
Look at this extraordinary rock,
this, it's a very strange rock
and What makes it very strange is,
is all these rock fragments -
big ones and small ones, round ones.
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.
lt must have been extremely difficult to interpret this rock.
The first geologists to map this in detail
spent some time in the northern hemisphere
and the Way they interpreted this
Was that these rock fragments
must have been deposite by some sort of ice sheet.
The ice sheet Would have moved over the bedrock,
picked up a lot of rock,
round it to a very fine poWdery matrix,
this grey-green material,
plucked up a Whole lot of rock fragments everywhere,
moved over a Water body
and as it sat over that Water body
and melted it Would have dropped all its material
and that reasonable explanation,
it seems to be the best explanation,
to explain this sort of chaotic set of fragments
in this rock floWer.
BeloW and above it We have fossils
and We knoW the age of these fossils
and it turns out that this particular tillite
is 300 million years old,
so that's, that's an amazing thing to think,
300 million years ago that this area here
in South Africa Was covered
by a very thick ice sheet.
Temperatures here in this climate
must have been very different,
an lce Age, a huge ice sheet covering this, this countryside.
The ancient tillite found in this part of South Africa
has also been found scattered in other continents
like Antarctica, lndia and Australia.
lt's all about the same age -
300 million years or so -
but this Widespread distribution
does make perfect sense,
as Maarten explains.
Well you, 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 super continent.
Over here, here's lndia and Madagascar,
can see Australia over there
and the centrepiece here stills makes this large land mass of Africa.
ln this frameWork and bunched together
the deposit makes sense in terms of a huge ice sheet
that Was covering part of,
or the largest part of this super-continent
called GondWana
and of course the centre
of this ice sheet Would have been over here,
so this part of it Would have been sitting around the South Pole.
The tillites have revealed that the Earth
has experienced an lce Age before.
On that occasion ice gripped the super-continent
for more than 60 million years.
This is not the only lce Age We've had.
We knoW in the far distant past
We've had more than one glaciation.
Even in southern Africa We have evidence,
good evidence for a glaciation in the Ordovician
430/440 million years ago
and further afield in Africa,
many parts of Africa We have
examples of glaciation 700 million years ago
and elseWhere in the World We can go back
as far as almost 21/2 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 lce Ages.
Looking at these tillites and combining them
With the evidence from the ocean floor
it's been possible t build up a picture
of Earth's changing climate.
For most of its history it seems to
have been Warmer than today,
but every noW and then Earth has been plunged
into vicious cold
so that huge areas have been covered
by ice for millions of years.
Then the ice is shaken off again
and the obvious question is; Why?
The first hint of the ansWer
came from looking at the ice remaining
from the current lce 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 3 months of the year
it's the home to a team of 30 scientists.
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.
lceland 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 disappeared completely except
for the Greenland ice sheets
on Which We are sitting right noW.
lt's very good that We have this ice sheet,
this remnant of the lce 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.
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 half Way.
There's one and a half kilometres to go.
Each metre of ice brought to the surface
is a step further back into the lce Age.
We see structural difference
betWeen fine grained Winter snoW
and coarse grained summer snoW
and these differences are preserved
in the ice so We are able to count annual layers
all the Way doWn through just like tree rings.
lf 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
and 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, the history of it
and the history of human beings
is really preserved in those ice cores
and it's really fantastic.
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 and makes sense,
because of the role that carbon dioxide plays
in controlling earth's climate.
lt's a greenhouse gas
that's alWays present in our atmosphere
and it acts as, as, 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,
from our understanding of the physics
and chemistry of the greenhouse effect.
ln 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 geo-chemist,
believes it is.
Before humans Were adding carbon dioxide
to the atmosphere
there Were other natural processes
that Were adding and sub-tracting carbon dioxide
from the atmosphere,
and these are 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
by CO2 coming out of volcanoes.
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.
This carbon dioxide in the soil
reacts With the rainWater that's percolating
into the soil to form an acid.
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.
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.
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 geologist knoWledge of earth's 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.
As you can see most of the time
carbon dioxide's been considerably
higher than it is at present
and you see high values in the earliest period
droppingto loWer values
in intermediate times
and to intermediately high values
betWeen that time and the present,
but the loWest levels you can see are the present
in a time about 300 million years ago.
This drop here is caused by one particular factor
in Bob's equations; plants.
Plants help 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 lce Age
left its imprint in the rocks of South Africa.
This is very gratifying
and indicates that this drop
l believe is due to the rise of the plants
and their acceleration of Weathering.
This acceleration of Weathering l believe caused the CO2 drop
Which lead to the glaciation at 300 million years ago
and Was l think the major cause of this glaciation.
lt is amazing to think that plants
could have triggered the lce 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 lce 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 to rock Weathering,
and rock Weathering goes on primarily
in the mountainous regions of the World.
We knoW that for the last 40-50 million year
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 a large part responsible for the reduction
in atmosphere of carbon dioxide levels.
NoW that's very interesting,
it's obviously been cooling over that time period
so We probably are seeing this causal link
betWeen the earth's tectonic activity
and the earth's climate.
NoW We can begin to understand
the pattern of climate change.
lt's a pattern Which changes to the rhythms
of evolution and the movement of the Earth's tectonic plates.
The climate Wanders from one lce 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.
lce Will never accumulate
if all the snoW of Winter melts the folloWing summer,
but if summers are cold snoW Will survive
from one Winter to the next.
Eventually the layers Will build up
and several decades of cold summers
Will result in a sizeable ice sheet.
lf summers Warmed up again
the ice sheet Would shrink back.
The idea that the pattern of fluctuating ice sheets
Was caused by fluctuating summer temperatures
Was taken up early this century
by a Serbian scientist called Milankovich.
He believed that the cause of these sloW repetitive changes
Was due to the Way 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 the noW the northern summer.
Over thousands of years the precise orbit
and the tilt of the Earth varies slightly.
lt Wobbles and dips making our summers hotter or colder
as the northern hemisphere moves toWard
or aWay from the sun.
Using this information Milankovich 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 Milankovich died,
the coral terraces of Barbados
Were dated for the first time.
This is a curve of hoW hot northern hemisphere
summers had been over the past 250,000 years.
This curve Was first calculated, predicted by Milankovich
and What you can see is that there are times
Where northern hemisphere summers are quite Warm
and at other times, for instance doWn here,
the summers are much cooler
and that these cooler summers,
these loW periods Would be glacial periods
and these Would be the Warm periods
With higher sea-levels and less ice volume.
NoW if We compare this curve to the Barbados data
What you see is that these coral terraces,
including the one We're standing on right noW,
their ages line up almost perfectly
With the time periods that Milankovich
Would have predicted Would be Warm,
therefore less ice and higher sea-levels
and indeed it Was the dating of these coral reef terraces
in the early 70s that proved seminal
for most people accepting the Milankovich hypothesis
that indeed this Was What caused climate change.
This tidy fit betWeen Milankovich's predictions
and the coral dates appear to complete the picture.
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 that 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.
ln 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
of 150 centimetres,
there are a number of changes in colour.
Here's a light layer here,
a little darker layer here,
an intermediate colour layer here,
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 ,000-2,000 years.
Having identified these strange bands of colour,
Gerard set to Work to find out What caused them.
lt Was a gruelling process.
Sample after sample of the cores
Were reduced to sand grains.
Each grain Was counted,
classified and re-counted.
After about a million grains of sand,
Gerard took stock of his curious findings.
We found six layers that had a marked increase
in limestone and dolomite fragments.
The origin of these appears to have been eastern Canada.
ln addition, When We look more carefully
at the sediment,
in-betWeen these six events
there Were additional layers.
These shoWed increases in volcanic ash
that came from lceland and increases
in grains mostly quartz and feltzspar
that Were stained With an iron oxide
that's called hematite and you see some of that here
and here and We believe that these particular grains
are coming from the Gulf of St LaWrence.
There's only one Way so many grains
could have been lifted from the bedrock of Canada and lceland
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.
What-ever caused this,
Work on the Greenland ice core
suggests the invasion of the ocean by icebergs
could have tipped the climate into turmoil.
Each of these slices of ice
represents6 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 l describe a single cycle,
you can imagine along the sloW cooling
and then there's a very rapid Warming
10-12 degrees Celsius
and that Warming happens
Within the span of a single human lifetime.
lt's quite astonishing
hoW quickly the regional climate of the north Atlantic changed.
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.
The oceans play a vital role in climate.
Much of the sun's energy is received here in the Tropics,
but ocean currents carry that heat toWards the Poles.
ln particular the Gulf Stream system
in the North Atlantic
carries a vast amount of heat
from the loWer, the loWer latitudes
up to the higher latitudes.
ln fact probably about 30% of,
of the heat delivered to Europe, Western Europe,
Great Britain is, is brought
by the Gulf Stream system.
This Warm Water is draWn up from the tropics
because of sinking cold Water in the North Atlantic.
ln 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.
lt's the sinking of this cold, salty Water
Which is the engine behind a 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.
NoW of course if you have
the North Atlantic choke full of icebergs,
all those icebergs are going to melt
and deliver a huge amount of fresh Water
to the surface Waters
and this fresh Water Would have loWered
the solidity of the surface Water, loWered the density
and shut doWn the conveyor belt.
By shutting doWn the conveyor belt
the heat supply to the north
Would have been cut off
and temperatures on land Would have plummeted.
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.
This is a time that agriculture's developed,
cultures have flourished,
but this is also a very unusual time for climate.
lt's been very calm, very stable.
We knoW from the geologic record
that in fact usually climate is much more dynamic.
We're going very quickly
betWeen colder periods and Warmer periods.
lndeed some climate changes can happen
in the course of a single human lifetime
as We knoW from the end of the last glacial
to the present Warm period.
Perhaps human civilisation only emerged
because this rapid change came to an end.
Today We may be reaping the benefit of a feW thousand years
of stable temperatures
but no-one knoWs hoW long this benign lull Will last.
All the factors that influence climate
in the past Will continue
to have an effect in the future.
NoW there is the additional concern
that mankind Will prematurely tip the balance
back into more turbulent times.
As they've travelled back in time,
geologists have uncovered a history
of climate change far more profound
than anything the SWiss pioneers could have suspected.
Everything they've found shoWs that climate change
is a natural result of the Way the Earth Works.
Climate has been changing one Way or another
throughout Earth's history
and there's no reason to believe
that it Will ever stop changing.