Nova (1974–…): Season 47, Episode 1 - Polar Extremes - full transcript

Paleontologist Kirk Johnson explores the polar extremes of the planet, including miles-high ice sheets and warm polar forests brimming with life.

Are you wondering how healthy the food you are eating is? Check it -
The Arctic...

the Antarctic...

our planet's ice worlds.

Vast, frozen, and empty.

♪ ♪

Yet hidden in these rocks,

buried under the oceans...

Control, boulder board.

...and trapped in the ice

are clues that reveal
a totally different past.

Oh my God, look at this.

Full of surprises...

It's like a whole forest.

I'm Kirk Johnson

and I'm headed out on
an adventure back in time...

Just walking around,
carrying a mammoth tusk.

...and around the globe,

from one polar extreme
to the other,

to discover an Earth
totally different...

It just looks like Mars.

...from the planet
we know today.

This place, this
is so totally amazing

An Arctic that was once
a warm, humid swamp;

Antarctica full of dinosaurs;

and a time when ice sheets
extended from pole to pole,

turning earth into
a giant snowball.

What powerful forces drove
the poles to such extremes?

And what does it mean
for our planet's future?


♪ ♪

Find out the true power of ice.

This is amazing out here.

"Polar Extremes,"

right now, on "NOVA."

♪ ♪

Major funding for "NOVA"
is provided by the following:

We're right at the mouth

of the Ilulissat Icefjord.

This place is a dream.

♪ ♪

It's almost unimaginable
to think

that I can paddle a kayak
around in a landscape

of floating, frozen ice.

Think about all the other places
you could be right now,

then think about where
you actually are.

♪ ♪

These icebergs are amazing,

and they come in all sizes,

from the size of
little, tiny ice cubes

all the way up to
the size of mountains.

♪ ♪

Believe it not,

we live on a planet whose fate
is determined by ice.

You wouldn't think it
living at mid-latitudes,

but here in Greenland,
it's really obvious.

♪ ♪

There is so much ice
in glaciers and ice caps,

mostly up here in the Arctic

and down in the Antarctic.

When you add it all up,

it's 70% of Earth's fresh water.

I'm paddling towards two of
the most spectacular icebergs

I've ever seen in my life.

This stuff looks so solid,

so vast, so permanent.

But looking around at
these giant floating mountains,

you've got to wonder,
"How did all this ice get here?

How long did it take to form?"

And, of course,
"How long will it last?"

For the whole time that humans
have been on this planet--

around 300,000 years--

there's always been ice
at the poles.

But if you look at the
entire history of Earth,

Homo sapiens' existence
has happened

in just the blink of an eye.

If we could travel
back in time--

hundreds of millions of years--

what would the Arctic
and Antarctic look like?

Ice at the polar extremes

is vital to the health
of our planet.

If it disappears, what can the
past tell us about the future?

How close are we
to a tipping point?

♪ ♪

That took me by surprise.

To answer these questions,

I'm joining scientists
working around the world...

Oh, yeah!


There's a perfect
three-million-year-old clam!

This is like
a window to the past.


You can actually see

the annual layers,
even at this level.

And probing...

DAVID HOLLAND: First we're going
to lower the probe.
Probe deployed.

To unlock the hidden history...

Okay, so it's cold.

You're better
than me, I'll tell you.

...of the polar extremes.

♪ ♪

Oh, yes!

I'm a paleontologist.

Get-get this edge right here.

Yes, sir.

I love finding fossils
of all kinds,

but I have a special place
in my heart for fossil plants.


Oh, my God.

Like this palm frond I found
in Alaska a few years ago.


(plane engine droning)

My first stop on
this journey is one

of the most remote places
on Earth,

with some surprising secrets
locked in its rocks.

I'm on my way
to Ellesmere Island,

high in the Canadian Arctic,
next door to Greenland,

and it's only 800 miles
from the North Pole.

We just crossed
over latitude 77,

and we just left Grise Fiord,

which is the northernmost town
in North America,

and we're going north.

♪ ♪

There's just clouds
and ice and rock.

It just looks like Mars,
it's a magical place.

This far north, there are
no roads, no airports,

and definitely no runways.

He's coming in right now.

Landing on uncharted ground
is a nail-biting experience.

Hold tight, hold tight.

Even for a seasoned Arctic guide

like Jason Hillier.

(plane droning)

There we go.

That's what we call an...

That's an Arctic landing
right there.

He hit the brakes pretty hard.


That was awesome.

Terra firma.


You got to have
a really good reason

to want to land a plane here.

It's a great place to be;
I mean,

this is a most beautiful spot.

(engine droning)

There's always the moment
when that Twin Otter leaves,

and you're standing here
realizing that

there's nobody for hundreds
of miles around.

You're all alone
out in the amazing, wide world.

Time to get down
to business.

I know.

I've come here to
try and figure out

what life was like in the Arctic
millions of years ago.

There's a two-month-long window
in summer

when conditions are right
for fossil-hunting.

This is also a trip
back in time for me.

I first came here in 1984,
when I was only 23 years old.

My first trip
to Ellesmere Island

changed the way I looked
at planet Earth.

It sealed my fate.

After that trip, I knew I had
to become a paleontologist.

Yeah, these are good memories.

It's great to be back here.

In addition to Jason,

Dave Briggs is here
to protect us

from any stray polar bears.


And completing the team

is fellow paleontologist
Jaelyn Eberle.

♪ ♪

Once camp is set up,

Jason is going to help me
find a special site

that I never got to see
the last time I was here.

It's just straight northwest

for four kilometers.

Just a little bit of a hike.

Then we, uh, sort of
head northwest, I guess.

♪ ♪

The beds are dipping
pretty steeply here.

Yeah, it's definitely something
we have to be careful of

with this mud.

♪ ♪

There's a bit of a precipice
over there,

although it's generally in the
direction of where we're going.

♪ ♪

Our target is several hours'
hike from our camp.

♪ ♪

This frozen tundra
may look barren,

but for me as a paleobotanist,
this is geologic heaven.

There are a few
small Arctic wildflowers

and some scrappy ground cover.

But the nearest living tree is
a thousand miles south of here.

You know, the coordinates
are, they're,

our best guess are
somewhere in this vicinity.

Finally, we reach
what I've been looking for.


Oh, my God, there's some
amazing stumps here.

Look at this.

Sticking out of the hillside are
a bunch of weird brown rocks.

These are petrified tree stumps.


One, two, three, four,

five, six, seven, eight,
nine, ten, 11, 12,

13, 14,

15, 16, 17, 18.

In one view,
18 trunks in place.

Ah, this is incredible, man.

This is, this is amazing.
It's like a whole forest.

Look at that one,
that one is just perfect.

You can see the whole
shape of the trunk.

It's quite an amazing thing--
just insane.

This is a stone tree trunk,
it's a petrified tree.

It used to be a growing tree,

then it got buried
and turned into stone.

Geologists have dated
these trees

to around 50 million years ago.

Back then,
this treeless hillside

would have been
completely different.

There's a lot of clues here

to how this forest
got to be the way it is.

The first is that all these
stone trunks are surrounded

by coal-- black rock,

which is a very strong signal
that these trees,

when they were growing,
were growing in a swamp.

The second clue are
these radiating roots.

In swamps, roots grow out
and not down.

♪ ♪

But what kind of trees
were these?

These rocks are just
chunky silt stones

that are quite near
the petrified forest,

and I'm breaking them apart

because I'm looking
for fossil leaves.

If I can find the leaves
that go with those trees,

I can put that forest
back together again.

Come to papa.

Ah, now, here we go.

Here's a close-up of that thing.

What have you got?

This is metasequoia,

little needles.

It's the dawn redwood,

it's a conifer, and these
whole sprigs will fall off,

as they've done here,
and land in the mud.

This used to be mud,
now it's a rock.

It would go with those trunks,
the petrified forest.

(blows dust off rock)

Together, these leaves
and the tree trunks

give us a window into the past.

50 million years ago,
this dry, barren tundra

was a warm, humid swamp...

(insects chirping)

covered with lotus plants,
ferns, and metasequoia trees

that were as tall as 150 feet.

♪ ♪

The area covered in ice today
was once a massive forest

that stretched all the way up
to the edge of the Arctic Ocean.

Water ran off the land, forming
a surface layer of fresh water.

Instead of sea ice,

the North Pole was covered in
subtropical floating ferns.

♪ ♪

The Ellesmere we see today
looks very different,

but one peculiarity of the
Arctic summer remains the same.

One of the things about working
in the Arctic

is that the sun
literally never goes down.

It just goes around
and around in the sky.

That means there's
never any night,

and it never gets dark.

It's midnight right now--
take a look outside my tent.

Yep, it looks pretty bright
out there.

But-but I want to go to sleep,
so I'm going to go to sleep.

Here's how we do it.

Close the tent.
(pulling zipper)

Zip up the sleeping bag
and put on the face mask.

Good night.

In the morning, I head out
to join Jaelyn Eberle.

While I've been out looking
for fossil plants,

she's been hunting
for the animals that lived

in this warm polar swamp
50 million years ago.

Maybe through here?
And then up and over

and then back down
the other side.

I think so, yeah.

I'll put my boots on then.
Do you have boots?

I've got sandals.

All right, you're better
than me, I'll tell you.

(both chuckle)

We're going.

Okay, so it's cold.

I wouldn't want it
to be any deeper.

There we go.

You beauty.

All right.

Jaelyn's spent many summers
up here searching for fossils.

I do love the Arctic.

I've been coming up for...
oh wow,

I think ten field seasons now.

And almost everything you
pick up is undiscovered,

that's something that

doesn't, to me, ever get old.

That's exciting.

Searching for animal fossils in
the permafrost is really tough.

They're little bits
and pieces,

so they're not going
to show up a lot.

I think we're going
to need to crawl.

The annual freezing and thawing

break up the bones
into tiny fragments.


Eh, probably not, but, um,

could be a little
piece of bone.

Tooth of anything right now

actually would be great,
'cause they're pretty rare.

Even the bits and pieces are
pretty rare at this site.

After around four hours
of crawling about--

today, nada.

But over the years,
Jaelyn's team

has unearthed a fantastic
collection of creatures.

These guys are
pretty impressive.

Looks like two jawbones
missing their teeth

Yes, that's right.
So it's a...

And those come from
a mammal called Coryphodon,

and there are no
Coryphodons living today.

An analogy
you could use today

would be something like
a pygmy hippo.

And living in the rivers
and stuff like that?

Yeah, probably,
living in the rivers

and the swamps and

on aquatic plants and...

Coryphodon was
a hippo-like mammal

with short tusks
and a tiny brain

that weighed just three ounces.

Standing around three feet high
and eight feet long,

Coryphodon was a vegetarian

that rooted around the swamp
for its food.

have found fossils

of all sorts of animals here.

With warm temperatures
all year round back then,

there were turtles, tapir,
and even alligators.

♪ ♪

The fossils we find
here on Ellesmere Island

tell a remarkable story.

The Arctic was warm and wet,
with no ice in sight.

It was filled with plants
and animals

similar to ones
you might see today

in the swamps of Louisiana
or the Amazon.

The frozen north was a
completely different world

50 million years ago.

♪ ♪

And that's got
to make you wonder,

"If it was like this
at the North Pole,

what was happening at
the other end of the earth?"

10,000 miles south of Ellesmere

is the world's
wildest continent,

the most extreme place
on the planet.

There's one word
that describes Antarctica,

and that word is ice.

And the entire continent is
one ice-bound mass.

♪ ♪

In fact, 90% of the ice
on planet Earth

is here in Antarctica.

With a record low temperature of
minus-128 degrees Fahrenheit,

this continent is officially
the coldest place on Earth.

Even in summer, the temperatures
rarely ever get above freezing.

Antarctica is the size of all
the United States plus Mexico,

and it's covered
with a sheet of ice

that in some places is
almost three miles thick.

That's a lot of ice.

♪ ♪

But has Antarctica always been
deep-frozen like this?

To investigate,
I've come to Nelson Island,

off the Antarctic peninsula.

I'm joining paleontologist
Marcelo Leppe

from the Chilean
Antarctic Institute.

Marcelo's taking me
to a small island...

one of the few spots
with exposed land,

where he and his team have
been hunting for fossils.

After a week,
we didn't find a,

a fossil.

you looked for a whole week
and you found nothing?

We started to make a hole
in the ground,

just close to the,
to the sea,

and we found the-the outcrop.
Just by accident.

Do you think we can
find it today?

Uh, I-I hope so.

(both chuckling)

♪ ♪

The outcrop we're after is
only accessible at low tide,

so we've had to time our landing
just right.

Let's drop our gear here,

It's cold.

Yeah, it's Antarctica.
It should be.

(both chuckle)

♪ ♪

Just like on Ellesmere Island,

the bedrock on this beach is
tens of millions of years old.

(shoveling continues)

♪ ♪


Not a great one,

but it's definitely
a fossil.

We're in the right spot.

Crawling around on this
rocky landscape today,

it's hard to believe that
anything could ever grow here.

Antarctica has no trees at all.

Just mosses and lichens

and only two species
of flowering plants.

So you find
a fossil plant here,

it's telling you about
a very different world indeed.

But after just ten minutes
of digging...


We find something incredible.

Ah, wow!

Just like that.

A beautifully preserved leaf
from an ancient beech tree.

That one looks like
a modern beech.

It's the size of
a beech leaf, wow.

And it's large.

You can see that it's
the middle section.

There's the main vein,
the lateral veins.

Leaf would have been about
that long and about that wide.

And these very
straight secondary veins

are typical of
the beech family,

a northern,
temperate-forest tree,

a tree that's deciduous,

you see it growing in
New York City and in London.

And here is a chunk of it

from the beach
in Antarctica,

next to an iceberg,

on a very cold day.

Oh, look at that one.

That's great.

We're finding
forest tree leaves,

and they're found in
great numbers, all together

like leaf litter, implying
the forest was right here,

where we are standing--
or kneeling, right now.

(digging and brushing continue)

These fossil leaves reveal
a very different picture

of Nelson Island from
the landscape we see today.

83 million years ago,

this frigid place was covered
in a verdant forest

of southern beech,
ginkgoes, and tree ferns,

with mild temperatures rarely
dropping below freezing.

♪ ♪

And it wasn't
just this one spot.

I've just been on the beach,

found these amazing
fossil leaves.

These leaves, on the other hand,

are from a different place,
they're from southern Chile.

We're here,
here's a 700-mile gap,

and here is southern Chile.

This leaf here is from
southern South America

and is very characteristic
of the beech family.

As is this leaf
from Antarctica,

that we just collected
this morning.

So we have Chile and Antarctica,

pretty strong evidence that
these two places were connected.

We've known for a while

that Earth's crust is broken
into plates

that slide and collide
on top of hot, flowing rocks

deep beneath the surface.

Evidence like matching fossils

from South America, Antarctica,
and even Australia

tells us that these
three continents

were once joined together and
covered with a vast forest

stretching across
the South Pole.

♪ ♪

So, with all these trees
and a nice, warm climate,

what kinds of creatures lived on
this ancient southern continent?

Although fossils have been
found in Antarctica,

some of the biggest clues
are in Patagonia.

♪ ♪

At a remote site in Argentina,

a team of paleontologists
has just discovered

the remains of a forest dweller.

Here we are.

It must have had
a very large appetite.

This guy was, you know,
all scattered around,

all the bones.

The leader of the
expedition is Diego Pol.

We found the entire forelimb.

Okay, that's great.
And the other hind limb

and then some vertebrae

and the tail and so on.

All these parts add up to a
69-million-year-old dinosaur.

Let's go.

I'm just going to start
chipping away at this thing.


The key thing to not
destroying a dinosaur fossil

is do not let a paleobotanist
in the dinosaur quarry.

'Cause we tend to break
big rocks with pickaxes

and sledge hammers,

and these guys use
little, tiny picks

and brushes.

We're not trying
to expose the bone,

we're just trying
to undercut it gently

and get the rock removed.

So, I'll just carefully pick
around here at the base.

(picks clanking)

See how delicate I am?

You know, I'm doing
a beautiful job

of carefully extracting this
bone from its rocky tomb,

where it's lain undisturbed
for 69 million years.

I need to watch my hammer
with this specimen,

because it's pretty unique.

This is a dinosaur
without a name.

It's a new species of dinosaur,
and that's a really cool thing.

Diego's team has just discovered
a new species of titanosaur...

(pick scraping)

...a plant-eating dinosaur

with a long tail,
long neck, and small head.

Titanosaurs could become so big

because there was
so much vegetation.

And some species grew to be
the biggest animals

that ever walked on land.

♪ ♪

Some people estimate these
animals were eating, like,

a thousand pounds
plant material per day.

A thousand pounds a day?
A thousand pounds a day.

Like half a ton
of vegetables a day.

The original vegans,

Oh, my God.

Yeah, these animals probably
had a pretty big home range.

They'd go and destroy
one forest for a while

At a time...

Then go destroy
another forest.

And then come back.

With South America and
Antarctica connected

into one continuous continent,

these munching monsters had
plenty of forest to roam

in search of a good meal.

Did they have any titanosaurs
from Antarctica?

Yeah, actually,
there is one.

I mean, they found
a single bone.

It was found in
Antarctic peninsula.

One bone?
A single bone.

A tail bone.

What a lucky find that was.

That's amazing.

So, we're really looking
at an animal

that could have walked

to Antarctica.
Absolutely, this...

For a good sandwich.

(picks clinking)

This fossil is a fantastic find.

But in 2014, not far from here,

Diego and his team uncovered
a creature even more awesome.

♪ ♪

This is the femur
we collected yesterday.

It's big, but it's nothing
compared the biggest dinosaurs.

This one is the
largest one ever found.

All right.

Check out the size
of those vertebrae.

Each single backbone is
about four feet tall.

I'm a big guy...

but this bone dwarfs me.

I'm lying next to
the eight-foot-long

titanosaur thigh bone
that Diego's team unearthed.

♪ ♪

And just in case you can't
picture how big this was...

This thing is
absolutely immense.

There's a life-size model
of this 75-ton beast

that greets you when you drive
in from the local airport.

I've seen the bones,

but I haven't seen the
reconstruction before.

♪ ♪

Patagotitan, the
world's largest dinosaur.

It's incredible to realize

that animals like this
once roamed

between South America

and Antarctica.

So what's going on here?

Dinosaurs near the South Pole...
(dinosaur bellows)

and swampy forests
in the North--

were the poles really
that warm back then?

Or could there be
another explanation?

What about those tectonic plates

that move the continents around?

Could this be the reason that

we find these
warm-weather fossils

in the polar regions today?

Sometimes people tell me
that, "Well, wait,

"plate tectonics mean that
landmasses can move around,

"and maybe those fossils
you're finding

"were deposited at mid-latitudes

then brought to the Arctic
by continental drift."

It's a good thing
to think about,

but we actually know
where the continents were

when they were there.

We actually know
those positions.

And we know for sure

that Antarctica was down
by the South Pole

and the lands at Ellesmere
Island were up by the North Pole

when they were forested
by these warm forests.

So, it's a good idea,
but the science says, "Nope,

there really were
polar warm forests."

♪ ♪

The entire planet
really was warmer,

and all the polar ice
we see today didn't exist

tens of millions of years ago.

That is pretty weird.

But there's an even weirder part

of the story of ice
on this planet.

This time, the evidence
isn't at the poles.

♪ ♪

It's in the hottest place
on Earth.

♪ ♪

Welcome to Death Valley.

I'm here at the National Park's
official weather station

at the appropriately
named Furnace Creek.

Ranger Alex Rothermel is letting
me take today's measurements.

So this is our rain gauge.

It hasn't rained
in a while.

Ow, it's always...
Is it hot?

Yeah, it's metal.

Hold this?
Okay, yeah.

We'll see if there's
any water in it.

Ooh, that is certainly hot.
Yeah, it's metal.

All right,
there's nothing in it.


(both laughing)

This is the
temperature gauge.

I see,
so it's in the shade.

Let's see on this...

I'm reading like
103 and a half,

is that what you get?

And you check this
every day?

Yeah, we do, and
then we record it.

July to August, it's been
something like 28 days

over 120 this year.

So you've got a pretty
legitimate claim

to be one of the hottest places

on the planet.

Oh, we are the hottest
place on the planet.

All right, so,

you actually hold the
all-time record here?

And you love this place?

This is the best
national park.

It's the best place on Earth,
I love it here.

And the hottest place
on Earth.

♪ ♪

But has it always been that way?

Geologist Francis Macdonald
has spent many years

battling the heat
in these desert hills,

searching for clues
that might reveal the secrets

of Death Valley's past.

Why I really like
coming out here

is that all of this
raw rock.

Here in Death Valley,
you're not limited by rock,

you're limited by how far
you can walk out there

in this hot weather.

The rocks here date back to a
time way before the dinosaurs,

long before trees
or even plants existed.

Why, in this rock formed
640 million years ago,

do we have these rocks
within the rock?

This one's a granite.

And it's surrounded
by this pink sand

and silt and mud.

And here's another
quartzite piece.

Here's a-a piece
of carbonate.

They're different
rock types,

they're telling you about where
they originally formed.

These rocks came
from a whole bunch

of different places?
All over the place.

What could have brought this
strange mixture of rocks here

and encased them in silt?

We need a process

that's just going pick up rocks
from all over

and dump-truck them
into a pile of mud and sand.

And we do know one process
that does that,

and that's glaciers.

So, we're sitting here
in Death Valley,

and it's about 95 degrees,
and you're pointing to a rock

and saying that

it was deposited by a
giant slab of glacial ice?

So work with me here

a little bit.

You've got to use your,
use your imagination.

♪ ♪

What these rocks tell us

is that 640 million years ago,

the climate here in Death Valley
must have been much colder.

So cold that what's now
a scorching desert

was covered in
giant sheets of ice

as big as you'd find
in the polar regions today.

The idea of an ice-covered
Death Valley is strange enough.

But these rocks tell us
something even more astonishing

about how far the ice extended.

Even though here
we're sitting today

at, say, about 35 degrees
north of the equator,

we know 640 million years ago
this was far further south,

and it was situated
very close to the equator.

♪ ♪

Earth's tectonic plates
have shifted quite a bit

over the last 640 million years.

Back then, Death Valley was
part of a huge landmass

that sat right on the equator.

So, if the ice made it to here,

it likely stretched all the way
from both north and south

until it met in the middle,

making the entire planet
a snowball Earth.

♪ ♪

For me, this is a pretty
mind-blowing realization.

Wherever you are in the world,

by uncovering evidence
in the rocks and fossils,

you can travel back in time

and reveal that Earth's climate

has undergone
incredible changes.

♪ ♪

You have to split a huge amount
of rocks to find a fossil.

♪ ♪

The only way to find them is
just to work your way down

through the layers.

♪ ♪

So, just sort of chop
a staircase down the hill.

Sometimes an entire hill.

(pick clinking)

In fact, we can use the
chemistry of rocks and fossils

to find the temperature
at the time those rocks formed.

Piecing together data
from sites across the globe,

scientists can build
a temperature timeline

going all the way back
to 500 million years ago.

Laying out Earth's temperature
like this,

you can see a pattern.

The climate fluctuates between
long periods when it's warm,

with no ice at all--
hothouse worlds--

and cooler episodes

with ice caps at
at least one of the poles--

icehouse worlds.

The hot climates are three times
as common as cold climates.

And yet, perhaps surprisingly,

we live in one of the
icehouse worlds today.

♪ ♪

Today, we live on a planet
that has ice at both poles.

We think that's normal,
'cause that's our world.

But when we look
at the fossil record,

we realize that our planet
has only had four episodes

where there's been glacial ice.

Only about 25%
of the last 500 million years

has our planet been
like it is today.

♪ ♪

So, why is this?

Why has Earth spent so much
of its past as a hothouse--

much warmer than it is now?

We can find clues from a close
neighbor in our solar system.

I'm looking at Venus.

It's the third-brightest thing
in the sky

after the sun and the moon.

It's also one of
our nearest planets.

But Venus has
a very unusual feature--

it's about 800 degrees

on the surface of Venus;
it's hot enough to melt lead.

♪ ♪

Why is Venus so hot?

It is a little bit closer
to the sun than we are,

but not enough to explain
the huge temperature difference.

♪ ♪

We found the answer
back in 1967,

when the Soviets sent the
Venera 4 space probe to Venus.

Just before it was crushed by
the huge atmospheric pressure,

the spacecraft beamed back
its precious data.

It had identified
one critical component

of the planet's atmosphere.

The reason Venus is so hot

is that its atmosphere is
composed almost entirely

of carbon dioxide, 95%.

♪ ♪

This thick atmosphere,
rich in CO2,

acts like an insulating blanket.

Most energy from the sun
passes through this layer,

but when it radiates back
from the planet's surface,

the carbon dioxide
traps the heat.

This is the greenhouse effect,

which drove Venus to get
hotter and hotter.

♪ ♪

It didn't just heat up--
it boiled.

Venus is a clear example of
a runaway greenhouse climate,

what happens when you get
too much carbon dioxide

in your atmosphere.

So if carbon dioxide
can warm up a planet,

is this what created all those
hothouse worlds in Earth's past?

(insects chirping)

How can we know how much CO2
was in the atmosphere

millions of years ago?

(gate creaks)

♪ ♪

The answer may be hiding
in some very special leaves.

And deep in a forest just
outside Washington, D.C.,

is an experiment to try
and unlock their secrets.

The experiment is being run
by one of my former students,

Rich Barclay.

Hey, Rich.

Oh, hey, Kirk.
How's it going, man?

Good to see you.
Hey, welcome to the Fossil
Atmospheres Experiment.

He's investigating
the ginkgo tree.

The ginkgo is pretty special,

because fossils tell us that
this species has survived

almost unchanged for
the last 200 million years.

Since all plants use
carbon dioxide to grow,

a plant that's been around
that long

might be able to tell us
how CO2 levels have changed.

Rich is growing them in
custom-made atmospheres

to see how different amounts
of carbon dioxide

affect the leaves.

So each of the chambers
in here

has a-a different
CO2 concentration.

This is a control tree,
at 400 parts per million.

the next treatment up.

800 parts per million.

The final tree
at 1,000 parts per million.

Can we go in there?

We can go into the,
into the chamber, yeah.

This tree is growing at
1,000 parts per million.

We put a tag around
the branch,

so you can just wrap
that around the branch.

And so I take the leaf
that's right above that one?

Yep, that one
right there is fine.

So, what we do
with the leaf

is we take it
back to the lab,

and we can look
at the-the details

of the leaves
under a microscope.

The leaves all look
pretty much the same to me.

But put them under a microscope,

and you see something
really cool.

This is the surface
of a ginkgo leaf

from this experiment.

You have to go in about
200 times magnification.

Now, you can see that the leaf
from today's atmosphere

is perforated with tiny holes
called stomata.

This is where the plants take
in the carbon dioxide

they need for photosynthesis

And what Rich is discovering

is that adding extra CO2
does something striking

to the stomata.

Then I've got this one.

And it's really obvious
to me

that there's far fewer
pores on this one.

That one's from the
1,000-ppm chambers.

As CO2 increases,
they don't need as many stomata,

they can become more efficient.

More CO2, less stomata.

More CO2, fewer stomata.

This happens with
all kinds of plants

that take in carbon dioxide.

But because ginkgoes have been
around for 200 million years,

they can preserve a snapshot
of Earth's CO2 levels

in the deep past.

This is a fossil of ginkgo.

It sure is,
it's like,

it really looks like ginkgo,
I mean, this leaf and that leaf

are almost identical.

How old is this one?

This fossil is
56 million years old.

And then when you take that, you
can put it under a microscope

and see the exact same features
as you find on the modern.

Right there on the fossil is

the pattern of stomata:

many fewer than
on the leaves today.

Fossils from all
around the world

help us estimate CO2 levels

going back more than
400 million years.

If we look back on
our temperature timeline,

we see that when CO2 levels
are high, it's hot.

And when CO2 levels drop,
it cools.

When this ginkgo was alive
56 million years ago--

about the same time
that swampy forests

were growing on Ellesmere Island
in the Arctic--

the amount of CO2 in the air

was roughly four times
what it is today.

So, where did all
that CO2 come from?

♪ ♪

One possibility is from
deep inside the Earth.

Here at Mammoth Lakes
in California,

the ground is belching out
steam and gas,

and the pools are literally
boiling beneath our feet.

Because right under
these mountains

is one of the largest

in America.

Wow, this place is
pretty cool.

Yeah, it's amazing,
isn't it?

Geologist Kayla Iacovino

studies volcanoes
all over the world.

We don't want to get
too close

to the edge
of the water here.

The ground is quite

If you did fall through,

that would be very,
very bad news, so...

Okay, I won't do that.

Do you want hold on to this,
carry it off...

Yeah, I hold this, right?
All right, here we go.

Kayla's taking measurements

to monitor the
volcano's activity,

starting with temperature.

It's like trout fishing.

You sneak out
to the edge of a creek,

you lower the line
over the edge...

And you wait, yeah.'re waiting.

Next, she needs to measure
the composition

of the gas rising
out of the ground.

So you want to use
this little tiny mud pot?

Yeah, this looks
like a good one.

This is a CO2 meter.

and it can tell us
the concentration

of carbon dioxide
in the gases that we measure.

The level of carbon dioxide
in the air

is around 410 parts per million.

But the reading from the mud pot
is much higher.

There we go,

3,000 parts per million
of CO2

The magma that sits deep
within the Earth

contains lots of carbon.

When this magma gets close to
the surface-- at a volcano--

that carbon is released
into the air as carbon dioxide.

Kayla is part of
a team of scientists

measuring how much CO2
is being released

from volcanoes
all over the world.

This is basically volcano
by volcano by volcano?

That's how it's done,

they actually look
at individual volcanoes?

Yep, and they kind of
add them up together.

Even volcanoes encased in ice,

like Mount Erebus in Antarctica,

are spewing out carbon dioxide.

Mount Etna in Italy is built
on carbon-rich rocks

and belches out more CO2
than almost any other volcano.

When geologists estimate
all the CO2

coming out of volcanoes today,

the total is around
300 million tons a year.

That might sound like a lot,
but it's not enough

to significantly change
global temperature.

The amount of CO2
coming out of volcanoes today

just doesn't even come close.

It's tiny compared
to the amount of CO2

that humans are putting out,

for example.

What it takes to
actually warm the planet

from a volcano,
from volcanic CO2,

is a massive amount
of CO2 put out

over a very long period of time.

♪ ♪

Today, Earth's volcanoes
are relatively quiet

and aren't cooking up
a whole lot of CO2.

But in the past, it's been
a very different story.

♪ ♪

This ten-mile-wide basin
looks peaceful today.

But many thousands of years ago,

it was the site of
a huge eruption

that spewed out 150 cubic miles
of lava and ash.

And this is just one spot.

We know that at certain points
throughout Earth's long history,

volcanoes and other
geologic activity

released much more CO2
into the atmosphere than today.

And sometimes this continued for
millions and millions of years.

That's what geologists think

must have kept the planet
so warm

during all those long periods
when Earth was a hothouse,

giving us warm polar forests
teeming with life.

♪ ♪

But if volcanic activity keeps
releasing carbon dioxide,

why hasn't CO2 built up
in our atmosphere,

making our planet overheat
like Venus?

♪ ♪

Earth must have something
Venus doesn't--

a way of taking carbon dioxide
out of the atmosphere

and putting it somewhere else.

It turns out here in
the mountains of Alaska,

I'm experiencing it.

What's falling on my hands
and my face right now

is rain in the mountains, and
it's got dissolved CO2 in it.

Rain absorbs CO2 from the air,

and then the raindrops
combine to form streams

that eventually become
fast-flowing rivers.

The dissolved CO2 makes the
water slightly acidic,

helping it erode
and weather rocks,

releasing elements like calcium,
magnesium, and silicon.

♪ ♪

Riding these rapids
gives me a sense

of just how powerful
this river is.



Off the edge, yes!

It's like a saw
that's cutting right down,

the canyons are
just cutting their way

into the mountain range
and just chopping them in half.

The water I'm rafting on now

is full of just the
right chemical elements

that will help lock up carbon.

♪ ♪


That was awesome.

Eventually, all the water
in this river

will end up in
the Pacific Ocean,

and with it, all of
those dissolved minerals.

Once they reach the ocean,

the dissolved elements are
taken in by tiny sea creatures

and used to build their shells.

Over millions of years,

these shells drop down
to the sea floor,

forming layers of limestone,

locking the carbon that used
to be in the atmosphere

into rocks.

It's the final stage
of a process

that's been driving our climate

for millions of years--
the carbon cycle.

There's a finite amount
of carbon on planet Earth.

When that carbon is
in the ground,

locked up in rocks or sediments,
then the planet is cool.

And when that carbon is up
in the atmosphere

as carbon dioxide,
the planet warms.

And the climate history of
our planet is this tug and pull

between carbon in the ground
and carbon in the atmosphere.

♪ ♪

We can see how
this balance plays out

on the planet's
southernmost active volcano,

Mount Erebus.

Here in Antarctica,

this mountain terrain is encased
in ice year-round.

There is no rain,
and there are no rivers.

The weathering part of the
carbon cycle is stalled.

Meanwhile, the volcano
continues to spew out CO2.

♪ ♪

Mount Erebus is a window
into what it was like

640 million years ago
during snowball Earth,

when the planet was covered
in ice.

With CO2 from ice-covered
volcanoes building up,

the planet eventually got warm
enough to melt the snowball.

If adding CO2 is
how we melt a frozen planet,

then how do we freeze
a warm one?

♪ ♪

About 50 million years ago,

after more than 200 million
years in a hothouse,

carbon dioxide levels
started to drop,

and the Earth began to cool.

Eventually, gigantic
ice sheets began to form,

first in the Antarctic,
and then the Arctic.

A new era had begun that
we still live in today--

the icehouse.

But where did all
this ice come from?

How do you build an ice sheet
a mile or two thick

when you start with nothing?

To take a closer look
at how ice sheets form,

I'm going to climb down
into a glacier.

Do that.

With a little help from
mountaineer Brian Rougeux.


Wish me luck.

Have fun down there.

There's actually an overhang.

Yeah, get your butt

as low as you
feel comfortable

and then take
a step down.

First, I have to hop over
this year's snow layer.

It's quite a step.


(Johnson laughs)

It's like a foot-and-a-half
overlap, man.

Nice work.

Beneath the overhang
are snow layers

from previous years.

Pretty amazing to be hanging
out here in this ice world

and to imagine how this ice even
got here in the first place.

Each year, it snows
in the winter.

In most places,
snow melts in summer.

But here, the summers
are so cold,

the ice never fully melts,

and next year's snowfall
piles up on top.

Snows falls
in the winter,

and it stays there
throughout the summer,

and the next summer,
and the next summer,

and the next summer.

Over many winters,

the snow pile gets higher
and heavier,

compressing the snow.

(snow pack crunches down)

The weight of the snow
pushes down

and compacts the
underlying layers into ice.

♪ ♪

Over thousands of years,

these layers build up
until they form an ice sheet,

and that's exactly
what happened at our poles.

When carbon dioxide levels fell,

around 50 million years ago,

the temperature started to drop,

and ice eventually took hold
at the bottom of the planet.

Our icehouse world started
34 million years ago,

here in Antarctica.

♪ ♪

But carbon dioxide
wasn't the only culprit

responsible for the deep freeze.

It turns out there was something
special about Antarctica--

its position on the globe.

In that direction, 700 miles,

is the southern tip
of South America.

In between me and South America

is what's called Drake Passage.

This stretch of water

is one of the most feared
passages in the world.

It's got tremendous storms.

♪ ♪

I feel really lucky
to be on a boat

where I'm not actually seasick,

'cause it's a nice flat,
calm day here

but if we wait just a few hours,
things can get pretty ugly.

The reason it's almost
always stormy here

is because there's
a powerful current

that constantly runs from west
to east through this gap.

But it hasn't always
been this way.

We know from fossils dating back
to the dinosaurs

that Antarctica used to be
connected to South America.

But around
30 to 40 million years ago,

the giant tectonic plates
beneath the continents

gradually pulled apart...

eventually creating
Drake Passage.

Once Antarctica was free,

powerful currents
started circling around

the entire continent.

This is the circumpolar current.

And it keeps the cold
in Antarctica,

and it keeps the warm
from Antarctica.

So it kind of keeps the
refrigerator door closed

on this mighty, icy continent.

Because the current
was then allowed

to go right around
the continent--

endlessly around,
endlessly around,

keeping it cold and frozen.

We think it's right
around this time

that summers down here
got colder.

So cold that the winter's snow
wouldn't melt.

Beginning at the South Pole,

the snow piled up
and glaciers grew,

and slowly Antarctica
started to freeze over.

But it would take
millions of years more

for the Arctic to get its ice,

mainly because
at the North Pole,

there is ocean rather than land,

and it's hard to form
ice sheets on water.

But eventually it cooled enough

that ice began to form
on the surrounding land.

And once it was there,
ice sheets could spread quickly,

sometimes, even reaching
Seattle, my home town.

This is my old neighborhood.

I lived here when
I was eight years old

until I went to college.

I used to play
in these forests--

my house was only
about a block away.

Walking around now, I see
things I missed as a kid--

clues in the landscape that
tell me what happened here.

Plopped right in the middle
of a nearby neighborhood

is something remarkable.

This is a huge rock.

I grew up in Seattle,

I never heard about this rock--
that kind of bothers me.

It's a gigantic boulder.

It's got trees growing
around it,

so it's been here a long time.

It makes you ask the question,
"How did it get here?"

To understand how,

you need to get the big picture.

And luckily, I know a place
with a spectacular view,

a spot I dreamed of climbing
when I was a kid.

Oh, yeah.

The top of the
Seattle Space Needle.

This is fantastic.

You need to clip me
in the back there?

Okay, I'm ready to go.

Oh, boy.

I'm 600 feet above the street,

no place to be if
you're scared of heights.

But for me,

it's the perfect way to
check out the city's contours.

♪ ♪

Oh, man, what a great place.

This is really phenomenal.

I never thought
I'd be able to climb

to the top of the Space Needle
in my hometown.

♪ ♪

And you really get to
understand the landscape

when you view it from above.

You can see things you don't see
when you're at human scale.

The hills have a grain to them--
north to south in this case.

They're all pointing
in one direction,

as if something powerful
flowed over them.

This is a landscape
created by snow,

accumulating year after year,

to form a massive,
moving ice sheet,

big enough to shape hills

and dump a huge boulder on
what's now the edge of the city.

Around 17,000 years ago,

an ice sheet once towered
3,000 feet,

five times the height
of the Space Needle.

Seattle was at the edge
of a vast ice sheet

that covered half
of North America.

In places, the ice was
over two miles thick.

This landscape is my
landscape of my childhood,

but it's also the
landscape of glaciers.

It's just killer.

What a day in Seattle.

When you think about this
whole area buried in ice,

it makes you wonder,

"How did anything live
through this ice age?"

♪ ♪

To find out, I'm heading north
to dig up some clues

buried at the edge
of the Canadian Yukon,

home of the Klondike gold rush.

They're still mining gold
here today,

but not with pans and shovels.

Now they use giant water jets

that make a firehose
seem like a garden sprinkler.

These melt away the frozen earth

to uncover the gold-bearing
gravels below.

But there are other treasures
buried here--

ice age bones.

Paleontologist Grant Zazula is
always on call to retrieve them.

Hey, Logan, I'm Grant.

I'm Kirk.

I'm Logan.
Good to meet you.

Can I have a go?
Oh, yeah.

What do I do,
just basically hold on?
Yeah, nice, slow movements.

Who doesn't love blasting
a fire hose at something?

This is really fun!

There's a buried gravel layer
that's got the gold in it.

And the silt that's frozen
has all the bones in it.

So there's a layer of bones and
a layer of gold-bearing gravel.

And if there wasn't gold
in this gravel,

then we'd have no ability to
actually find any of these bones

because there's no way
you're going to get through that

with a trowel, you know?

Oh, look at that, that layer of
ice is amazing I'm exposing.

So that's 25,000- or
30,000-year-old ice in there.

25,000-year-old ice?

This permafrost layer is ground
that's been permanently frozen

since the last ice age.

Once the hose breaks it up,
we go hunting.

Oh, here we go.

Oh, yeah.


Little bit of a scapula.

I think it's a horse.

Oh, wow.

Permafrost is amazing stuff,

because it's cold enough

to preserve the tissue
of buried animals

as if they were in a freezer.

We've got a big beast over here,

It's a big piece of mammoth.

Oh, it's huge.

Yeah, it is.

You'll have to
shovel on your side.

Oh, it's still...
it going in quite a ways.


Smells like the Pleistocene.
Smells good.

It's the pelvis
of a woolly mammoth.

This is still bone.

It's not like a dinosaur fossil

where it's been replaced
by minerals.

This is still actual bone

even though it's thousands
and thousands of years old.

We'd call it a fossil.

It's just not
a petrified fossil.

Yeah, absolutely.

What have you got there,

It's probably
a moose antler...

Oh wow, a moose!

Here's a tooth.

Is it a horse tooth?
Oh yeah.

There are fossils
everywhere here.

And this is...
Oh, horse tibia,
horse tibia.

This is like name that bone.

Oh my God, this is...

This is an
upper horse molar.

I'm not leaving.
(laughs): Yeah.

I am not leaving.

Oh, what'd you get?

It's the chunk of
the skull of a wolf.

Wolf! Hey!
First carnivore.

Three beautiful molars in here.

This is like taking candy
from a baby.

Don't take my candy.

Oh, here's your candy.

(both laughing)

Oh, there's a tusk, right?

That's really cool.
It's solid ivory in there.

We just found the tusk
of a woolly mammoth.

There's the...

Just walking around
carrying a mammoth tusk.

Like nothing's going on,
on a summer's day.

These are blades of grass that
were collected by an ice-age

Arctic ground squirrel,
30,000 years ago.

So this is a 30,000-year-old
piece of ground squirrel poo?

Yeah, absolutely, yeah.

I think everything that
could be found has been found.

♪ ♪

It's an impressive haul.

♪ ♪

We've got bison,
mammoth, horse,

moose, caribou,


Carnivores-- a wolf and a lion.

So what can these bones tell us

about life
in the last ice age?

They're so perfectly preserved,

we can figure out
how the animals

adapted to survive
the harsh, cold climate.

(pen scratching)

Take woolly mammoths.

They would have stood
about ten feet high

with giant tusks
up to 12 feet long.

They evolved from
the same ancestor

as modern-day elephants.

But to survive
the colder climate...

(woolly mammoth bellows)

woolly mammoths developed
long hair and small ears

that prevented them from losing
too much of their body heat

through their skin.

There was no shortage
of mammals living here.

228 bones.


That's a... that's a big haul
for a single day.

228 bones in four hours.
Yeah. Yeah.

♪ ♪

So, how did all these
different mammals survive here

when we know that much of North
America was buried under ice?

To thrive here
they needed to eat--

and this means something
must have been different

in the Yukon.

When you have animals
like woolly mammoths,

and bison, and horses,

which are animals that are
big grazers.

They need to consume
a lot of grass.

This is telling me
that this landscape

is covered by
expansive grasslands.

And there's like, there's no
Arctic grasslands today at all.

No, absolutely,
this is an extinct ecosystem.

This is an ecosystem
that we have no analogues to

in the present-day environment.

But if there was grassland here
during the Ice Age,

that means--
unlike the rest of the north--

this part of the continent
was ice-free.

Places like Boston, and Seattle,
and Chicago were covered by ice.


Most of Alaska was not,

and the part of the Yukon
that we're in,

was not covered by ice.

it's telling us that it was

almost too cold and too dry
to support glaciers.

While most of North America
was icing over,

here in the far northwest,

there wasn't enough snow
to build an ice sheet.

And with more of the planet's
water now locked up in glaciers,

sea levels fell,

exposing new ice-free land
between Alaska and Siberia--

the Bering Land Bridge.

This was a great place

to be during the ice age.

There was glaciers only probably
100 kilometers away,

but this is a little...
little bit of an ice age oasis

for these animals because
it was highly productive.

♪ ♪

What's now forest and tundra

would have been more like a
prairie covered in wild grasses,

and in summer,
small flowering plants,

like poppies, buttercups,
and sage.

♪ ♪

The ice eventually started
to retreat,

leaving an ice-free corridor
that allowed ice-age animals,

and later people, to migrate
between Asia and North America.

♪ ♪

We still live
in an icehouse world,

with ice at the poles,

but it's obviously
nowhere near as cold

as the most extreme ice ages.

So, what's going on?

We can find the answer,

thanks to a large meteorite...

that struck the Arctic

three and a half million
years ago,

creating a ten-mile-wide crater
that became a lake.

Ever since the lake formed,
even when it was covered in ice,


all the stuff floating or living
in the water--

has been settling down to the
lake bottom, forming layers.

Locked inside these thin layers

is a detailed history
of our current icehouse,

stretching back three and a half
million years.

Today, Lake El'gygytgyn
lies 60 miles north

of the Arctic Circle in Russia.

Back in 2009, scientists set out
to unearth the secrets

buried deep beneath
the frozen lake.

But it wasn't easy.

And good.

One more.

Julie Brigham-Grette

was a leader
of this extreme expedition.

There's lots of lakes
in the world,

but why that one?
Turns out
this meteorite hit

right smack dab in the largest

unglaciated area
in the entire Arctic.

We put a 100-ton drill rig
out on the lake ice

and then drilled
from that platform.

The team only had
a short window,

in the very depths of winter,

when the lake ice was strong
enough to support the drill rig.

Over 50 scientists
from four countries

camped out on the frozen lake,

working in temperatures often
30 to 40 degrees below zero.

♪ ♪

This was the one of the most
difficult and dangerous

drilling projects
ever attempted.

Slide it just
a little bit.

So I feel privileged
that Julie's letting me handle

these precious cores.

This is a nice look... okay.

So this goes around
this side.

This 1,000-foot length of core
is the longest continuous record

of Arctic climate we have.

So a single layer, like this
little layer right here,

that's telling you
what was happening

to the climate on that day.


We're extracting
the climate history.

So you've basically got
a mud record

of 3.6 million years of climate
in the Russian Arctic.


This is just a small part,

but we have the entire core

represented here
in photographs,

which allow us to then look
at the entire core over time.

That's great,
you can scroll through it.

♪ ♪

Layer by layer, Julie has pieced
together an astonishing story.

We can zoom in on this

older part of the record.

Pollen, spores, and other
fossils in the mud layers

from three and a half million
years ago,

reveal a climate warmer
than today.

Back then, this Arctic lake
was surrounded by hemlock

and hazelnut trees,

and the water was teeming
with life.

Creatures tunneled into
the lake bottom,

stirring up the mud layers,

and leaving them smooth.

But then here,
around 2.6 million years ago,

there's a sudden change.

These dark striped layers
come from a time when the lake

was completely frozen over
all year round,

with no plants or animals
living on the lake bed.

You could have ice skated
across the lake in July.


That's cold.

Here, Julie can pinpoint
the moment when this part

of the Arctic froze over
2.6 million years ago.

This isn't found anywhere
earlier in the lake history.

All of a sudden, boom,
comes the first glaciation.


And here's what's really cool.

As we move along the length
of the core,

these contrasting periods
of warm and cold

pop up again and again.

The temperature swings between
very cold periods,

called glacials,

when the ice sheets extend
down over the continents,

and warmer inter-glacials,
like today,

when there's still ice,
but it's confined to the poles.

But what causes
so much variation

within the icehouse world?

Why does the ice grow and shrink
in such a regular rhythm?

♪ ♪

(birds twittering)

There's some changes
in the Earth's climate

that are periodic-- we don't
think anything about them--

things like the change
of the seasons.

♪ ♪

Let's call that fire sun,

and here's... I've got a little
bit of a globe right here.

And we know that the sun
is 93 million miles away

from planet Earth.

And what makes the seasons

is that the Earth is tilted,

so when the Northern Hemisphere
is tilted towards the sun,

you have Northern Hemisphere

Six months later

as the Earth rotates
around the sun,

you have the Southern Hemisphere

And that change of the seasons
makes lots of sense to us.

But there are
longer-term variations

in the Earth's climate too.

And these changes
are also driven by the way

our planet moves around the sun
and receives energy from it.

Our orbit is distorted
by the gravitational pull

of the bigger planets--
Jupiter and Saturn.

Those big planets
are pulling on Earth,

and they're bending and flexing

the Earth's orbit
around the sun.

♪ ♪

The shape of our orbit
gradually changes

from more circular to more oval.

And there are two other things
that drive these climate cycles.

The tilt of the Earth shifts,
and it wobbles on its axis.

All these changes over time
add up to a repeating pattern

of warming and cooling,

over tens to hundreds
of thousands of years,

where the ice grows and shrinks
over and over again.

These swings in climate

can make it pretty challenging
for life to thrive.

Even for a highly adaptable
species like us.

The first humans appeared
about 300,000 years ago

and they had to deal
with very erratic climates--

cold to hot to cold to hot.

But around 12,000 years ago,
something changed.

From the depths of the last
ice age, our planet warmed,

and the glaciers that covered

much of North America and Europe

As the ice melted,
sea level started to rise,

flooding river mouths
around the world

and building up deltas
like the Nile,

and flood plains
like the Indus Valley.

The flat, well-watered, fertile
ground was ideal for farming.

And the spread of agriculture

would alter the course
of human history.

Successful farming
triggered the expansion

of civilization
around the world.

And that depended on
a stable climate.

We live in a world that's just
right for human civilization--

not too much ice
and not too little ice.

♪ ♪

A perfect inter-glacial balance.

But now, the Earth's
natural climate cycle

is being disrupted.

After hundreds of generations
of humans have settled

across this icehouse world,

the planet's temperature
is rising,

and the polar regions
are changing.

So, what's happening,

and what does the future hold?

You might think of a glacier
as a thermometer for the planet,

because it's basically
a block of ice.

♪ ♪

In most places around the world,
glaciers are diminishing.

Where glaciers that once
were very extensive

now are shrinking
back up their valleys.

Nowhere is that more dramatic

than at the mighty Jakobshavn
Glacier in Greenland.

(people talking indistinctly)

I'm going to get
my camera, I think.

I'm joining
Denise and David Holland

and their team of scientists
from New York University.

MALICK (on radio):
Hello guys, are you all ready?

Yes, sir,
we're ready.

We're flying up to the edge
of the massive ice sheet

that covers almost
all of Greenland.

Oh, that's a huge iceberg
down there!

The scale of this thing
is, is amazing.

The team wants to understand
how the ice behaves here.

BRIAN ROUGEUX (on radio):
if you're able to drop me

and then take off for a bit,
that, that makes it

quite a bit easier for me.

Mountaineer Brian Rougeux

has to venture out
between the crevasses,

to place GPS trackers
at different spots

on this fast-moving,
treacherous glacier.

MALICK (on radio):
I'm going to put it
on the ground.

I just need to find a spot.

Lots of little crevasses here.

Seems flat,
but it's not flat at all.

Pretty windy up here.

The 45-mile-per-hour winds

are too high for the pilot
to risk touching down

completely on the ice.

Malick, I'm going
to open the door, okay?


Brian will have to make
a quick exit.

So we're going to deploy

Take care of the rotors, okay?

Yep, Brian got the message
on the rotors.

We're going to circle back
for him later

in a few minutes, okay?

I wouldn't want to be him,
but I trust your judgment.

The solar-powered GPS
instruments will measure

how fast the glacier is moving.

Make sure that we know
where he is.

♪ ♪

The little lake
is a good landmark.

Malick, do you read, it's Marco.

Yeah, I read you.

We are doing a circle
around the position.

That wind is howling out there.

Is he coming onboard again?

He's now boarding, Brian's
now on board, closing door.

All secure.

Well done.
MALICK: Check.

That's something.

Data from the trackers
will be downloaded

back at the research base,

on a rocky outcrop
near the glacier.

The team spends several weeks
here each summer.

Aw, this is amazing out here!

It's awesome.

♪ ♪

When you come to a place
like this, you're blown away

by the vastness of the scale
of the ice.

I'm right at the very head
of the Jakobshavn Ice Fjord

and off in that direction is the
massive ice sheet of Greenland.

And this glacier is the fastest
moving glacier in the world,

and it's dumping icebergs out
at a staggering rate.

♪ ♪

At the point where the glacier
meets the ocean sits our camp.

This is the calving edge

where icebergs are launched.

The one that sank the "Titanic"
probably came from here.

(iceberg rumbling)

One of the big questions here
is: is this rate of ice flow

off Greenland unusual or not?

And the only way you can tell
that is to watch it.

David Holland
has a special camera

that takes an image of the
calving edge every day

all year round.

This is a camera in a metal tube
with a glass front on it.

That's exactly it.

You just pulled
the SD card out.

And that's it,
so that's the...

That's a year's worth
of data?

So let's have a look at it
and see what we got.

Okay, definitely.
Let's bring it back.

♪ ♪

This is the last
how many months?

So, this is from last August
until, I think,

approximately last December.

What you see is here in summer
the glacier has retreated.

So it was way back there.
It's way back there!


And as it becomes colder

in the environment,
the glacier is advancing.

You can kind of see a wall here
that's moving forward.

Now it's in the middle
of the screen.

And you can see it moving here
as the winter comes?

The wall is moving downstream.
Yeah, yeah.


I mean,
when you look at a glacier,

it looks like
it's just sitting there,

but this makes it look
like a river.

But the bigger picture
we're looking at

is for the last 20 years,

this cycle has retreated
many, many miles into the fjord.

♪ ♪

Since the 1850s,

the Jakobshavn Glacier
has been slowly decaying,

the calving edge inching its way
backwards up the fjord.

And this century,
it's been speeding up,

losing more than ten miles
in a decade.

When I was here in '96,
I would have seen it

way down the valley.
That's right, that's right.

And now it's w... it's moved up.

The year after you were here,

the, the major retreat
started, 1997.

I didn't do it,
I promise you.

So what is happening here
to cause this glacier

to retreat so quickly?

To try and find out, the team
is taking to the air again.

This time they need to drop
probes into the water

just in front
of the calving edge.

♪ ♪

They'll measure how the water
temperature changes

along the length and depth
of the fjord.

The first hole you see.

We'll do that and then every
three nautical miles.

DAVID: Yup, yup.

That's not to say that
I'm going to get it in the hole.

This could be my lucky year.

Is it possible to go
slightly lower?

Okay, preparing probe.

The helicopter hovers
just above the surface.

After hitting the water,
the probe sinks...

Probe deployed.

...and sends back

live temperature readings
as it descends.

400 meters, temperature 1.6.

600 meters, temperature 1.7

Okay, that's it, hit the bottom,

790 meters,
temperature 1.9.

The team takes readings
from 12 different positions

along the length of the fjord.

400 meters,

temperature 1.5.

Okay, all done, Denise.

The probes don't disappoint.

They reveal an invisible attack
on the glacier from below.

Over recent decades,
a deep current,

slightly warmer
than the surface,

has been eating away
the base of the glacier.

The ocean seems to be in control
of this 20-year retreat.

When we looked at
the last few decades,

it's really quite remarkable.

The waters all along

the west coast of Greenland
were cool right up until 1997.

And in one sudden year,
warm water arrived,

so it really correlates well

with this sudden jump
and retreat of the glacier.

(iceberg rumbling)

When the warm water reaches
the calving front,

it melts the glacier from below,

causing the ice above
to collapse.

Later in the summer,
David and Denise

witnessed one of the biggest
calving events

ever captured on camera.

Over a period of just
30 minutes,

this iceberg four miles across--

half the length of Manhattan--

broke off from the ice sheet.

As the glacier's calving edge

the huge mass of ice behind it
can accelerate down the valley,

toward the sea.

And it's not just the ice
in glaciers

that's reached a tipping point.

Millions of square miles
of sea ice

that forms when the ocean
surface freezes

in the polar regions
is under threat.

Each year, the sea ice
at the poles melts and refreezes

with the seasons,

growing in the winter
and shrinking in the summer.

But over the last 40 years,

the area covered by summer sea
ice has been getting smaller.

With real consequences for
people living in the Arctic.

♪ ♪

This is Shishmaref,

a small island off the
northwest coast of Alaska.

You must be Corey, yeah?

Welcome to Shishmaref!

This is our ride?
All right.

Looks good to me.

(engines starting)

♪ ♪

This place really is the
front line of climate change.

This little coastal town
in Alaska is only 30 miles

south of the Arctic Circle.

About 600 Inupiat people
live here in Shishmaref,

on the shore of the Chukchi Sea.

Sea ice used to hug the shores
of this remote island

for ten months of the year.

This community depends
on the ice

to hunt for walrus and
bearded seals for their food.

We're here in summer,
but summer's getting longer,

For people who depend on sea ice
for their food,

that's a big problem.

What year were you born?

Born in a tent
five miles from here.

Growing up,
what do you remember

about sea ice
when you were a kid?


all these years, you know,
the sea ice was dependable.

It used to get real thick
because it used to freeze

before Thanksgiving.

The sea ice has gotten thin

and freezes later.

Last year it froze in January--

we had waves in January

and it doesn't get thick
at all--

it breaks up fast.

When's the ice
usually go melt off?

It used to melt off
end of June,

because on my birthday,
June 26, we...

me and my dad
and my grandpa

we used to be hunting
amongst the ice.

Not anymore, it goes away
June 6, June 9.

When the ice is thin,

it's really dangerous
for the hunters.

This is footage captured
last winter

by a local drone pilot,
Dennis Davis.

During the hunting season,
he flies his drone

to help the villagers find the
safest path across the sea ice.

So this was back...
Oh wow.

...January 14.

There's almost
no ice at all!

It's like 50 feet of ice, not
a mile and not 15 miles of ice.

This is as far as you can go.

Look at this,
we'll zoom in.

You can't even see
any ice out there.

As far as you can see,
there's no ice--

I mean there's thin ice,
but there's no real, real ice.

All this ice out here,

it's supposed to be
frozen solid,

between three and six
feet thick.

Is that a kind of unusual
condition for January?

It's starting to be a normal.


Where the ocean is open,
when it's supposed to be frozen.

♪ ♪

And it's not just the ice
that's disappearing.

The permafrost underneath
this village is also thawing.

Warmer oceans are creating
more violent storms,

and without the sea ice
to protect it,

this coastline
is now eroding fast,

up to 50 feet in a single year.

Every now and then, another
house falls into the sea.

♪ ♪

Mo Kiyutelluk's house...

Hey, Mo.

...was very nearly one of them.

How's it going man?
Good to see you.

So this is the spot where
the house used to be, huh?


That was your house?

There was permafrost
under that-- nothing but ice.

Yeah, yeah.

Then apparently when the...
as it melted,

it, it just kept on falling off.

How did they
save your house?

Whole town took part
in pulling it.

What-- they were pulling it
by hand?

Yeah, the whole town!

And that kept it
from toppling in.

So this is your house, huh?

That's the one.

You saved it

from the water
and you moved it

a long ways away
from the water.

Mo's house may have been
saved for now...

but the elders realize
that Shishmaref will eventually

be swallowed by the sea.

For the villagers,
it will mean the old ways

of walrus and seal hunting
are fading.

They're planning to move
to a new site on the mainland.

Here there are plenty of berries
to pick.

Coastal hunters
turned gatherers.

It's going to be a really
different lifestyle

for these guys.
I think so, yeah.

♪ ♪

Shishmaref is just one example.

Communities all over the Arctic
will need to adapt

to a different way of life.

So what's driving
this dramatic transformation?

We've seen that sea ice
is retreating

and ice sheets are shrinking.

The answer to why that is
happening, though,

is actually held
in the ice itself.

♪ ♪

At an Arctic base, scientists
from all around the world

are analyzing ancient ice.

The East Greenland Ice Core
Project, or EGRIP,

is a huge experiment
to drill down into

one of the biggest pieces of ice
on our planet.

They want to understand
how it's flowing and changing.

Here, in the middle
of the Greenland ice sheet,

glaciologist Jim White is one of
the frontline polar scientists

trying to decipher the history
of climate locked in the ice.

Hey, chief!

How you doing, my man?

Good. Good to see you.
Good to see you!

We know that in the deep past,

carbon dioxide seeping
out of the ground

warmed Earth
and created a hothouse.

But what's been happening
to our atmosphere lately?

(machine cranking)

Jim and his team drill cores
from deep inside the ice sheet.

The rig sits 23 feet
beneath the surface of the ice

to shield it from the weather,

but it can still reach
30 degrees below zero down here.

We're looking at ice that's
about 10,000, 11,000 years old.

You can actually see
the annual layers,

even at this level.

Each ice layer represents
a single year of snowfall.

When snow compresses into ice,
it traps air,

forming bubbles
of ancient atmosphere

that preserve the exact
conditions when the snow fell.

This is from one of the cores
we just brought up.

What you see in here
are little bubbles of air--

time capsules of the atmosphere
from 5,000, 6,000 years ago.

We can take this ice
back to a lab, crush it,

release the air and measure
how much carbon dioxide

was in the atmosphere
10,000 years ago.

Using ice cores from Greenland,

and even older ones Antarctica,

scientists are able
to read ancient CO2 levels

that go back more than
800,000 years.

Our planet over
the last 800,000 years

has really behaved
within two boundaries--

an upper boundary of CO2

that has never gotten more than
about 300 parts per million,

and a lower boundary
that never got much below

180 parts per million.

For the most part,
the ups and downs in the CO2

seem to be in sync
with the natural cycles

we see in our climate history.

But as we get into the more
recent past, something changes.

The amount of CO2 that
we've added to the atmosphere,

that's now raised us
up to 400 parts per million,

that takes us way out of
whatever boundaries existed

over the last 800,000 years.

This occurred since, basically,
the industrial revolution.

So it's the last 150 years
or so.

Mostly in the last 50 years.

At some point
in the recent past,

human beings
became demonstrably

the largest agent of change

when it comes to
greenhouse gases on this planet

in the last million years.

♪ ♪

Throughout most
of Earth's history,

every time carbon dioxide
goes up,

our climate gets warmer.

And it's this rapid rise of CO2
over the last 150 years

that's warming our planet

and causing the ice
in our polar regions to melt.

The only difference is this time
the source of the CO2 is us.

♪ ♪

But is it really possible that
human activity can be producing

enough CO2 to change
our atmosphere--

and our climate--
so dramatically?

Many people find it hard
to believe that humans

are acting as a geologic force,
outstripping even volcanoes,

as a contributor
to global climate change.

♪ ♪

Have you ever thought about
how much carbon

is in a gallon of gasoline?

Let's weigh it and find out.

A gallon of gasoline weighs
about 6.3 pounds;

87% of that is carbon.

And that means there's
five pounds of carbon

in a gallon of gasoline.

Think of that like a five pound
bag of charcoal briquettes.

(engine rumbles)

When you burn gasoline, you
create carbon dioxide and water.

Carbon dioxide is an odorless,
invisible gas,

but think about this:

what if the carbon
came out of your tailpipe,

not as carbon dioxide,
but as solid chunks of carbon?

Kind of like car turds.

So if the average American car

gets around 25 miles
to the gallon,

that means that every 25 miles,

you're dumping five-pounds of
car turds out of your tailpipe.

The average mileage for each car
is about 12,000 miles a year,

so that releases
over a ton of carbon.

In the U.S., there are a total

of about 100 million cars
on the road,

releasing about 330,000
tons of carbon per day.

♪ ♪

When we include the rest of the
world's one billion cars,

we reach three million tons
of car turds every day.

And that's just cars.

When we add in power plants,
factories, aviation,

and agriculture
and multiply it by 365,

the total carbon released
by human activities in a year

is a massive 12.5 billion tons--

enough to leave a pile
of car turds four miles across

and over a mile high.

(penguin squawking)

Many scientists say we've
entered a new geologic age:

the Anthropocene,

with humans now altering Earth's
climate on a geologic scale.

♪ ♪

But what exactly will all this
carbon dioxide do to the planet?

Rocks and fossils help us piece
together the surprising history

of our planet and its life.

But can they help us
predict the future?

Today, the carbon dioxide levels
are 410 parts per million.

When was it last
about that level?

Oh, about three million years

So let's go back
three million years

and see what Earth
was like then.

♪ ♪

Back then, though there was ice
at the South Pole,

the North was mostly ice-free.

♪ ♪

So, what impact did this have
on the world?

To find out,

we need fossil beds
that date back to that time.

♪ ♪

There's one 90 miles inland
from Virginia Beach.

There's lots of white,
crumbly stuff in here.


Geologist Maureen Raymo
studies places like this

to try to predict what a warmer
climate might have in store.

Super cool.

They didn't build rock hammers
for digging in clay,

I'll tell you that much.

To find what Maureen's
looking for,

we need to dig deeper.

And my rock hammer
just isn't going to cut it.

(backhoe beeping)

Digging tool of choice.

Yeah, beats a shovel doesn't it?


We're trying to reach
a layer of mud that dates back

three million years.

Can you bring a chunk of that
black stuff up off the bottom?

We'll just dump it
on this side.

Just like a little scoop.

Ooh, yeah!
Oh, look at that thing.

Every geology department
should have one.

(Kirk chortling)

Thanks to the backhoe,

we hit a fossil jackpot.

Hey, thanks!

That's perfect.

That's awesome.

Look at that!

That's a perfect

three-million-year-old clam
right there.

Oh man, that could have been,
like, alive yesterday!


That's beautiful.

Places like this are pretty
special for paleontologists.

This is great.
RAYMO (laughing):

A window to the past.

We're on a beach.

We're on a beach
three million years ago.

♪ ♪

It's incredible to think
that this quarry,

90 miles inland today,

was once a beach teeming with
corals and other marine life.

There was warm ocean water
lapping against this shore.

Warmer world, warmer fossils,
coral reefs.

It's amazing.

It's right before we slid into
the ice ages.

It's just solid black mud,
full of clams.

Three million years ago,

when the Earth was three
or four degrees warmer

and there was no ice
in the Arctic,

a lot of the water that is now
locked up in glaciers

was in the ocean,

which means that the global sea
level was about 60 feet higher.

According to Maureen's

this is how the East Coast
would have looked.

If Earth continues to warm,

the coastline will start
moving inland,

as the ocean returns
to places it covered long ago.

It doesn't seem like much,

but it gets you back to a time
when you had shorelines

that were 80 miles
further inland.


Three million years ago
is the last time

the CO2 in the atmosphere
was as high as it is today--

over 400 parts per million.

This is our window
into our future.

So, how come our present
sea level isn't right here now?

The ice sheets
are out of equilibrium

with the atmosphere right now.

The atmosphere's warming,

we're adding more CO2
every year.

You could think of them
like a frozen lasagna

you put in a pre-heated oven.

You know, and the oven
is our atmosphere, warmer,

and the ice sheets are melting
just like the lasagna

would slowly melt,
and it take a while.

So, in the same way
the lasagna's eventually

going to be ready to eat,

we'll eventually
have higher sea levels.

We will.

So, the real question is:

how long it takes

for the ice sheets to melt,
which conditions, right?


That's the question
that is driving research

all over the world.

♪ ♪

Thanks to us,
CO2 levels are now rising past

410 parts per million
and beyond,

at a rate the planet hasn't seen
for millions of years.

So, what happens to life
when things warm up so quickly?

♪ ♪

It turns out that the planet
ran a similar experiment

56 million years ago.

And the Bighorn Basin in Wyoming

holds the secrets
to what unfolded.

It's also one of my favorite
fossil-finding spots.

♪ ♪

I'm joining paleontologist
Jonathan Bloch

to see what we can discover.

It's warm enough out here.


It might look like barren
scrubland today,

but the fossils here
suggest that these Badlands

were once teeming with
all sorts of creatures.

♪ ♪

That is a tiny little bone.

♪ ♪

Sometimes we just wash
these little tiny fossils

in our mouths and hope
that it's not

rabbit poop
or something like that.

I keep finding primates.

Oh, yeah.
What did you find?

Let me see, it's got...
maybe it's a lizard.

I have a jaw here,
if you want to see one.

Oh, cool.

This is a lemur-like primate.

Oh wow.

One of the first primates
in North America.

I want to find my own jaw now.
(chuckling): Yeah.

Is that another primate?


I've found three primates
so far.

The fact that we're finding
all these animals,

tells me that this landscape

used to look very different
from today.

So where to from here?

Let's just keep following
around this ridge.
Other side? Okay.

For a start,
if there were primates,

there must have been trees.

Around 56 million years ago,

this stretch of Wyoming
was a warm, lush floodplain,

covered in a subtropical forest
of laurels, legumes, and palms--

a bit like northern Florida

There's one animal that was
very abundant in these forests.

If I just gave you
one of these teeth,

you would be able to say,
"Oh, that's a horse"?

Right, I could tell
it was a horse just based on

the way the cusps and the crests
are organized

on the crown of the molar.

The teeth would also tell you
how big the animal was?

Right, so the first horses
look just like

what you're holding there.

So you've got a jaw there
with a bunch of teeth in it

that are the size of some of

our modern domestic dogs,
maybe about this big.

By any standard it is
a small horse.

These are the earliest horses
to evolve on Earth,

56 million years ago.

And they lived right here
in Wyoming.

These first horses
were much smaller than today's.

(pen scratching)

They had a short muzzle, and
three- and four-toed padded feet

with hooves at the end
of each toe,

perfect for browsing
in the damp forests.

(birds chirping)

But not long after these
early horses appeared,

something dramatic happened,

and the story is written
in the rocks.

You can see this
very strong red bed.

It's just like a red strike
across the hill.

Very persistent, really strong,
it's really notable.

That's really towards
the end of the event here.

The beginning is...

you have to drop down
about 50 feet over there

to get to where it starts
and it's marked by

kind of a gray bed there.

This represents
how much time?

It represents maybe between
100,000 and 150,000 years.

♪ ♪

In the gray layer of rock is
evidence of a cataclysmic event.

When CO2 levels tripled,

skyrocketing to as high as
2,000 parts per million,

the hothouse got even hotter.

And this had a surprising effect
on the animals

that were living here,

especially the horses.

What was really exciting for us

was as we were collecting
through these rocks

that are stacked up
through time,

this is what they look like

when we found them
at the base of the section.

As we kept following
higher and higher,

we noticed the horses
became smaller.

Smaller than small?

Quite a bit actually.

What, like, 30%?
Maybe 30%?


So, we're going from like
a cocker spaniel to a chihuahua

or something like that?

But we found lots of them,
hundreds of fossils,

and they...
they show that pattern.

So it's a real pattern
that emerged.

So, the big question was...

Why did these small horses
get smaller?

What we can tell through

the same interval of time
as they're getting smaller

is that it's getting
a lot warmer.

Global warming seems to be

controlling the size
of the horses.

Over just a few thousand years,

rapid warming
turned the lush vegetation,

from subtropical wetlands

to drier forests.

Even minor variations
in temperature

cause very rapid
evolutionary change.

(pen scratching)

On land, horses and other
mammals became smaller.

There's an evolutionary

smaller bodies lose heat
more quickly.

In the oceans, warmer,
more acidic conditions

wiped out many marine species

This is one of the fastest
warming events

we see in the fossil record.

And what scares me
is that we're now warming up

our planet even faster.

So one of the challenges
for you

is to understand
just how similar this is

to our present situation?

And I guess in terms of what it
does to the animals and plants?

that's exactly correct.

And that's why it's important to
look throughout Earth's history

at events like this

to look to see if there are
any rules governing,

you know, how plants and animals
might respond to climate change

to help inform us about
the things we might be able

to expect in the future.

♪ ♪

What happened back then took
tens of thousands of years.

♪ ♪

The warming on our planet today
is happening

over just a few hundred years.

♪ ♪

Are we on our way to ending
today's icehouse

moving into a hothouse,
with no ice on the poles,

for the first time
in human history?

If that happens,
it's not just the wild,

natural world
that will be transformed,

with species disappearing

The crops we depend on for food

will be vulnerable to the
extreme hothouse weather.

And with all of Antarctica's
ice melted,

sea levels around the globe

could rise by more than
200 feet.

But we wouldn't even need to go
all the way back

to a full hothouse Earth
to feel the effects.

Just melting Greenland
and some of Antarctica

would move the shoreline
far enough inland

to flood major cities

like Washington, DC,


and Shanghai;

and to cover
an awful lot of Florida.

Today, we are approaching
a tipping point.

But how close are we?

Hidden deep in the
Canadian Rockies is a place

that could give us an early
warning of what lies ahead.

It's a unique cave
that for geologists

could act like a canary
in the coal mine.

Although I'm a thousand miles
south of the Arctic Circle,

this cave stays cold
all year round.

The rock and soil beneath the
surface are permanently frozen

all through the summer.

(metal rattling)

I'm here with geologist
Jeremy Shakun

and mountain guide Dave Stark.

Getting a little tight, huh?

I'm crawling already.

The ground inside this cave
has been frozen

since the last ice age.

This is 10,000 years
of permafrost.

You can feel it
getting cold fast.


To preserve this unique cave,

it's closed to the public.

Even scientists restrict their
visits to once every few years.

This is really cool back in here
now, it's opening up.

Whoa! Look at this chamber!

Now we're in an auditorium,
with a ginormous rock fall.


15 minutes in, and there's
a spectacular change.

Oh my God, look at this!

You crawl in
hands and knees.

Watch your heads,
I'll go in first.

Cold knees.

It's like a gigantic igloo!

How deep is the ice
you're crawling on?

I don't know,
but you can see way down.

Oh God! Holy moly.

Be careful.

You got to see these.

Super weird.

♪ ♪

It's like--
Oh (bleep)!


That's crazy though.

I mean this is like...

it's like being inside
rock candy.

This is unbelievable.

This is one of the most
amazing places I've ever been

on this planet.

The ground's so cold here,

any moisture in the air
freezes to the cave walls,

forming enormous crystals
of ice.

They're... but they're big,
they're like five inches across.

Like the size of your hand,

I've never seen ice crystals
like this.


There's some that are like
big dinner plates.

I feel like I'm in
a crystal chandelier factory.

This stuff looks like glass,
not ice.

♪ ♪

Holy cow!

This place

is so totally amazing,
I can hardly believe it.

♪ ♪

And actually it's pretty crazy,
some of these

are dripping just a little bit.

It's very clear

that just our bodies in here,
if we stay much longer,

are going to change
the temperature of this place.

And we're looking at kind of
a remnant of an ice world.


It's... it's amazing,
this is an ice world

that's changed into
a non-ice world.

Let's, uh... let's duck and go.

(microphone rumbling)
All right...

Stay low.

This crystal cavern
is a reminder

that we are still living
in an icehouse world,

but it looks fragile,
on the cusp of change.

This is treacherous
going in here, man.

Slick rock.

Slick, jagged, loose rock.

If we go deeper
into the cave system,

we can see what happened
when it warmed in the past.

So, the farther back
in the cave,

the further back we go in time.



Oh, wow, that is totally
peeling away from the ceiling.

Like right over your head.

like this ice at our feet,

seems like it came
from up there.

Wow, that's crazy.

In here, we find the flip side
of the ice chandeliers,

evidence from a warmer
inter-glacial world,

the last time
this crystal cave melted.

See, that's the kind of stuff
that would be so cool to date.

This is the stuff
that gives us a glimpse into

a past warmer world

when this whole cave
was thawed out

and there wasn't any ice.

It's not forming now
because it's cold in here

and we don't have
running water.

But if we were in
some warm time in the past

when this cave was thawed,

so the water comes down
from the surface above,

dissolves some of that rock,

you'd have sheets of water
running down this

and it has little minerals
dissolved in it,

and when they run down
the surface,

they deposit these crystals.

And layer by layer
they build this thing up.

There're beautiful things.

Oh, it's awesome,
So let me show you this.

Check this out here.

Here's a flow stone
that's actually from this cave.

It looks like toffee.

Yeah, absolutely, right?

Layers of caramel-colored stuff
in there, right?

And so the way
this one worked is,

oldest right here,
and then with time

it kept adding more and more
and more layers to it.

So it would have been something,
you know,

growing like that
out of the wall.

I got you.

And just layer upon layer
gets added.

What are you measuring?

So, basically,
we're measuring

when there's water flowing

It tells us
when this thing was growing.

Like this one,
for instance,

from this cave,
we dated it.

It's 400,000 years old.


How much warmer was it
400,000 years ago?

How much heating did it take

to thaw out this permafrost?

What's the danger line?

And, interestingly,
400,000 years ago,

the world was warmer,

but just like
a couple degrees.

It's sort of like the
where we expect to be

middle, later part
of this century.

♪ ♪

If history repeats itself,
the permafrost in this cave

could be on the brink
of melting again.

What's wild about it
is it contains

a ton of frozen carbon,
um, and it's, it's a ton.

It's twice as much carbon
as already in the atmosphere.

But were is that carbon
coming from?

It's just old dead stuff.

It's just old plants and animals
that at one point were alive.

They have carbon in them,
they die,

it goes down into that soil.

So, right now, it's frozen,
it's turned off,

it's not going anywhere

until you dial up the
temperature a little bit.

Open the freezer door,

it starts to melt...

And all that meat
will just start rotting.

Burping out methane
and warming things up even more.

How much global warming
can you do before these caves,

this Arctic permafrost,

thaws out?

And it was dripping
when we walked in here.


Which I think means
we're tipping back.

Come back in 50 years
or something

and these things are going
to be regrowing again.

Okay, yikes.

Yeah, that's serious.

♪ ♪

If the permafrost thaws in here,

and across all the frozen land
at the polar extremes,

a massive release of methane and
CO2 will speed up global warming

all over the planet,

creating an unprecedented threat
to humanity.

♪ ♪

There have been thousands
of generations of humans,

and here I sit at the moment

that humans have flipped
the switch

that might take planet Earth
from an icehouse climate

to a hothouse climate.

The clues to what could happen
next are out there,

hidden in the most remote spots
of the Arctic

and the Antarctic.

This is...
this is amazing.

♪ ♪

This has been
an amazing journey,

from pole to pole
and back in time.

♪ ♪

We've seen climates were
much warmer than they are today

and much colder.

♪ ♪

We've seen that in the past,

more carbon dioxide...

Perfect three-million-year-old
clam right there!

...warmed the planet

and raised sea levels.

These snapshots
from Earth's history

show us the direction
we could be heading.

♪ ♪

Now we have to ask ourselves:
is this the world we want?

There's almost no ice at all.

Humans are geology,
we are impacting this planet.

And the future
is dramatically uncertain.

♪ ♪

It's the first time that
a mammal has actually changed

the composition
of the earth's atmosphere

and driven a dramatic change
in the earth's climate.

It's us.

The question is:

are we clever enough
and forward-thinking enough

to flip that switch back?

♪ ♪

♪ ♪

To order this program on DVD,

visit ShopPBS
or call 1-800-PLAY-PBS.

Episodes of "NOVA" are available
with Passport.

"NOVA" is also available
on Amazon Prime Video.

♪ ♪