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The Arctic has always been the toughest test,

no more so than in the far corner of Alaska

called the Arctic National Wildlife Refuge.

Thousands of caribou face the cold...

the terrain...

and formidable predators to reach one special place

at just the right time...

a vast, beautiful plain on the edge of the Arctic Ocean.

Here, they give birth.

This is the story of the last great land migration

in North America.

And of how every Arctic creature is now on a daunting journey

from a timeless past into a vastly different future.

On the top of the world,

it is still the ice age.

It's northern Alaska in winter.

The temperature drops to 50 below.

Frozen rivers curl north from the Brooks Range

and down to a plain stretching to the Beaufort Sea

and the Arctic Ocean, now locked in ice.

This vast stretch of white mountains,

tundra, and icy rivers,

forms one of the world's most important wildernesses...

the Arctic National Wildlife Refuge.

It's a home for courageous creatures from another time.

This wildlife community will face a year

of difficult decisions.

Hundreds of miles southeast, bravest of all perhaps,

is a pregnant caribou.

She's part of the Porcupine caribou herd

and will undertake the longest annual land migration

on the planet.

In her lifetime, she's likely to walk the same distance

as a complete circuit of the globe.

It's December, and the caribou are waiting,

eating lichen in the twilight.

The journey ahead needs precise timing.

She'll march through forests,

over mountains, across rivers,

to the coastal plain.

It's a round trip of almost a thousand miles.

Some 160,000 caribou

will walk from their wintering grounds

in the Yukon and Northwest Territories

to their calving grounds in the Wildlife Refuge.

The Brooks Range stands in their way,

running parallel to the coast.

The sun sets for the last time until the New Year.

The only sunlight is reflected by a wintry moon.

Hundreds of miles above,

solar radiation fires up the ionosphere,

and the ghostly aurora.

This is a world of huge planetary

forces in constant flux.

This is nature at its most spectacular and extreme.

A dawn twilight returns in January.

Blizzards blast the tundra.

Most animals have left or are hibernating under the deep snow,

but musk ox remain.

They are loyal to a home range and won't leave.

They rarely visit the valley.

The snow is too deep,

and they can't dig out the frozen grass.

Even up here, it's hard work.

Winters seem to be more erratic recently,

on average more than eight degrees warmer

than 50 years ago, which sounds better,

but now there's freezing rain on top of heavier snowfall.

The caribou are becoming restless.

Her belly is beginning to swell with her unborn calf.

She's searching for reindeer lichen, her winter food.

She's a reindeer as much as a caribou.

They are the same species.

She digs through snow like the musk ox.

For a prospective mother,

finding enough food is her principal focus.

But it's not her only worry.

It's a lone wolf.

The caribou's instinct is to stay on the move.

Somewhere has to be better than this.

But migration is not blind instinct.

It's a series of collective decisions based on subtle clues

to weather patterns and journey times.

Every year is different.

The wolf has choices to make, too.

He can follow the caribou, but they move fast,

and the journey is too far.

The first birth of spring.

It's not a caribou,

but a newborn musk ox, a 30-pound baby,

delivered straight onto the snow.

The family surrounds her,

a protection from dangers outside their circle.

But within hours, the calf wants out.

There's a beautiful new world to explore.

Every calf is precious.

Musk ox were hunted to extinction in Alaska

in the 1860s,

but were brought back in the 1930s.

There are now a few thousand here in Alaska.

A little river gully is a new adventure for the calf.

Once in, getting back out is harder.

The mother won't follow.

She doesn't know how deep the snow is

and could easily get stuck.

She tracks her calf from the bank.

The calf discovers...

ice.

There's a physical urgency to find your feet here...

...but perhaps a greater mental urgency for the calf

to figure out her world.

The patient mother waits...

and her calf finds a way out.

She's got maybe another two years

looking after this youngster.

The herd of about 15 will protect the calves

and search for food around their large home range until summer.

The caribou have left the winter woodland far behind,

and are heading north.

By April, the snow is starting to melt,

and many rivers run earlier than a decade ago.

Spring is moving north faster, and the caribou need to keep up.

The herd heads on for about a hundred miles,

then will turn west

towards the Arctic National Wildlife Refuge.

It's the end of April. They are over halfway.

The mothers will all give birth

within days of the third of June.

Six weeks to go.

Golden eagles nest along the route.

They came up from the south

and established territories in March,

feeding on hares, ptarmigan, or marmots.

The eagle pair returns to the same spectacular nest each year.

They are loyal to a site decided over a decade ago.

But the calendar has changed a lot since then.

By early May, the chicks are near fledging,

and the caribou herd has already passed.

A ground squirrel is breakfast.

It will take a new generation of eagles

to make the huge adjustment to the changes unfolding here.

A lone wolf has stuck with the herd for many miles.

Caribou from a broad area either side of the Canadian border

are converging on the same route.

They'll navigate around the eastern end of the Brooks Range,

about 40 miles from the Arctic shore.

In a few places, the route has bottlenecks

around rivers and lakes.

This migration of Arctic caribou is compared to wildebeest

on the African savanna, and so the coastal plain

is sometimes called America's Serengeti.

As the herd funnels through the foothills

of the Brooks Range,

the wolf briefly gains an advantage.

The weaker calves from last year are struggling to keep up.

There's been little time to stop and eat.

As long as the herd stays watchful

and maintains a distance, they're safe.

Caribou are faster.

The wolf is a tactician,

but tactics are hard if you work alone.

Here is an unparalleled amount of food on the hoof,

but to this lone wolf, it might as well not be here at all.

By May, spring has reached the Arctic National Wildlife Refuge.

Life here has slept under snow for eight months.

Plovers and redpolls arrive to nest,

part of a massive aerial migration.

Millions of snow geese leave California or Texas

and are on their way to the Arctic Islands of Canada.

They'll stop off here on their way back.

Sandpipers are part of the worldwide event.

Golden plovers set off from Argentina.

Terns, loons, and songbirds come from Antarctica

or Africa or East Asia.

Hundreds of species cross the planet

on their way to the Arctic north.

A world of birds is heading to the summer paradise of Alaska.

The golden plover couple have arranged a nest on the ground,

laid their eggs, and are raising their chicks.

The parents provide some protection against Arctic foxes

or predatory birds,

but it's mainly just warmth under a camouflaged wing.

Once thawed out, the chicks have to get back to finding food

for themselves.

The mother leaves after a couple of weeks,

and the father follows a week or so later.

But the chicks stay longer

and have to find their own way back to South America.

In the meantime, the chicks catch insects

for as long as they can,

then come running back to warm up again.

Their mother knows it's not just the cold that will get them.

Arctic foxes live in tunnels in the layer above the permafrost.

The parents returned a few months ago

from their winter wanderings in snowy white coats

to give birth and turn brown for summer.

The cubs were born here in early May.

Now, a few weeks later,

they enjoy the remarkably warm days.

They need to grow up fast.

The cubs are already eating meat

and seem constantly hungry.

Ground squirrels and nesting birds are here

in large numbers in May and June.

One delivery is not enough for them all.

The others don't notice the mother's return

until it's almost too late.

Oh, well.

Dad will be back soon with more.

It's a constant meal service during days

that never quite seem to end.

Nesting on the ground is dangerous,

but there is one bird that would fight off any fox... the jaeger.

Jaeger means hunter in German.

The caribou are beginning to arrive.

They are tired, hungry, many are about to give birth,

and they could so easily tread on a bird's nest.

The jaegers are not going to let that happen.

They start with some gentle warnings,

but the caribou ignore them.

Yet the jaegers are fearless.

The caribou mother has come hundreds of miles,

faced raging torrents,

been jammed by lakes, escaped wolves,

and now there's a determined parent bird

that doesn't want her here at all.

The caribou respond to the swooping bird ballet

with strange dance moves of their own.

As more of the herd arrives,

the jaegers battle tirelessly to keep their eggs safe.

For the birds, and for all life here, there's little rest.

Even the sun keeps going,

skimming above the horizon for most of June.

This midnight sun warms the caribou

as they all finally reach their destination.

These are their calving grounds,

in the heart of the Arctic National Wildlife Refuge.

She must arrive precisely, so that she's in the right place

when, a week to 10 days after the snow melts,

the sedges and grasses are at their most nutritious.

Somewhere is a mother giving birth...

one new calf among 160,000, anonymous to all but each other.

Almost all the mothers give birth within a few days.

The calf doubles in size in two weeks.

The first green flush of sedges is briefly high in protein

and sugars and coincides within a few days

of when the calf needs most milk.

A week or so later, the grass will become coarser,

bitter, and indigestible.

The caribou slowly spread out,

surfing a wave of melting snow and growing grass.

The successful flood of energy

transforms the character of the caribou.

The first people here feel part of it.

Humans and caribou were created as one, they believe,

and each still contains a piece of each other's heart.

To the Gwich'in, this is the sacred place

where life begins.

It's late June, and paradise suddenly closes in.

A plague of mosquitoes emerges.

Each one may take just a drop of blood,

but the warmer summers favor the insects.

They emerge earlier and breed faster.

The mosquitoes take so much blood

and distress the caribou mothers so much

that the herd spends the rest of the summer

attempting to avoid them

by searching for somewhere cooler.

Life becomes a daily balancing act of finding

food for yourself

and not being food for mosquitoes.

The consequences of the hotter summers may be subtle

and complex for the caribou,

but the effects to the north are much more obvious.

Over the last 40 years, the decline of sea ice

in the Beaufort Sea

is one of the most dramatic changes anywhere.

One of the animals affected most is the polar bear.

During the last ice age,

they would probably spend all year on the frozen sea.

Since the ice age ended, polar bears hunted seals

on the Arctic ice in winter and spring,

and in late summer, rested on land,

snoozing and living off their fat.

Nowadays the ice on the Beaufort Sea is erratic.

It melts faster, and the bears arrive early and thinner.

By July, they are hunting for food along the shore.

They're not a huge threat to the caribou herd.

They are lumbering on land, and easily overheat.

Years are more variable now,

and polar bears are showing unexpected flexibility.

They search for smaller prey

and scavenge or fight over scraps.

The Arctic Refuge is a safe home,

but there is no refuge from the outside world.

It's too hot for them.

The July record here is now 85 degrees Fahrenheit.

It's often hardest for a mother.

Polar bear cubs are born in midwinter.

This coast is one of the most important onshore denning areas

in North America.

These cubs are almost two years old

but still need their mother's milk.

Polar bears have declined in the southern Beaufort Sea.

There are about half of how many were here 35 years ago.

These are desperate times.

North Alaska has the largest oil field in America.

The base at Prudhoe Bay is next to

the Arctic National Wildlife Refuge.

It's a stark reminder

of the cause of the changing seasons.

Pipelines extend south

and cross several caribou migration routes.

The polar bears search for food across land

now scarred by oil exploration.

For the caribou, the green surge is over,

and they are eating willow shrub

and searching for anywhere cooler and mosquito free.

For some, that means the coast.

Others head south into the foothills of the Brooks Range.

After enduring the heat of July, by late August, it's cooler.

A mother and calf turn south, with the protection of the herd.

Waiting for them is a grizzly.

Grizzlies and caribou meet each year.

The herd is named after the Porcupine River they must cross.

The bear's journey here is equally hazardous.

In summer, grizzlies hunt all the way to the Arctic Ocean.

The Porcupine caribou herd

is heading south into the Brooks Range.

Away from the water, the caribou start to relax.

It's a mistake.

Caribou may be faster than bears on the open plain,

but they must also rest the calves.

Grizzlies and polar bears are almost the same species

and increasingly overlap and interbreed,

but grizzlies are a more serious threat to the herd.

The bear has learned to chase off the mothers

and then search for calves hiding in the scrub.

He's found two.

In a moment of indecision...

he loses both.

The calf's mother gallops to his side.

It's a huge victory after so much hardship.

Though not for the bear.

The journey south crosses the Brooks Range in fall.

In spring, this route is blocked by snow,

but now they can return straight over the mountains.

For the calf, this is his first taste of the wider world,

with some new characters... Dall sheep.

They are cousins of bighorns, little bighorns,

and mountain specialists.

Like musk ox, they don't migrate,

but they do move around the mountains looking for grass

and shrubs between the rocks.

The ewes give birth to a single lamb in late May.

By September, they are all enjoying the last of the summer.

The mothers gather in flocks and watch for wolves and eagles.

This is the migration route south,

familiar to more than just caribou.

The fledged golden eagle

has been shadowing the caribou herd south.

As the herd turns east,

the caribou face the mountains' wolves.

In a small valley, the herd slows to rest.

A lone female plans her hunt.

She doesn't try to hide.

The mountains are on her side now.

A calf becomes separated.

To its mother,

almost home, the trek has lost its meaning, its purpose.

The snow geese arrive here from the northernmost Arctic islands.

This is an important stopover on their way back to California,

Texas, and Mexico.

Snow geese numbers have grown over the last 20 years.

There are now 20 million of them.

In a world of so much decline,

it's good to know some species are doing well.

The autumn storms are sweeping in.

The weather changes very quickly.

Ptarmigans have been caught in their summer plumage.

They'll turn white to an out-of-date schedule.

As long as the snow isn't too deep,

they can feed on willow buds and birch.

Ptarmigans are here all year.

That makes them a reliable target for a red fox.

The many eyes of a flock

and their sharp calls protect them for now.

Red foxes are thriving in the Arctic

and out-compete Arctic foxes if the weather is milder.

By October, the rivers are frozen

and are buried under snow.

The musk ox have to dig for vegetation again.

The male musk ox have other concerns.

Before winter arrives,

they must sort out mating rights to the harem.

The rut started in August

and a clear victor has yet to emerge.

The musk ox are also related to bighorn sheep,

but are much bigger.

Males weigh in at 800 pounds,

and their closing speed is close to 60 miles an hour.

Their skulls are heavily reinforced

and the brain protected by fluid.

They back away, but only to prepare for combat.

On the right, the older waits,

then charges.

There's probably no other animal on Earth

that could survive this battle.

The winner will now mate with the herd.

By October, the land is frozen solid.

Two polar bear cubs have survived the summer

thanks to their mother.

The sea won't freeze solid until November,

but some coastal bays and river estuaries freeze earlier.

It's their playground while they wait.

On warm days the surface

starts to melt and becomes slippery

and tides weaken the new ice.

The surface is still fragile.

By midwinter, they'll be sniffing out seals

in a freezing twilight far from land.

This is their ice age home at last,

a timeless scene that leaves behind a world of change.

Then the ice breaks, and reality returns.

The sea melts and refreezes

and will melt again for two frustrating months.

The polar bears of the southern Beaufort Sea are doing better

than many others around the Arctic.

The Arctic National Wildlife Refuge protects the dens,

and mothers have become expert scavengers.

Their sense of smell is probably the best of all animals.

And look what they've found.

It's a bowhead whale... what's left of it.

With unpredictable winter ice ahead,

this bowhead carcass is a salvation for the family

and for all the local polar bears.

They will feed for weeks.

Bowhead whales are increasing, maybe thanks to longer summers.

Everything is changing, and everything is connected.

There is regulated whale hunting here,

and this carcass has actually been left

on the beach for the wildlife.

For the polar bears, it's a lifeline,

but maybe little more.

They are ice bears, evolved in another world.

Across millions of years, the Earth has seen huge changes

many times.

The most recent was the end of the Ice Age, 12,000 years ago.

This extraordinary wilderness

protects these Ice Age survivors...

the polar bears, musk ox, arctic foxes,

and the spectacular caribou,

holding out on the top of the world.

Now, even for them, the ice age may, finally,

be coming to an end.