Nature's Great Events

The power of the sun

drives the seasons, transforming our planet.

Vast movements of ocean and air currents

bring dramatic change throughout the year.

And in a few special places, these seasonal changes

create some of the greatest wildlife spectacles on Earth.

Here in the Arctic each summer, the sun begins to melt the winter ice.

Nearly three million square miles of ice will disappear,

opening up a narrow window of opportunity for millions of animals.

For many, it's their best chance to feed and breed.

But for polar bears, it's the hardest time of the year.

They have to survive the greatest seasonal change on the planet.

Winter in the Arctic.

The northern lights flicker across the sky.

It's a land of continuous night

where temperatures plummet to minus 40.

Polar bears are in their element,

hunting for seals on the frozen sea.

But the long night is coming to an end.

In February the sun rises

for the first time in four months.

In the coming weeks the strength of the sun

will power an enormous change.

But for now its rays offer only a little warmth.

Despite the sun's return, six million square miles of the planet's far north

is still cloaked in ice.

Deep snow covers the mountains.

Even the sea is frozen solid, many metres deep.

Groups of ringed seals haul out through holes in the ice

to bask in the weak sunlight.

But they are vulnerable and have to keep a lookout.

With the sea still frozen, it's easier for predators to get close,

and the seal's greatest enemy is the polar bear.

A mother bear and her four-month-old cub are hunting.

Seals make up most of the bear's diet,

and to find them she must lead her youngster out onto the ice

for the first time.

Scientists, looking at how a changing climate is affecting bears,

have fitted the female with a radio collar.

The ice here is thinner,

and she must spread her weight to avoid breaking through.

For her cub, it's all just a game.

By sticking close to Mum, he'll learn how she hunts for seals,

a critical lesson for his future survival.

At this time of year the frozen Arctic is empty of life

and only a few hardy residents can survive.

For a female arctic fox, the winter has been a time of hunger.

Food is scarce and she's had to wander far and wide,

scavenging from the remains of bear kills.

But her fortunes are changing.

A dead reindeer is a lucky find.

If no other scavengers come along,

this could sustain her for the rest of the winter.

With the sea frozen, polar bears are busy hunting.

This bear has caught a seal.

He waited for it to surface through a hole in the ice

and his patience has been rewarded.

He must catch a seal about once a week, and this is the best hunting season.

As long as there is sea ice the bears won't go hungry,

as walking on the ice is the easiest way for a bear to get close to a seal.

But the ice will soon start to break up and hunting will get harder.

The calories the bear takes in now

will have to see him through hard times ahead.

For polar bears it's the survival of the fattest.

At this time of year there are still only five hours of daylight.

But as the sun rises higher, each day lasts 40 minutes longer.

It's March, and with increasing sunlight

the first in a huge wave of migrants are arriving.

Millions of seabirds are travelling north to reach the frozen coast.

Huge flocks of little auks and guillemots

fly across many miles of ice from the nearest open water.

Their destination is the still-frozen sea cliffs.

It's an inhospitable place,

but in a few weeks everything here will change.

It pays to be early, even though the snow has yet to melt.

The first arrivals get the best nesting ledges and a head start.

They have just 50 days from laying their eggs to raise their chicks.

The Arctic's silent wilderness is coming to life.

It's three months since the sun's return and its power is growing daily.

The air temperature rises slowly

and once above zero, at long last, the melt begins.

Melting snow feeds freshwater streams which pour off the land.

In just a few short days, the melt unveils a whole new landscape.

The coastal cliffs now teem with nesting seabirds,

and it's not long before they attract unwelcome attention.

The fox's white winter coat has disappeared with the snow.

Her new camouflage will allow her to change from scavenger to hunter.

The short summer will be her best chance to raise a family.

The snow may be disappearing from the land,

but the sea ice has yet to melt.

The coastline is still locked in ice.

The islands of the Arctic

are surrounded by thousands of miles of white frozen ocean.

Because the sea ice is so thick, it won't begin to break up

until the temperature stays above zero for a number of days.

Until then, few creatures can penetrate this icy barrier.

The sun reflects from the white frozen sea,

creating a desert of icy mirage.

The bear can still walk on the ice to hunt for seals,

but not for much longer.

It's getting warmer by the day.

The bear's world is about to melt away.

It's now June and the sun beats down 24 hours a day.

The temperature remains constant above zero degrees.

The frozen sea begins to melt.

Pools form across the surface, absorbing more of the sun's heat,

speeding the thaw.

Sunlight penetrates the frozen surface,

illuminating a strange world beneath the ice.

The frozen barrier has split.

Mysterious voices echo against the icy ceiling.

Giant cracks, or leads, form at weak points,

creating a pathway for new arrivals.

Narwhal.

Known as the arctic unicorn because of their strange spiral tusks,

narwhal are some of the most secretive and elusive animals

in the world's oceans.

The leads provide passage for thousands of these mysterious whales.

Each summer they travel 600 miles north,

navigating through the ice to reach rich fishing grounds.

More whales travel along the edge of the ice where it meets the open sea,

to search for the openings of leads.

It's a hazardous journey. As mammals, they need air to breath

and if the ice closes above them they could suffocate.

They're looking for holes in the ice where they can surface.

A single breath will last them for 1 5 minutes.

The further they travel, the harder it becomes to find holes in the ice.

They snatch a breath and then travel on.

But ahead, the ice forms an impenetrable barrier.

The narwhal use their heads and their long tusks

to break the ice and enlarge the breathing holes.

For now they've reached a dead end and must wait.

The ice will need to melt further if they are to continue.

All across the Arctic the sea ice is beginning to retreat.

The leads widen, forming tracts of open water.

Wind and ocean currents shift the loosening ice, breaking it up.

As the ice melts, thousands of miles of open ocean become accessible,

providing a bonanza for millions of seabirds.

In just four weeks a colony of guillemots

can devour one and a half million tonnes of arctic fish.

As the ice melts, their journey to their feeding grounds in the open sea

shortens by the day.

They gather at the edge of the retreating ice

and dive to hunt for arctic cod and capelin.

The shoals are found 80 metres down in the murky depths,

and the guillemots must hold their breath for nearly two minutes.

With their crops stuffed with fish, they return to the colony

to feed their hungry youngsters.

With 24 hours of daylight, they go on fishing around the clock.

Warmed by the sun and driven by winds and currents,

the sea ice is now fragmenting.

That is a big problem if you need the ice to hunt from.

A polar bear can smell a seal from over a mile away.

But the prey he depends on is hard to find

in this constantly moving landscape.

He must take to the water to navigate through the drifting maze of ice.

Bears are excellent swimmers, but he's no match for a seal.

As the ice melts, finding seals gets harder

and with more open water, it's easier for a seal to escape.

The melt has caused a shift in power.

Now it's ''advantage seal''.

The bear's chance has slipped away, and his hunger grows.

The ice barrier broken,

strange marine visitors begin to arrive.

They've swum 600 miles to get here, and with the ice gone

they can make their way into the shallows of freshwater estuaries.

Beluga whales.

As more and more belugas arrive,

a strange annual ritual begins.

For belugas, this is a very special event.

The violent thrashing against the river bed loosens their year-old skin

and rids them of unwelcome parasites

they may have picked up on their journey.

In freshwater, warmed by the strengthening sun

and with the smooth pebbles in the shallows to rub against,

the whales whistle with pleasure.

But the ice-free summer will be short.

The whales can only enjoy their arctic spa for two weeks.

Soon the belugas must leave the shallows

and make the most of what's left of the summer

to hunt the shoals of arctic fish.

It's early July,

and at the bird cliff the fox has caught a nesting fulmar.

She has a family now, eight tiny mouths to feed,

and one bird is not enough to go round.

The cubs bicker over their dinner.

They're only three weeks old and in just another two weeks

they'll have to be ready to fend for themselves.

The winter ahead will be so harsh that only two of these eight cubs

are likely to gain enough weight to survive.

Their lives depend on every mouthful.

When food is scarce the most dominant cub will feed.

And the smallest will go hungry.

The seabirds have been working round the clock to keep their chicks fed

and to prepare them for the first big challenge of their lives.

If they are to escape the winter,

they'll have to leave the cliffs right away, and get far out to sea.

But it's only been 20 days since they hatched

and their wings are still too weak for them to fly.

So far their feet have never left the ground