Planet Earth

A hundred years ago,

there were one-and-a-half billion people on Earth.

Now, over six billion crowd our fragile planet.

But even so, there are still places barely touched by humanity.

This series will take you to the last wildernesses

and show you the planet and its wildlife as you have never seen them before.

Imagine our world without sun.

Male emperor penguins are facing the nearest that exists on planet Earth,

winter in Antarctica.

It's continuously dark and temperatures drop to minus 70 degrees centigrade.

The penguins stay when all other creatures have fled

because each guards a treasure,

a single egg resting on the top of its feet

and kept warm beneath the downy bulge of its stomach.

There is no food and no water for them

and they will not see the sun again for four months.

Surely, no greater ordeal is faced by any animal.

As the sun departs from the Antarctic, it lightens the skies in the far north.

It's March, and light returns to the high Arctic,

sweeping away four months of darkness.

A polar bear stirs.

She has been in her den the whole winter.

Her emergence marks the beginning of spring.

After months of confinement underground, she toboggans down the slope,

perhaps to clean her fur, perhaps for sheer joy.

Her cubs gaze out at their bright new world for the very first time.

The female calls them,

but this steep slope is not the easiest place to take your first steps.

But they are hungry and eager to reach their mother,

who's delayed feeding them on this special day.

Now she lures them with the promise of milk,

the only food the cubs have known since they were born, deaf and blind,

beneath the snow some two months ago.

Their mother has not eaten for five months

and has lost half her body weight.

Now she converts the last of her fat reserves into milk for her cubs.

The spring sun brings warmth, but also a problem for the mother.

It starts to melt the sea ice.

That is where she hunts for the seals she needs to feed her cubs

and she must get there before the ice breaks up.

For now, though, it's still minus 30 degrees,

and the cubs must have the shelter of the den.

It's six days since the bears emerged

and spring is advancing rapidly.

But even now, blizzards can strike without warning.

Being so small, the cubs are easily chilled

and they would be more comfortable resting in the den,

but their mother must keep them out and active.

She's becoming weak from hunger and there's no food on these nursery slopes.

The sea ice still holds firm, but it won't last much longer.

Day 10, and the mother has led her cubs a mile from the den.

It's time to put them to the test.

They've grown enormously in confidence,

but they don't have their mother's sense of urgency.

At last it seems that they're ready for their journey

and only just in time

for, a few miles from the coast, the ice is already splitting.

Now the mother can start hunting for the seals they must have,

but she's leading her cubs into a dangerous new world.

Nearly half of all cubs die in their first year out on the ice.

Summer brings 24 hours of sunlight and a thawing, shifting landscape.

Further south, the winter snows have almost cleared from the arctic tundra.

Northern Canada's wild frontier.

Here, nature stages one of her greatest dramas.

Every year, three million caribou migrate across the arctic tundra.

The immensity of the herd can only be properly appreciated from the air.

Some herds travel over two thousand miles a year in search of fresh pastures.

This is the longest overland migration made by any animal.

They're constantly on the move.

Newborn calves have to be up and running the day they're born.

But the vast herds do not travel alone.

Wolves.

Packs of them, eight to ten strong, shadow the migration.

And they're hungry.

It's the newly-born calves that they are after.

Running directly at the herd is a ploy to generate panic.

The herd breaks up and now it's easier to target an individual.

In the chaos, a calf is separated from its mother.

The calf is young, but it can outrun the wolf if only it manages to keep its footing.

At this stage the odds are even.

Either the caribou will make a mistake

or, after a mile, the wolf will give up.

Midsummer on the tundra and the sun does not set.

At these latitudes, the sun's rays are glancing

and not enough of their energy reaches the ground to enable trees to grow.

You need to travel five hundred miles south from here before that is possible.

These stunted shrubs mark the tree line, the beginning of the Boreal forest,

the taiga.

The needle-shaped leaves of the conifers are virtually inedible,

so this forest supports very little animal life.

It's a silent place where the snow is unmarked by footprints.

In the arctic winter, snow forms a continuous blanket across the land.

But as spring creeps up from the south, the taiga is unveiled.

This vast forest circling the globe

contains a third of all the trees on Earth

and produces so much oxygen, it changes the composition of the atmosphere.

As we travel south, so the sun's influence grows stronger

and at 50 degrees of latitude, a radical transformation begins.

Summers here are long enough for broad-leaved trees to replace conifers.

Broad leaves are much easier to eat and digest

so now animals can collect their share of the energy

that has come from the sun.

It's summer and these forests are bustling with life.

But the good times will not last.

Broad leaves must be shed in winter for they're damaged by frost.

As they disappear, so the land becomes barren with little for animals to eat.

The inhabitants must migrate, hibernate or face months of near-starvation.

The Amur leopard, the rarest cat in the world.

Here, in the deciduous forests of eastern Russia, the winter makes hunting very difficult.

Prey animals are scarce and there's no concealing vegetation.

The cub is a year old and still dependent on its mother.

Deer are frequent casualties of the harsh winter

and these leopards are not above scavenging from a corpse.

African leopards could never survive here

but the Russian cats have thick fur to shield them from the cold.

There are only 40 Amur leopards left in the wild, and that number is falling.

Like so many creatures, the cats have been pushed to the very edge of extinction

by hunting and the destruction of their habitat.

The Amur leopard symbolises the fragility of our natural heritage.

The future of an entire species hangs on the survival

of a tiny number of mothers like this one.

All animals, rare or common, ultimately depend for their energy on the sun.

In Japan, the arrival of the cherry blossom announces the beginning of spring.

The sun's energy brings colour to the landscape.

The Earth, as it makes its annual journey round the sun, spins on a tilted axis

and it's this tilt that creates the seasons.

The advance of the seasons brings constant change.

As the sun's influence diminishes in the north,

so the deciduous forests of America begin to shut down,

losing their leaves in preparation for the dark, cold months ahead.

One season hands over to another.

Some organisms thrive on decay,

but most must make special preparations for winter and a life with little sun.

Whole populations of animals are now forced to travel great distances

in pursuit of food and warmth.

Three hundred thousand Baikal Teal gather to escape from the Siberian winter

by migrating south to Korea.

The world's entire population in a single flock.

But there are parts of the world that have no seasons.

In the tropics, the sun's rays strike the Earth head-on

and their strength is more or less constant all year round.

That is why the jungle grows so vigorously and supports so much life.

This forest covers only 3% of the planet's surface

but it contains more than 50% of all its plants and animals.