Original subtitles

[narrator] Earth is a restless place.

[thunder crashing]

Its seasons driven

by the orbit around our sun.

An epic journey

that sets the rhythms

of all life.

Humans honour this passage through ritual and celebration.

[fireworks crackling]

Nature marks the cycle with its own patterns.

Over the course of a year,

the planet's most spectacular events of migration,

rebirth

and transformation unfold.

[roaring]

Moment by moment,

[roaring]

month by month,

throughout a wild year on Earth.

Early July sees the sun's abundant energy

focused on the northern hemisphere.

Intense light and heat

act as catalysts on the ice caps of the Arctic,

as they start to shift and thaw.

Water carves vertical rivers

that plummet deep into the frozen abyss.

[water rushing]

The icy landscape accelerates its flow towards the sea.

Huge fault lines appear.

Something has got to give.

[crashing]

This is iceberg calving season.

The ice sheets in Greenland

shed billions of tons of ice a year during the melt.

In recent years,

the melt season has started earlier and is lasting longer.

Now, more ice melts in the summer

than gets replenished over winter.

The overall loss of ice has become catastrophic.

30 years ago,

Greenland's ice sheets

would melt 25 billion tons of ice each summer.

Last season,

more than 600 billion tons of ice was lost in the melt.

The climate is truly changing.

The glacial melt

adds to some of the world's most nutrient-rich waters in the Svalbard archipelago.

[squawking]

By July,

millions of seabirds have migrated from the south

to feed around the clock in 24-hour daylight.

It's an ideal place for birds to breed and raise their young.

Auks have chosen to nest in the scree slopes

at the base of the island's cliffs.

For Arctic foxes that have spent the winter scavenging,

it's a perfect time to hunt.

[ominous music playing]

The eggs are well protected,

but the adult birds still have to venture out.

[squawking]

[dramatic music playing]

Though, attacking them head-on rarely works.

The auks are just too quick off the mark.

What's required

is some of that famous fox cunning.

[chirping]

And a lucky break wouldn't hurt.

A gull is also on the hunt.

[suspenseful music playing]

The threat from above distracts the auks for a split second.

And a split second is all the fox needs.

He has outfoxed the auks.

[squawking]

Many auk species avoid predation

by nesting on the precarious cliffs.

The more perilous their perch, the better off they are.

Each pair only have one precious offspring.

It's only three weeks since the chick has hatched.

But already, it's eager to leave the nest.

It's quite the maiden flight.

Hundreds of metres to the safety of the ocean.

And every summer,

one brave chick is the first to take the leap.

[heroic music playing]

It's more like falling than flying

as their tiny wings act as a parachute.

But their fathers follow them down.

Helping to steer them towards a water landing.

Soon, the sky begins to fill with jumpers.

And their chaperones.

In just a few days,

100,000 chicks will take the leap.

None do it alone.

But not all make it.

[crying]

Some fall short on a dangerous landing strip

filled with hungry predators.

Luckily, Dad is about.

And from here, it's a quick dash to the sea.

The chicks that make it to the water

have no time to rest.

Not strong enough to take to the air again,

the chicks now commence a heroic migration,

swimming to waters off Iceland.

Even as far as North America.

Over 1,000 kilometres away.

[birds cawing]

In Alaska,

the land is also shedding its icy cloak,

as streams splash towards the sea.

These changes trigger the arrival

of another great traveller.

Sockeye salmon are returning to Bristol Bay,

the place they were born.

They have migrated thousands of kilometres

from the deep ocean,

using the earth's magnetic field like a compass.

The smell of their home stream

has been imprinted in their memory since birth.

[water flowing]

Their arrival is keenly anticipated by brown bears,

who start gathering along the banks of the bay

as the salmon return.

They've come for the ultimate teddy bears' picnic.

As the salmon exhaust the last of their energy to spawn,

they reach the end of their life cycle.

What seems like a river of death

is actually a vital regenesis.

Rivers usually take nutrients out to sea.

But here,

the annual spawning returns nutrients to the rivers.

Feeding not just the bears,

but also, the trees for kilometres around.

It's a cycle that is reset every year,

where nothing goes to waste.

Across North America,

summer warms the land.

California's valleys are iridescent with growth.

Its beaches bustle.

Inland, its deserts sizzle.

In the rolling sands of the Sonoran Desert,

there's no relief under the cloudless skies.

[rattling]

The sidewinder snake

must change her behaviour in summer.

To avoid the blistering heat,

she limits her time searching for prey,

venturing out only in the early morning and dusk.

Shunning the hot sand,

she restricts her contact to two body parts at a time.

The peculiar mode of locomotion

helps make her the fastest rattlesnake on sand.

Leaving a J-shaped signature as she hunts.

Dunes like mountains become a Sisyphean task.

[eerie piano music playing]

[sand flowing]

Persistence is the only option for this hungry snake.

But with little moisture to carry scent,

sniffing out prey is hard.

Using special temperature pits on the side of her head,

she scans the environment for life's heat signal.

Able to pick out the slightest variation

between the hot sand

and a hot lunch.

A gecko is cold-blooded,

making it virtually invisible to the snake's thermal sensors.

But that only works if it stands still.

One false movement, and it's game over.

Fortunately, the sidewinder

can survive an entire month without food.

Enough time to catch a meal.

Deserts cover almost one quarter of the Earth's surface.

But not all deserts are scorching sand dunes.

The arid steppes of Central Asia

are an empty, windswept place.

But as high summer draws close,

scattered rains bring the promise of greener pastures.

Drifting with the season,

the nomads of Mongolia are always on the move.

When grazing is scarce,

they must travel longer distances.

Over hundreds of kilometres to find pasture.

Their lives are inextricably tied to their horses.

It's been part of their existence

for over 3,000 years.

Children learn to ride

almost as soon as they are able to walk.

And summer is the time for gathering

and racing across country.

The nomads believe to be without a horse,

is like a bird without wings.

[screaming]

The nomads depend on their livestock

for all of life's essentials.

So they will stay here

as long as there is enough grass.

Using the sun and stars, the hills and mountains

to find their way.

For these people,

life is in perpetual motion.

Further westwards,

summer is also transforming

the mighty Himalayas.

The rooftop of the world

reaches over 7 kilometres into the sky.

[triumphant music playing]

Known as the Third Pole,

the Himalayas are home to the largest body of ice

outside the Arctic and Antarctica.

The heat of the summer

causes meltwater from the mountains

to spill into ravines and fill rivers

running towards the lowlands.

By July,

the migration of water from mountaintops to valleys reaches a crescendo.

In the Terai region of Nepal,

floodplains receive rich deposits of Himalayan silt,

providing nutrition

for some of the tallest grasses in the world.

July's lush growing season

also cultivates another

of nature's seasonal flourishes.

[buzzing]

Insects.

One particular insect

stirs from its hibernation state.

Hatching from water-borne wriggler

into its winged form.

The warm, wet conditions

are perfect for mosquitoes.

Females make a higher pitched buzz than males,

and it's the females that cause vexation.

Because only they seek a blood meal.

It's not just mosquitoes.

Ticks, gnats, flies

all boom at this time of year as well.

It's impossible to keep up

with the endless swatting of pesky freeloaders.

The greater one-horned rhino

may appear to have impenetrable armour.

However, its plates are nothing but big folds of skin.

And it's the folds the flies love best.

For these cracks

are the sweatiest parts of the rhino's body,

full of bacteria and dead skin cells.

Delicious.

Living with a blanket of bugs

is a constant summer time irritation

for the rhino.

But help is at hand.

Mynah birds love to forage

on creepy crawlies.

Even the baby rhino

becomes a walking dinner table.

[dramatic music playing]

Despite the irritation of insects,

for the mynah bird,

they provide a veritable feast.

[buzzing]

[rain pouring]

July and August

are monsoon months across South East Asia.

[water flowing]

The rains reach into Cambodia,

causing the mighty Mekong River to flood.

Swollen with so much water,

the river starts to flow backward,

away from the sea.

The inflow of water is so immense

that it triggers the largest freshwater lake

in South East Asia to balloon.

Lake Tonle Sap

grows an astonishing six times its former size,

becoming a vast inland sea

that swallows a fifth of Cambodia.

It's one of the biggest seasonal variations

faced by any people on the planet.

Roads become rivers.

Houses built on stilts can now only be reached by boat.

[rowing]

The water teems

with the largest gathering of freshwater fish in the world,

filling both the lake

and the surrounding flooded forests.

[chirping]

As the lake grows,

millions of birds migrate here

to feed on the seasonal feast.

They'll stay here

till the life-giving waters start to recede.

But for now,

it's a fisherman's paradise.

[chirping]

While the natural world is on the move, humans are too.

It's the last lunar month of the Islamic calendar.

Millions of pilgrims travel to the Holy City of Mecca

in Saudi Arabia,

the spiritual centre of Islam.

All Muslims strive to make The Hajj

at least once in their lives.

Circling the Kaaba seven times, all sins are dispelled.

Each year,

more than two million pilgrims gather

to join the sacred sea of people.

The Islamic calendar

follows the motion of the moon, not the sun.

It's one of many calendars across cultures

recording the passing of time since antiquity.

For the natural world,

the calendar is unwritten.

Length of day.

Falling temperature.

A change in current announces, "It's time."

Southern right whales

rely on these natural timekeepers

to know when to move.

Each year,

as winter takes its grip on the South Pole

and the days become shorter,

whales migrate from the waters around Antarctica

to the safe harbour of Australia.

In the warm shallows, protected from predators,

a young calf takes a vital breath.

[uplifting music playing]

Here in the calm winter waters,

the calf fattens up on mother's rich milk.

In a few months when the days are longer

and the calf is stronger,

they'll embark together on the long return journey

to the feeding grounds of Antarctica.

Further south,

in a bay of island Tasmania,

the churning surface of the sea cloaks what stirs below.

A spindly army of spider crabs

march to the beat of their primordial drum.

Winter sea temperatures

have triggered their advance.

In places,

they are ten bodies thick,

forming a mountain of crustaceans.

Hundreds of thousands of crabs

migrating to the shallows to transform.

An itch here.

A scratch there.

A sign that the old coat

is ready to come off.

One of the first to moult,

this male secretes enzymes

to lift and separate his old shell.

He cracks through

to reveal his new coat,

bright orange and soft.

His shell offers no protection until it hardens.

Now, at the most vulnerable moment in his life,

the very crowd he sought for protection

turn against him.

[dramatic music playing]

All around, newly moulted crabs are hunted and ripped apart.

This mutilated crab

limps desperately searching for cover.

But he's an easy target.

Even with four missing limbs,

he fights back.

Perhaps these angels will give him sanctuary.

Safe at last.

In time, he'll eventually regenerate.

His attackers

may not be so lucky.

Smooth stingrays arrive for this all-you-can-eat buffet.

It's hard to know where to start.

Entree.

Main.

Dessert.

And with a flick of his tail,

he is back for seconds.

The crabs' only defence is in their sheer numbers.

By dawn,

hundreds have been hoovered up.

[suspenseful music playing]

Finally satiated,

the rays slink into the murk.

[sad music playing]

The surviving spider crabs

march solemnly towards the depths,

stepping over fallen comrades.

Or is it leftover lunch?

This mob will meet again next year

for the same gruesome event

in Tasmania's winter waters.

On the continent of Africa,

it's the movement of rain that sets the pulse for life.

By August,

one of the natural world's greatest migrationsis now underway.

On the border of Kenya and Tanzania,

wildebeest are on the move.

Having left the Serengeti,

they have travelled over 500 kilometres north.

Towards the savannah grasslands of the Masai Mara,

storms on the horizon are their beacon.

They know instinctively that where there's rain.

there's greener pastures.

As the living tributaries converge,

the wildebeest grow in numbers.

[stampeding]

Soon the herd is over one million strong.

By August,

the herd reaches the Mara River.

The crocodiles have been expecting them.

For some,

this is the first time

they've encountered such an obstacle.

Others know all too well what's to come.

Herd leaders assess the danger,

searching for a crossing point where the odds seem just a little bit better.

They pause at the water's edge,

waiting for someone to break rank.

Somehow,

he makes it past the waiting jaws.

Then, the dam breaks.

[triumphant music playing]

In an hour,

thousands will cross at this one point.

Most will make it.

Some will not.

The river takes its sacrifice.

As the rain clouds move on,

so too do the wildebeests.

To the lust pastures of Kenya.

[calming music playing]

Further south, in Zambia,

the lack of rain and relentless sunshine

sucks the life from the earth.

The clear blue skies

are welcomed by Carmine bee-eaters.

Every year,

they migrate hundreds of kilometres southward

to converge here,

on Zambia's Luangwa River.

As the water level drops, it exposes the riverbank.

This is what the birds are seeking.

For it's where they nest.

[triumphant music playing]

[chirping]

Each bird rushes to secure a spot.

But they won't necessarily get their first choice.

Resident white-fronted bee-eaters are already here.

They live year-round in the region.

And recently moved to the riverbank to breed.

They've had a head start making burrows.

And won't be evicted without a fight.

[chirping]

[triumphant music playing]

But the migrants haven't travelled this far

for nothing.

[chirping]

A prime patch of real estate is worth every flying feather.

Both bird species dig into the riverbank

to lay their eggs.

The location of the burrow is key.

Too close to the bottom

and reptiles can barge through the door.

Too close to the top,

and they're in easy reach for other egg thieves.

The middle of the bank is optimum.

Passers-by can do just that.

Thankfully,

all neighbourhoods have access to food.

Even mid-dry season,

permanent waterholes of the Luangwa River

harbour aquatic insects.

And the birds are nimble hunters.

But their favourite food is along the riverbank.

In August,

the sausage tree is in bloom.

That's a boom for bees.

And bee-eaters.

[chirping]

The only problem with this diet

is the sting in the tail.

But that's quickly disabled.

Like all halcyon days,

they'll soon be over.

The bee-eaters must hatch and fledge their chicks

before the rains return and the river rises.

When this happens,

the riverbank nests will vanish underwater.

And the birds will once again, take to the wing.

The great animal migrations on the African continent

are rivalled by similar migrations in the African seas.

Along the KwaZulu-Natal coastline in South Africa,

the arrival of a cold-water current

sweeps up the coast,

carrying with it a bloom of plankton.

This cool, northwards flow

helps propel billions of sardines in search of food.

At their peak,

shoals can span over seven kilometres,

providing an irresistible lure for predators.

Army of common dolphins, 15,000 or more,

arrive ready to strike in a coordinated assault.

They separate the school of sardines,

corralling the fish

by blowing a barricade of bubbles.

Weaving them into tighter and tighter balls.

[clicking]

As they push the sardines to the surface,

the tiny fish have nowhere to escape.

The oily scent of the battle attracts more hungry predators.

Dusky sharks move in

to take advantage of the dolphins' hard work.

[dramatic music playing]

Jaws slash through the bait balls with precision.

The churn on the surface signals to the aerial attack force.

Gannets hover.

Then they too, join the fury.

[intense music playing]

Diving at 100 kilometres an hour,

they strike the water and torpedo to depth.

[intense music playing]

But the feast is worth it.

The predators along this coastline

cannot always rely

on the sardines passing their way.

[clicking]

Warming oceans means the cool currents

which trigger the sardine run

have been happening later and later.

Some years,

the run has failed completely.

For many species across our planet,

the timing of natural events is changing

because of rising global temperatures.

For the creatures adapted so precisely

to nature's biological clock,

climate change presents a new rhythm they must shift to.

If they can.

If they cannot,

many of the great migration events on our planet

may diminish,

or simply disappear.