Original subtitles

A Perfect Planet.

We live in a world with a violent atmosphere.

Rain clouds form, and powerful winds carry their fresh water around the globe.

Without this, our perfect planet simply couldn't function...

for all life on land depends on the weather.

We all know that the weather can change on a daily, even hourly, basis.

But annual weather patterns have been stable for millennia...

and it's this reliability on which life depends.

Every year in October, the weather brings a dramatic transformation

to this tiny patch of forest in Zambia.

Storm clouds begin to build.

And right on cue,

straw-coloured fruit bats start to appear.

They've travelled hundreds, if not thousands, of miles to be here.

And they always arrive at exactly the same time.

They know that these rains will transform this forest,

allowing countless fruit trees to ripen.

And the bats gorge on the new-found abundance.

For just 12 weeks every year,

this forest becomes home to ten million fruit bats.

One of the greatest gatherings of mammals on Earth,

all crowding into an area no bigger than a dozen football pitches.

From this hub, the bats fan out,

consuming more than 300,000 tonnes of fruit,

spreading the seeds and regenerating the forests.

Africa's largest wildlife migration - entirely dependent on the rain

that comes to this small corner of Zambia

at the same time each year.

Every second, over 13 million tonnes of water

evaporate from our oceans, forming clouds.

The spin of the Earth and the prevailing winds

determine where these clouds blow.

As a consequence, some places get less rain...

and others much more.

The Amazon rainforest...

one of the wettest places on Earth.

It gets three metres of rain a year...

so life has learned to deal with the near-daily downpours.

Every year, for thousands of years, the persistent rains

have flooded vast tracts of the Amazon...

turning the jungle into a seasonal wetland.

Anything living here must either climb or swim.

But there's one extraordinary animal that has turned this deluge

from a disaster into an opportunity.

This fire ant has reached the edge of her known world.

Her way forward has now become blocked by the rising waters.

She's one of many, and in a few minutes

their underground home will be submerged.

Forced to evacuate, workers and soldiers try to save the larval young.

As their nest floods, the colony seems to be doomed.

But from chaos comes order.

The fire ants band together, each locking legs with its neighbours.

They're building a living raft.

Each ant is covered in fine hairs that trap air against its body.

As the raft forms, this air creates a cushion for them all to float on.

This is the power of the colony.

By working together, they've become unsinkable.

And no-one gets left behind.

Pulled from the shallows, the colony is now at the mercy of the flood.

Holding tight to the larvae

and keeping their queen hidden at the centre of the raft,

the colony drifts into deeper water...

where they're vulnerable to fish that now swim through the flooded forest.

Those that lose their grip must get back to the safety of the raft.

With jaws that join to become a surgical needle,

this pond skater could suck an ant dry.

But together, the colony is strong, and it's time to repel boarders.

The ants can travel like this for weeks.

They've even come to depend on the flood to carry them

to new feeding grounds.

At last, the ants can make their raft secure.

Surging forward, they claim their new home...

evicting current tenants without hesitation.

One of the first to safety is their larval queen,

carried from the heart of the raft.

This palm will be their home for the next three months,

until the floodwaters recede and they can head back underground.

The average temperature of our planet hasn't varied

by more than a single degree Celsius in 10,000 years.

This has made our climate stable,

allowing animals to fine-tune their behaviour

around predictable weather patterns.

October in the Amazon.

The heaviest rains have come to an end,

and the falling river level exposes a new landscape.

Three weeks ago, these mile-wide sandbars didn't exist.

In another three months, they'll be back under water.

But now they provide a crucial opportunity for giant river turtles.

They've been coming here at precisely this time of year for generations.

50,000 of them - a quarter of the world's population.

They're all here for one reason.

But their window of opportunity is short, so timing is everything.

Scaling the three-metre-high banks will take this female higher and drier

than she's been for months.

She's come here to lay her eggs.

But others have beaten her to it.

The early arrivals have already claimed nearly every inch of the beach...

and there are still many more turtles to come.

Wherever she turns, the space is taken.

But being fashionably late does have an advantage.

Had she arrived earlier,

her eggs might have been dug up by those searching for somewhere to lay.

She's found her spot.

Settling down, she lays nearly 100 eggs.

Burying them keeps them safe from predators,

and the warm sand incubates the clutch.

They must hatch before the river rises again and floods the sandbar.

Any turtles that lay after her may have left it too late.

Hopefully, she has got her timing right.

Two months on, and the hatchlings emerge into the world.

They were laid at the perfect time.

Hundreds scramble for the river

just as the water starts to rise.

Those still to hatch aren't so lucky.

This year, the heavy rains have returned early.

The nesting beach is beginning to flood.

Most hatchlings can escape in time.

Those still below ground...

will drown in their nests.

For thousands of years, giant river turtles have timed their breeding

around the Amazon's reliable cycles of rain.

But today, our weather is changing.

Disastrous early floods like this used to happen once every 20 years.

Now the turtles have to deal with them every four.

Winds may carry huge amounts of rain to the Amazon,

but they don't provide this service to every corner of the planet.

A third of the land receives less than 25 centimetres of rain a year.

Deserts are scarred by desiccating winds and baked by the sun.

But even weather as challenging as this can be perfect for the locals.

As darkness falls in the coastal deserts of South Africa,

the dunes come alive...

with the call of a surprising desert dweller.

A rain frog - not what you'd expect

in a place that gets just seven centimetres of rain a year.

To survive here, she has to avoid the heat of the day

buried in the sand.

But each night, she heads out on her hunt for moisture.

Short stumpy legs and a plump body are ideal for digging...

but not so good for getting around.

She can't hop, so she waddles across the dunes.

The world's slowest steeplechaser.

Obstacles are tackled with dogged perseverance.

She can travel a whopping 38 metres a night,

no mean feat when you're the same size and shape as a marshmallow.

Termites.

They are 75% water...

a meal and a drink in one.

But she's not the most agile predator.

Maybe if they just came a little closer...

Oh, dear.

Perhaps if they stop moving...

At last!

But now she has another challenge.

To swallow her food, she must squash in her eyeballs to push it down her throat.

Termites may be juicy,

but they don't provide her with quite enough water to survive.

It rarely rains here,

but coastal deserts do get another reliable source of moisture.

At night,

cool moist air is blown in from the ocean,

forming fog banks that shroud the desert in mist.

It condenses on vegetation, wetting the sand beneath.

So it's here that she waits, absorbing moisture through her skin.

Rain frogs are so dependent on this water

that they can only survive in places with at least 100 days of fog a year.

And on the foggiest nights, when other frogs have emerged onto the dunes,

she takes her opportunity to find love.

Male rain frogs are smaller than the females,

and his stubby legs make it hard to hold on,

so he glues himself onto her back.

Still locked in their embrace, the pair head back beneath the sand,

where they'll lay their eggs, safe from the heat of the desert sun.

But there's one desert that gets so cold

that liquid water simply can't exist.

Winter in Mongolia's Gobi Desert...

40 degrees below freezing.

One of the driest and most extreme environments in the world.

Most animals can't survive weather like this...

but there's one unique mammal that manages to eke out a living here.

The Gobi is home

to the last 1,000 wild Bactrian camels on Earth...

surviving at the very edge of what's possible.

Even camels need moisture, but any water here is now frozen solid.

It's down to the bull to find a solution.

Setting off with his herd, his search could cover thousands of square miles.

Driving them forward, he can keep going for days on end.

Herds come together from across their vast range...

all looking for the same thing.

These long-distance specialists can smell moisture from 30 miles away...

and they've picked up a scent.

Even in the Gobi, one of the driest places on Earth,

wind can provide a rare source of moisture.

Snow is blown to this desert from Siberia.

Where it settles is impossible to predict, and it may not last long.

But for now, at least, this bull has found what he's been searching for.

In the cold, dry air,

snow doesn't melt...

so it must be eaten.

The Gobi's windblown snow is a lifeline

that will sustain his herd throughout the winter.

Even in this most hostile of places,

weather still provides just enough for survival.

The largest, most unpredictable weather on the planet

starts far out at sea.

Hurricanes, typhoons and tropical cyclones

are violent, short-lived megastorms.

But there's a far more reliable weather system

that originates over the ocean...

one that is eagerly anticipated by all in its path.

Monsoons are seasonal winds that blow at the same time each year...

and they carry colossal amounts of water from the ocean to the land.

Christmas Island.

A remote speck of rock in the Indian Ocean.

For the last four months, its forest interior has remained dry and dusty.

Life here is waiting for change.

In November, the first storms of the monsoon reach the island.

They're a welcome relief for those that have suffered through the dry.

And they are vital for the island's most numerous residents.

The red crabs of Christmas Island.

They've spent the dry season sheltering in damp burrows

beneath the leaf litter.

Crabs breathe through gills that must be kept moist.

So it's only with the return of the annual monsoon

that these land crabs can finally emerge.

With the humidity now high enough, they can begin an epic trek to the coast.

Every red crab comes together from across the island,

and they do so in their millions.

50 million, to be precise.

They've come to release their eggs that can only develop in the ocean.

There's just one small problem.

They may be crabs...

but they can't swim.

They're scared of the sea.

But they don't have a choice.

They must release their eggs on the highest tide.

Each carries around 100,000 eggs,

so over just two or three nights,

trillions are released into the ocean...

though some are more successful than others.

Once their work is done, the crabs will return to the forest.

Then, four weeks later,

on the next highest tide...

having hatched out to sea,

a new generation of red crabs returns to the same beach.

In good years like this one,

countless millions arrive back on the shores of Christmas Island,

turning the beaches red.

Like their parents, they also need moisture in the air to breathe...

so before the rains move on,

the young crabs must undertake their own epic journey into the forest,

where they'll wait out the next few months of dry.

The Christmas Island crab -

entirely reliant on the return of the monsoon.

The annual rhythms of the weather shape life across our planet.

In some parts of the world,

seasonal cycles of wet and dry

can bring dramatic change.

And there are few places where this is more apparent than Southern Africa.

The Zambezi River.

It flows across six countries and drains half a million square miles.

At the peak of the wet season,

5,000 tonnes of water cascade

over Victoria Falls every second.

And yet, each year when the rains stop,

these falls run dry.

It's almost as if someone turns off the tap.

May.

December.

Wet season.

Dry season.

It's happened like this for thousands of years.

But there are signs that the weather here is changing.

In recent years,

the volume of water flowing over Victoria Falls has dropped by half...

a concern for animals that have come to rely on

the natural rhythms of wet and dry.

Zambia's Luangwa River is a major tributary of the Zambezi.

Every year, as the dry season takes hold,

it dwindles into pools.

Accommodation starts to get tight for the locals.

With no other water around, the river becomes a magnet for visitors.

Millions of red-billed quelea flock to the water's edge.

These seed-eaters need to drink every day...

and Nile crocodiles know it.

They may not have eaten for six months.

For them, the dry season is the start of the good times.

And they are not the only ones to depend on this time of year.

Carmine bee-eaters have flown hundreds of miles

from the rainforests of the Congo...

arriving at the peak of the dry season,

when steep sandbanks are exposed by the falling water.

They're a perfect place to nest, out of reach of predators from above.

Bee-eater burrows can extend for two metres into the soft river sand.

Already home to 6,000 nests,

real estate at Bee-eater Towers is in high demand.

But it pays not to build too close to the neighbours.

As more birds arrive, competition for the best nesting sites heats up.

It's thirsty work.

In the sudden commotion, they're vulnerable from below...

and above.

African fish eagles.

They are permanent residents here.

But at this time of the year, they don't just eat fish.

The safest place to be is inside a burrow.

But an eagle fly-by sends shockwaves of panic through the colony.

The eagle's strategy is to pin their prey against the river bank.

With the colony in disarray, the predators have the advantage.

These fish eagles also choose to breed in the dry season

because they need the bee-eaters as food for their chicks.

But this year's dry season shows no sign of letting up.

Parts of the river are now bone dry,

forcing huge male hippos from their home.

With nowhere left to turn,

they must converge on the few remaining patches of water.

But these are already occupied.

The resident bull here

can't tolerate another big male in his shrinking pool.

The newcomer is banished.

Still, the sun bakes the land...

with devastating consequences.

Hundreds of nests are wiped out.

Southern Africa has experienced dry seasons

for thousands of years.

But this is something different.

Record high temperatures and the lowest rainfall in nearly 40 years

have thrown parts of Africa into one of the worst droughts in a century.

Across the planet, our once stable climate is changing.

Weather is less reliable...

and is getting more extreme.

Wildfires are becoming more frequent.

Droughts are more intense.

And storms are more ferocious.

All this from a recent rise in global temperatures

of just a single degree Celsius.

Yet if climate change continues unchecked,

we may see a rise of almost five degrees by the end of the century.

We can only imagine what this will do to our weather.