Wind How Earth Made Us

Our planet is full of astonishing natural wonders.

Look at that!

Oh!

It has immense power.

And yet, that's rarely mentioned in our history books.

I'm here to change that.

I'm looking at four ways that the power of the planet has shaped our history.

The power of fire,

the source of great technological breakthroughs.

Water...

Oh, my gosh! You're getting all wet there.

..our struggle to control it has directed human progress.

The deep Earth...

Blooming heck! That really is deep.

..that provided the raw materials for our conquest of the planet.

But this time I'm looking at the power of the wind.

For thousands of years,

the wind has shaped the destiny of peoples across the globe.

It has built fortunes and brought ruin.

Even today, we're still at the mercy of the wind.

WIND WHISTLES

People have exploited the wind for thousands of years,

on land and, most of all, at sea.

And to really experience its awesome force, this boat is the place to be.

This is one of the fastest sailing boats ever built.

It's capable of up to 50 miles an hour.

And when you're down close to the water,

you can really feel that phenomenal speed.

But what makes this thing really special is when it starts to fly.

Whoo!

HE LAUGHS

But the real key to this craft's phenomenal breakneck pace is up there.

The sail. There's enough of it to actually cover a tennis court,

every inch of it grabbing every bit of energy from the wind

and converting it to pure power.

This is the power of the wind, the atmosphere in motion,

one of the most powerful and least understood forces on Earth.

We tend to think of the wind as chaotic and difficult to predict.

But when you look on a much bigger scale,

at the global picture over time,

a very different view emerges.

Weather systems, and with them the winds,

follow the same routes around the planet again and again.

The discovery of these patterns,

and sometimes the failure to understand them,

lie at the heart of some of the greatest adventures in human history.

To see a remarkable example of how powerful the wind can be

in changing people's lives,

I've come to a small town in the middle of the Sahara Desert called Chinguetti.

Today, it's almost lost in a sea of shifting sand dunes,

but once it was so much more.

There's a timelessness about this.

Some of the buildings are over 700 years old.

There's only a few thousand people live here now,

but in its heyday, this place heaved with 20,000 people.

And twice as many camels!

Hidden away down the back streets of this crumbling town,

there's a reminder of Chinguetti's glorious past.

-Bonjour.

-Ah, bonjour.

-Ca va tres bien?

-Ca va, ca va.

The Al Ahmad Mahmoud Library

has been run by the same family for over 300 years

and contains hundreds of ancient manuscripts.

What is the oldest...? Plus ancien livre?

Ah. Le plus ancien livre chez moi...

-LAUGHS:

-It's in a shoebox!

-Ah.

It's not hermetically sealed.

-MAN SPEAKS FRENCH

-Oh, wow.

Look at that.

Ah. What is this?

Ca, c'est le plus vieux Coran en Afrique de l'Ouest.

It's the oldest Koran in West Africa?

Dixieme siecle.

It dates back to the 10th century.

Oh, look, the writing's tiny.

This priceless book is one of thousands stored in dozens of libraries

throughout Chinguetti.

Ca, c'est les arabesques.

Arabesque, yeah, yeah. The colour is beautiful.

Chinguetti's glory days were over 500 years ago,

and it owed its existence as a thriving town to the wind.

Chinguetti is in the heart of the Sahara.

It's a barren, inhospitable wilderness.

The largest desert on the planet.

Ah.

Look at that.

It just goes on and on.

The Sahara is so hostile that crossing it is dangerous and difficult.

Searing heat, no water, immense distances.

It's effectively a climate barrier.

WIND HOWLING

Well, there's another reason why deserts and dunes are so hard to cross,

and that is, they simply don't stand still.

They are constantly on the move.

In fact, these are some of the most dynamic and rapidly changing landscapes

on Earth.

HE COUGHS

There are few reliable landmarks,

so following a route across the desert is incredibly hard.

But it's not only the shifting sand that's controlled by the wind.

The entire Sahara Desert itself was created by large-scale wind movements.

These winds begin at the equator.

This is where the sun is at its hottest, so the air is continually rising.

As it spreads away from the equator, it cools,

until between about 20 and 30 degrees latitude,

the air sinks back to Earth, heating up again in the process.

This pattern of winds creates a band of hot, dry deserts around the world

on either side of the equator,

including the Sahara and Arabian deserts.

In an era when travelling was done by foot,

the desert was a formidable barrier.

For most of human history,

different corners of the world have evolved as if in parallel universes,

hemmed in not just by mountains and oceans,

but by the desert that made climate a barrier too.

But about 1,000 years ago, nomads were forging routes through the Sahara.

Chinguetti was an oasis town along one of these routes.

To the south was gold and ivory.

To the north, the markets of Europe.

Chinguetti's fortune was made

because it was a gateway connecting two worlds

that were separated by the power of the wind.

But this city's great days didn't last.

The winds that created the desert barrier had brought it riches.

But ironically, its decline was also due to the wind.

In one short period, about 500 years ago,

the world was entirely remade,

transforming the fate of people around the globe.

And it was all down to a pivotal discovery about how the winds work.

This is the Gold Coast in Ghana, on the west coast of Africa.

Today, it's dominated by bustling fishing ports.

Everyone's got piles of fish!

But in the 15th century, it was an important centre for the gold trade.

Europeans began to trade with the rich empires of West Africa,

and the Portuguese built this fort, Elmina,

to protect their commercial interests.

And you could say it was here that the remaking of the world began.

You know, if you'd been looking out from this spot in 1482,

you'd have seen a Portuguese ship hove into view

carrying materials to build this fort.

On board was a man who would end up inadvertently changing the destiny

of this whole region.

And he did that not with swords and with cannons,

but with a discovery about how the Earth's atmosphere worked.

He also happened to discover a new continent.

His name? Cristoforo Colombo.

Christopher Columbus visited these shores

at an important moment in European history.

In the 15th century,

the nations of Europe were competing to find quicker, easier routes

to the riches of Asia.

Christopher Columbus was a man with a plan,

because he reckoned he knew a shortcut route to the Far East.

As he'd been sailing up and down this coast,

he'd been keeping a close eye on the winds.

Now, the West African coast juts out into the Atlantic,

so sailors here were sometimes forced into the open ocean.

Columbus realised that out there, among the rolling waves,

the winds seemed to be always blowing in the same direction -

away from Africa.

Columbus reckoned he could use that wind to blow him all the way round the world.

Columbus had no way of knowing

whether the wind he'd encountered along the West African coast would carry on

or peter out, leaving him stranded in the middle of the ocean.

But in 1492, he headed west into the apparently endless ocean

in search of his new route to the Far East.

It's hard to appreciate today

just what an epic leap into the unknown this voyage was.

It took five tough weeks,

but as we all know, Columbus's hunch was right -

there was a wind that blew right across the Atlantic.

The thing is, his grasp of sailing was much better than his grasp of geography.

It wasn't the Far East he'd landed in. It was the Bahamas.

As far as Europeans were concerned, he'd discovered a new continent,

and for that, his name is known throughout the world.

Yet for me, America wasn't his greatest discovery.

Columbus's real genius was his instinctive understanding

of the way the winds blow across the Atlantic.

He had discovered what we now call the trade winds -

winds that blow steadily in a south-westerly direction.

It was the trade winds that took him all the way from the African coast

to the Bahamas.

Getting across the Atlantic was all well and good,

but now Columbus had to find his way back home.

And that was going to be tricky,

because if he just tried to retrace his steps east,

then that would carry him straight into the wind

that brought him here in the first place.

Instead, Columbus headed north, along the American coast,

and here he picked up another wind

that blew consistently in the opposite direction, from west to east -

what's known as a westerly.

At the time, it must have seemed he was just outrageously lucky with the winds.

But luck had nothing to do with it.

To prove the point, Columbus sailed back to America three more times.

Each time, he found the same winds.

Between 20 and 30 degrees latitude, the wind blew east to west.

Between 40 and 50 degrees, it blew in the opposite direction.

You know, Columbus was wrong about the continent he'd discovered,

but he was right about something far more important -

how to repeatedly use the circulation of the atmosphere

to cross the Atlantic Ocean and get safely home.

Today, we know that the trade winds and westerlies that Columbus exploited

are part of one system,

the same atmospheric circulation that creates deserts over continents.

At the surface, the descending air flows back towards the equator.

These are the trade winds.

They close the loop and form what's known as an atmospheric cell.

It's the spin of the Earth that deflects these surface winds

so that they move towards the Americas.

Each hemisphere has three giant atmospheric cells

which define the prevailing surface winds around the entire Earth.

Once people knew about the prevailing wind patterns,

it spurred them on to set sail for other new lands.

The fate of nations now depended on where they lay in relation to the winds.

The Dutch connected with the westerlies in the Southern Hemisphere

to reach the Far East

and ended up in control of the Dutch East Indies,

or Indonesia, as it's now known.

The trade winds took them home.

In the Atlantic,

Columbus's voyage formed the basis for a triangular trade route,

connecting Europe, Africa and the Americas for the first time.

The Spanish crossed the Pacific using the easterly trade winds,

so their ships made landfall at the Philippines,

which became a Spanish colony.

To get home, the Spanish picked up the westerlies,

bypassing Japan, which preserved its isolation,

and landed in California.

Now, you can still see the legacy of that distant Spanish influence

in the names that are so familiar to us today.

San Diego,

Los Angeles

and San Francisco.

Within 150 years of Columbus's voyage,

a network of trade routes had spread out across the world.

It was the start of globalisation.

For Europeans, the conquest of the winds and waves was a triumph.

But there was a terrible price.

Many other civilisations were devastated by European contact.

Perhaps the biggest impact was here, back in Ghana.

And you can trace those changing fortunes

in the story of the Elmina fort.

By the early 1500s, the function of this trading fort had changed dramatically.

Gone was the bartering for ivory and gold,

and instead the storerooms here

were swollen with a very different kind of commodity.

These dark cellars had once contained the stock for the gold trade.

Now the fort of Elmina had become a staging post for the slave trade.

You know, it's really ugly to think of this place

as a storeroom for gold and ivory and all these beautiful riches

and then, just within a few years, changed into a prison.

While Europe boomed,

Africa's place in the world had been changed for ever.

It looks like a way out, and in a perverse kind of way, it was.

Because after spending a couple of months locked up in the cells,

you'd be taken down this long, low passageway to this -

a gate barely one person wide.

This was the door of no return,

because when you left here, blinking into that sharp African light,

probably completely unaware of what your fate was,

you'd go onto a gangplank and you'd be shipped to the Americas as slaves.

In the 400 years after Columbus made his epic voyage,

nearly 12 million slaves were shipped across the Atlantic.

The impact of new ocean trade routes even reached as far as Chinguetti,

in the Sahara.

Sailing ships now bypassed the old desert trade routes,

so the town was eclipsed

by human exploitation of the very winds that had made it great.

The atmospheric cells are the framework for winds around the planet.

But there's another global wind that influences the climate,

and with it, the course of human history.

High in the atmosphere are giant conductors

that orchestrate weather patterns around the world.

They're called jet streams.

Jet streams are powerful currents of fast-moving wind

that whip along the boundary between two cells.

They're several hundred kilometres wide but only a few kilometres thick.

They snake around the globe in wavy loops,

directing the course of weather systems below.

We're only really aware of their significance

when they stray from their normal path.

If the jet stream strays southward,

it can send deadly tornadoes across Florida,

far from their usual route to the north.

In 1998, a jet stream wandered off course

and sent a devastating ice storm across north-eastern America,

leaving 45 people dead and forcing hundreds of thousands from their homes.

But perhaps the most catastrophic example of the power of the jet stream

was on the High Plains of the United States in the 1930s.

Today, towns like Capa in South Dakota

lie empty and abandoned.

But in the early part of the century,

farmers were rushing here to claim new land.

Then, in the 1930s, disaster struck.

Powerful winds, intense drought

and dense, choking dust storms.

It became known as the Dust Bowl.

Millions of acres of farmland turned to wasteland.

Half a million people were uprooted from their homes.

Most never returned.

At the time, it seemed like a freak accident,

but we now know that the jet stream was the trigger.

For several years, it had drifted hundreds of kilometres south

from its normal course,

taking the rains with it.

The jet stream controls the short-term patterns of wind and weather

across the world.

But perhaps the most significant way that the wind has affected history

is by defining the climate and character of entire continents

over thousands of years,

imposing limitations for people in some parts of the world,

and for others, offering huge opportunities.

Take China.

Today, China has become a world superpower.

But China's civilisation is one of the oldest in the world,

and its success was built on something delivered by the wind.

This is central China.

It's known as the cradle of Chinese civilisation,

because this is where

the wealth and power of China's ancient dynasties began.

High above the Yellow River is what made it all possible.

A resource that was the key to China's earliest beginnings.

This plateau was the foundation stone for China's ancient agriculture.

But what made it that wasn't a stone at all.

It's what's under my feet.

It's soft and crumbly.

When you crunch it, it just turns to dust,

which is exactly what it is, except it's called loess.

This dust is rich in minerals

and combines with rotten plant matter to form a light, fertile soil.

Chinese farmers settled here more than 10,000 years ago,

and it was the first sites of rice cultivation in the world.

And the reason all this loess is here is because of the winds.

50 million years ago, India collided with Asia,

and that pushed up the Himalayas.

These mountains created a completely new pattern of winds.

The Himalayas are so high that air is forced up, forming clouds and rain.

But when the wind reaches the far side of the Himalayas,

it's bone dry.

It's called a rain shadow,

and it forms some of the driest and dustiest places on Earth -

the Taklamakan and the Gobi deserts.

So China is surrounded by giant reserves of dust,

and the prevailing winds act like a huge conveyor belt

that blows it all the way to central China.

Because the plateau is so vast,

farming could develop here on an enormous scale.

That meant surplus food,

and surplus food is the first and most important prerequisite

for any self-respecting empire.

Over 3,000 years ago,

the first of China's famous dynastic empires was formed.

It was based in the centre of the loess plateau.

The Great Wall of China was built across the northern edge of the plateau

to safeguard the empire's heartland.

The importance of the loess plateau

has also shaped China's cultural heritage.

In the 5th century, they built these - the Buddhist temples at Yungang.

Carved into solid rock beneath the layer of loess

is a honeycomb of 250 man-made caves,

the walls covered with over 50,000 Buddhist statues.

But the crowning glory of the loess plateau is this.

The 8,000-strong Terracotta Army.

Not only are they buried in the loess,

the terracotta from which they were created

is itself made from loess.

So what began with loess led to empires and dynasties, art and religion,

and it was all made possible by the winds.

China was lucky.

It found itself at the end of a wind pattern

that delivered some of the finest-quality soil in the world.

Not everywhere was so fortunate.

Perhaps no continent on Earth has been more limited by the wind than Australia.

Nothing quite prepares you for the sheer scale of the Australian outback.

It's very, very barren.

I wouldn't like to be a farmer out here.

It's also amazingly dusty. I can feel it.

Bitter taste in my mouth.

Australia's Red Centre couldn't be a harsher place to live.

If it wasn't for the odd shrub,

it could be mistaken for the surface of Mars.

But at this watering hole

there are signs that people settled here a very long time ago.

Carvings up to 30,000 years old.

And well-crafted stone tools as well.

Flat, round stones like these

were used for grinding up millet seeds and tubers.

It's a very similar technology as that used by the first farmers

in Asia and the Middle East.

You know, it's fascinating to think why this didn't lead to the type of farming

that emerged elsewhere.

About 10,000 years ago,

the development of agriculture on other continents

led to complex, large-scale societies.

But here, farming never really took off.

You might think that's because it's parched and dry.

But it's just as much to do with the wind.

Here you can see the effects of the wind down at ground level.

Now, what you'd normally expect to find

is a kind of mixture of sand, gravel and clay,

all jumbled up with plant debris to give us soil.

Instead, here you get something that looks rather bizarre.

You can see a kind of mosaic of larger fragments,

where the finer stuff's just been blown away by the wind.

And what it produces is an armoured cap to the land surface -

what we call a desert pavement.

This crust makes it very difficult for plants to grow.

It isn't just a localised problem.

The winds strip dust and soil away across much of the continent.

So, what causes this stripping action?

To understand the answer, you need to be in the centre of the continent

and you need to get up high.

This tabletop mountain is called Attila, also known as Mount Conner.

It's a huge natural monument right in the centre of Australia.

HE CHUCKLES

Oh, that makes it all worth it.

Look at that.

That's a hell of a view.

Whoo!

You know, when you're down there, it's just so flat.

You don't get a sense of the sheer scale of this landscape.

It's only being up high that you can just see how...how big it is.

You also appreciate from here

that for the people that had this landscape, being so precious to them,

that being able to get up here,

and seeing the land laid out almost like a map,

must have made these high places just so special.

Mount Conner sits at the geographical and spiritual heart of Australia.

But it also lies at the centre of an amazing wind system.

The incredible thing about the atmosphere above central Australia

is that there's a giant circular wind pattern

thousands of feet above my head.

The prevailing winds swirl in a great anticlockwise spiral

around the continent.

They've been stripping the fertility from the soil

for hundreds of thousands of years.

In China, fertility was carried in by the wind.

But here in Australia, fertile dust and nutrients were simply blown away,

leaving sand and stones behind.

The sand has been shaped into vast fields of dunes,

which circle the centre of Australia, lined up with the path of the winds.

It's a process that continues to this day.

Giant dust storms regularly engulf eastern Australia.

In 2002, the largest ever recorded was more than 2,000 kilometres long.

Nearly 5 million tons of dust were removed in just this one storm.

Most of it ends up in the ocean, where its nutrients create huge algal blooms,

an essential part of the marine food chain.

So the climate and the winds dealt a tough hand

to the ancient Aboriginal peoples.

With large areas of the continent bare and arid,

continuing with the hunter-gatherer lifestyle made more sense

than taking up farming.

You know, you realise that the people here were ingenious and adaptable.

For a start, rather than relying on one or two intensive crops,

they instead diversified into a wide range of wild food sources.

And also, instead of living in permanent, settled communities,

they lived instead in small, mobile groups,

always able to move in search of food.

The differing fate of Australia and China

is down to large-scale wind patterns over continents

that are stable over thousands of years.

But the wind has had some of its most dramatic effects on human history

when it interacts with the energy of the oceans.

It's an interaction that can have major long-term consequences,

but it can also bring short-term disaster.

The sea acts as an immense store of the sun's heat.

There's more energy in the top three metres of the ocean

than the whole of the atmosphere - enough to power America for 50 years.

By pumping this energy into the air,

the ocean is constantly influencing the wind...

..a principle that is graphically demonstrated each year.

Hurricanes are the most extreme storms on Earth,

the ultimate example of the violent partnership

between the atmosphere and the ocean.

The hotter the ocean, the faster the air above rises,

drawing the wind inwards in a vicious spiral.

Each one degree rise in sea temperature

increases wind speeds by more than 20 kilometres per hour.

Around the eye of the hurricane,

the clouds build up like the inside of a stadium,

leaving a calm centre around which the winds rotate.

It's the spin of the Earth that gives a hurricane its distinctive spiral shape.

And as they move across the surface of the globe,

hurricanes are caught up in the same atmospheric circulation

that drives the trade winds and westerlies.

Their tracks cluster in bands of destruction

on either side of the equator.

Devastating as hurricanes are,

on a planetary scale, their effects are relatively minor and short-lived.

But it turns out that the ocean affects winds

over much larger areas and longer timescales,

and that discovery has answered a great puzzle

in the story of the human conquest of the globe.

The Pacific is the largest ocean on Earth.

The only land is a scattering of tiny islands,

some of the most inaccessible places on the planet.

Ever since modern humans left Africa several tens of thousands of years ago,

our distant ancestors have spread across the continents.

But there's always been a bit of a gap - the Pacific Ocean.

Long after the rest of the planet was colonised by humans,

the Pacific lay empty.

With its scattering of tiny islands,

it's little wonder that the Pacific remained unexplored for so long.

If you were a would-be explorer heading out into the unknown,

the chances are you'd run out of food or water

long before you reached the next tropical paradise.

Then, just over 3,000 years ago, sailors set off from Asia

and began to spread to nearly every island in this vast ocean,

ending up in the distant, far-flung islands

of Hawaii, New Zealand and Easter Island.

It was a journey that took them a quarter of the way around the world.

You know, it's not just the distances that people travelled that amazes me,

it's also the direction.

This is my crummy map of the Pacific.

Here's Asia over here, with Japan.

This is supposed to be the Americas here. Australia down here.

It's thought that this whole area was peopled by going from west to east,

but the thing is, in this region, the winds blow in the opposite direction -

from east to west.

Trying to sail into the wind from such long distances

would have taken a lifetime.

So quite how they did this has always been a big mystery.

The answer lies in that turbulent link between the atmosphere and the ocean,

and the best place to see it in action is in the middle of the Pacific.

An island like Yap.

A tiny dot of dense rainforest

over 1,000 kilometres from the nearest continent.

The question is, how did people get to islands like Yap

and then move on to the other islands of the Pacific

when they were heading into the prevailing winds

and all they had were these - wooden outrigger canoes?

TRUMPETS

These boats have barely changed

since the first sailors set off across the Pacific.

So how did they sail across the entire ocean against the wind?

Normally, sailing into the wind would involve taking a zigzag route

called tacking.

The problem with sailing into the wind is this -

you keep needing to tack all the time,

which means you need to move the sail from the front to the back

by swinging the mast and the boom round,

so that the front of the boat becomes the back.

And then... It's actually quite tricky and quite dangerous.

By moving this sail from the front of the boat to the back,

these canoes can indeed tack back and forth across the wind,

gradually moving forward.

But it's a slow and difficult process.

It's good? Yeah?

I always get slightly nervous.

For you, thousands of times. For me, this looks dangerous.

Ali Haleyalur is the chief navigator.

So in the past, when your predecessors made lots of long journeys,

how did they do that against the wind?

If it's really far, it's not safe to go east,

because within that four or five days that you tack in it,

you still cannot arrive,

and then another storm hits you there.

So it's better you have to wait when the westerly wind comes.

There are always short periods when the wind blows from the west

due to seasonal changes,

but not long enough to undertake long voyages.

But the ancient navigators realised

that there are certain times when the winds change direction

and blow consistently for long periods from west to east.

The secret of this change lies in the relationship

between the Pacific Ocean and the winds.

Every few years, warm water from the west Pacific

surges into the cooler waters of the east.

This warm water heats the air above, changing air pressure

and making the trade winds weaken or swap directions completely.

Today we know this phenomenon as El Nino.

These changes over the Pacific have a huge impact on the weather...

..causing flash floods on the American continent.

Meanwhile, in places as far apart as Australia and Africa,

temperatures soar, causing wildfires.

But for the ancient Pacific colonisers, it would have transformed their options.

With the wind blowing consistently from west to east,

the exploration of the Pacific would have been much easier.

So what happens to the winds during El Nino years?

I realised that during the El Nino years,

the wind is extended very long and very strong.

It remains coming from the west. That's what I see during that time.

So the westerlies stay for longer.

-Yeah, kind of stay for a longer time.

-Right.

And this may be the key to the mystery of how the Pacific was colonised.

El Ninos tend to come in phases.

It now seems that in the past,

each El Nino phase coincided with a wave of colonisation across the Pacific.

And so the most epic journeys in history,

journeys that took people to the most far-flung corners of the world,

were at least partly the result of how the ocean affects the winds.

It would be nice to think that the ocean and winds

always had positive effects on history. But the reality is more complex,

because El Nino is just one phase in a larger climatic system

called the Southern Oscillation.

This oscillation in the Pacific is so powerful

that it's had profound effects on civilisations across much of the planet.

Chaco Canyon in the south-west corner of the USA,

once home to a people who built a sophisticated civilisation.

Oh, wow! Look at that.

She's beautiful.

That is so big!

I mean, that's what really strikes you - this is a big landscape,

and still this jumps out at you.

You can just tell that this place was built to last.

It looks like the people here figured they'd be here for a very long time.

At the heart of the canyon are the remains of a structure

called a "great house".

Pueblo Bonito.

It was built by the Anasazi over 1,000 years ago.

Ooh!

Must have been a wee bit smaller than me!

Pueblo Bonito was the centre of the Anasazi civilisation.

Thousands of people lived nearby in the surrounding farms and villages.

You know, there's a good reason why the people at Chaco Canyon

built their settlements at the base of these massive cliffs,

and that's because the water is from up there.

There's hardly any rainfall around here,

but the rain that does fall lands on the mesa behind here, runs off into ravines

and then comes cascading down into the valley.

Rather than let it drain off into the river,

the Anasazi would build dams and channels to pool the water

or to divert it off to where it was needed.

But by 1300, this whole region had become effectively deserted,

and the big question was why.

The answer lay thousands of kilometres away.

Unknown to them, they were at the mercy of the Southern Oscillation

in the distant Pacific Ocean.

When unusually warm water moves to the west of the Pacific,

it changes the winds,

taking rain and storms away from the Americas

and leaving communities inland parched.

Normally, this isn't enough to have a lasting impact,

but around 1300 AD, the climate got stuck in this phase,

leading to a series of mega droughts lasting decades.

It wasn't just the Anasazi civilisation that was affected.

Each time the Southern Oscillation got stuck in this position,

the result was a similarly devastating mega drought.

The Fremont, Mogollon and Hohokam cultures

all declined at the same time as the Anasazi.

In South America, the Tiwanaku and the Sican,

and in Central America, the Toltecs and the Zapotecs

were all weakened or collapsed

because of changes in the Southern Oscillation.

And droughts caused by the Southern Oscillation

also brought to a close the first era of the mighty Mayan empire.

Severe droughts weren't the only factor

behind the collapse of these civilisations.

At Chaco Canyon, the people were living close to the limits of their resources,

so they were highly vulnerable to climatic changes.

For me, that's a message that still resonates today.

The impact of the winds on human history has been subtle and often unseen,

but extraordinarily powerful.

They define climate zones that, for thousands of years,

set the limits for human development over much of the world.

Then, paradoxically, the winds set us free from these limits.

Now, as our climate is changing,

we can expect significant changes in wind patterns,

altering the distribution of heat and moisture around the world.

How we cope will depend on how close we are to our own limits.

Whether it's on land or at sea,

we've gained so much by exploiting and adapting to the rhythms of the wind.

But we've never really mastered it.

We can only ever be one step behind.

I mean, even today,

when we can virtually track every twist and turn of the air above our head,

the atmosphere is still mysterious, still erratic

and ultimately still shapes our future.

Next time - fire.

Oh!

It's deadly and yet it's also the power behind human progress.

Our dependence on fire means that events deep in the Earth's past

have changed the course of human history.

Ah...

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