Original subtitles

Earth...

the only planet orbiting the sun

that has continents,

oceans,

plate tectonics,

and the sole planet with an atmosphere

that has sustained life for billions of years.

How did Earth get this way?

What's behind the power, the violence

that cracks solid rock in an instant,

that shatters buildings

and sends tsunamis racing for the shore,

that erupts seas of lava and billowing clouds of ash?

Earth has been making headlines lately.

Powerful images of destruction never seen in our lifetime.

People are asking,

"has Earth entered a period of intense geologic upheaval?"

Scientists have new answers to questions about our home,

the third planet from the sun.

It's a story that, just a few decades ago,

could not have been told.

About 10 billion years

after the big bang created the universe,

our solar system began to form.

Scientists calculate that this happened

4.567 billion years ago,

and that was when planet Earth was created.

A long time ago, about 4 1/2 billion years ago,

the Earth formed from this nebula around the sun.

The sun in the center, and the nebula swirling around,

and the bits of the nebula accreting,

that is, sticking together and colliding with each other,

and it was a very turbulent time.

And the Earth gradually came together,

and as it became more massive,

the gravitational force became larger,

and it could attract more of the small pieces,

and the Earth grew and grew.

It took about 100 million years

for Earth to reach its present size,

but before it did,

our planet experienced its largest, most dramatic event.

So after the early Earth formed,

probably tens of millions of years afterwards,

you had a Mars-sized object that actually hit the Earth.

The giant planetoid impactor was called theia.

The Mars-sized impactor hit the Earth,

and it spewed off a lot of the material from the Earth,

and part of that formed the small...

The moon that we now have.

And also we generated a lot of heat from this impact,

and so it might have remelted a lot of the early Earth.

Scientists only recently learned

of this massive impact

by studying rocks collected on the moon.

It turned out that the moon rocks were very similar

in composition to Earth rocks.

After the impact, the moon became a satellite,

while the Earth was left a molten ball of fire.

And so the early Earth was, like, this molten mass.

That must have looked liked maybe one giant volcano

or lava lake.

It slowly solidified and formed this outer crust

that is where we stand today.

As the crust formed,

collisions and bombardments died down,

but the meteorites continue to fall to this day.

We start with some breaking news this hour.

People in the Urals in Russia

have seen burning objects raining down from the sky

after a meteorite exploded above the Earth,

causing a meteor shower.

Eyewitnesses reported several blasts.

Despite the shock that a meteor shower can cause,

meteorites are extremely valuable to scientists

who study when the Earth was created.

They are collected and studied worldwide.

But the best place to find these rocky aliens

is one of the Earth's most unearthly places:

Antarctica.

On the sleek, blue ice expanse

of the central antarctic ice sheet,

the surface is swept clean by relentless winds.

A meteorite sits exposed for an average of 20,000 years,

sometimes for several million years,

before scientists might spot it and collect it.

A type of meteorite called a chondrite

contains the same basic elements

that make up the sun and planets,

so they tell US a great deal

about where we originally came from

and when.

To figure out how old meteorites and the Earth

actually are,

scientists use high-precision instruments

to measure their radioactive properties.

But why must scientists rely on meteorites

to figure out how old the Earth really is?

Why don't they simply study the oldest rocks

that were formed on the Earth itself?

From the Earth's formation around 4.6 billion years ago

up to about 3.8 billion years ago,

is an era called the hadean,

when the Earth was undergoing heavy bombardment,

and there's virtually no rock record

from that period of time at all on the Earth remaining today.

The hadean era,

Greek for underworld, or hell.

Nothing survived this inferno...

Or did it?

It was a missing link hundreds of millions of years long

between Earth's creation and its oldest rocks,

like these 3.6 billion-year-old rocks found in Australia.

Beautiful.

But breakthroughs in analyzing these rocks

finally opened a window into the hadean.

Interestingly, we do have some materials from that time.

They are small zircon grains, 4.4 billion or so of age.

Zircons are the oldest terrestrial material

ever found on Earth.

But rocks containing zircons that are billions of years old

have only been discovered in a handful of places on Earth,

like Antarctica.

The rest have been obliterated.

Studying those rare vestiges of the early days of Earth,

scientists were astonished to find evidence

that very early on, Earth developed a characteristic

that it and no other planet in the solar system possesses:

Plate tectonics.

Plate tectonics has been around on Earth

from the early days,

at least 2 1/2, 3 billion years ago.

So more than 3/4 of the life of the planet.

Mars nearly developed plate tectonics,

but failed.

Mars is smaller than Earth.

It cooled faster.

It may well have had a style of plate tectonics

early in its history, but it clearly does not today.

Venus, considered Earth's sister planet,

is also missing Earth's trait of plate tectonics.

Venus doesn't have plate tectonics

in the style that we recognize

with very clear, linear boundaries between rigid plates,

perhaps because there's no water on the surface of Venus.

It is the striking and distinguishing characteristic

of Earth's surface,

and scientists are finding that this substance is the key

that started Earth's early crust

into constant, ever-changing motion.

One of the reasons we have plate tectonics,

which we don't see on any other planets,

is that we have a lot of water on the Earth.

So we have water not only in the oceans,

but there's approximately an ocean's worth of water

locked up in the rocks in, say, the upper 100 kilometers

of the Earth.

When you put water into a rock,

it makes the rock weaker by maybe a factor of 100

than it would be without the water.

Weaker rocks make for plate tectonics

because you have to have plate boundaries

where rocks either slide by each other,

or one goes down underneath the other.

And so unless you have rocks that are weak enough

to be able to slide past each other,

you're not gonna have plate tectonics.

But where did Earth's vast amount

of liquid water come from?

In the early history of the Earth,

when there was a lot of volcanism going on,

volcanoes actually brought gases up

that created the early atmosphere.

And magma has water in the form of vapor

that it can bring up.

Icy meteorites and comets

were the other likely source

that infused Earth's crust with water

and got it moving.

And once plate tectonics started,

it never stopped.

It has influenced everything that has happened on the surface

including life.

The oxygen, the minerals, everything that we get

comes from the interior of the Earth.

We all have internally within US

a whole suite of minerals and things that...

We're from the Earth.

Essentially, that's where we...

Our chemical makeup is from the Earth.

And if you had no way of getting that outside of the Earth,

then essentially you'd have a dead planet.

Plate tectonics became recognized

as being a crucial ingredient for life,

as is water.

This was an important piece of knowledge

for scientists who were studying Earth's ancient rocks.

If the conditions for life were present

when Earth's ancient rocks formed billions of years ago,

then wasn't it possible that life also took hold

in the early dawn of Earth's formation?

The search was on.

Some scientists zeroed in on the 3.5 billion-year-old rocks

in the outback of western Australia.

Amazingly, it turns out that fairly soon after,

people started discovering some potential signs of life

in those rocks,

and one of the major discoveries

was structures called stromatolites.

Stromatolites are peculiar rocky structures

built by colonies of cyanobacteria,

or blue-green algae.

They are extremely rare on Earth today,

growing in only a few places,

like laguna Bacalar in Mexico.

But they were the first life forms to appear on Earth,

and they gave rise to all other forms of life,

including people.

It took awhile, though.

Stromatolites dominate the first 3 billion years

of the history of life on Earth.

So from 3 1/2 billion years ago

to up to about 1/2 a billion years ago,

the fossil record consists

almost entirely of stromatolites.

Stromatolites lived in oceans and lakes.

For that period, no fossils have been found

of creatures that lived on land.

Does that mean there were no continents

for the first 3 billion years of Earth's history?

Once again,

Earth's ancient rocks yielded clues to this mystery.

We don't know for sure when plate tectonics started,

but we do know for sure when we first had continents.

We have intact areas that we can go and map in

that go back as far as 4 billion years.

That's nearly 90% of the age of the Earth,

so we're quite sure about the fact

that we had continents back that far.

No one knows what the continents were like

4 billion years ago

or why we ended up with seven continents

that ride around on a dozen large crustal plates

and many microplates in between.

But the question that plagued scientists for years was this:

What was the prime force that made the plates move?

There were two contending forces.

The first was discovered aboard seafaring research vessels

mapping the sea floor decades ago.

Scientists learned that the sea floor

was spreading apart in the middle of the oceans

at submarine mountain ranges called mid-ocean ridges.

Submersibles were sent down

and came back with footage of hot gases welling up

in the rift zones.

Magma was rising from below here,

and it was believed that it was pushing

the oceanic plates apart.

The key to plate tectonics was at last discovered...

Or was it?

Seismic experiments of Earth's deep crust and mantle

changed scientists' thinking.

A second force was discovered

where plates collided head-on,

as they do under the Andes mountains of Ecuador.

One plate was imaged diving underneath the other.

The was called a subduction zone.

The sinking slab appeared to be pulling

the rest of the plate behind it,

making the entire plate move.

A great debate arose over which force was more important

in driving plate tectonics:

Pushing from the rising magma or pulling in subduction zones.

There now appears to be a winner.

When people first recognized mid-ocean ridge spreading

as the key to understanding plate tectonics,

the view was that the magma, the mantle,

rose up in the center and forced the plates apart.

But that view has changed over the years

and has largely been replaced by the view that it's the pull

of the subduction slab that matters most.

Bob, can you please hand me the pvc pipe, please?

Sure.

So scientists looked harder at subduction,

and as they did, more geologic wonders were uncovered.

For one, seismic studies of subduction zones

revealed that oceanic plates are very thin

compared to continents.

The sea floor is 3 to 6 miles thick,

while continents average 30 miles thick

and swell up to 50 miles under great mountain belts.

This has a major influence

on how the plates affect each other,

especially in subduction zones.

Where subducting plates meet continents,

Earth's tallest mountains are built:

The Himalaya of Asia,

the Alps of Europe,

the Andes of South America.

Subduction created them all.

But subduction can create havoc on the surface.

As seismic networks improved around the world,

it became apparent that subduction was the culprit

behind Earth's most powerful earthquakes.

First the earthquake itself...

A grocery store in the capital at the moment of impact.

The tohoku earthquake of 2011

was the most powerful ever known in Japan

at magnitude 9.

It struck where the oceanic pacific plate

subducts under a piece of the north American plate.

Where subduction is occurring,

you see a very well-defined zone of earthquakes,

essentially between the downgoing oceanic crust

and the continental crust,

and it forms a line of earthquakes

where these two crusts are in contact with each other

and slipping past each other.

Subduction quakes can also generate

devastating tsunamis

because of their proximity to coastlines.

One of the worst earthquakes ever...

The Japanese quake and the 2004 Indian ocean earthquake

off sumatra are striking examples.

Earthquakes and tsunamis are not the only action

spawned by subduction.

Subduction also creates volcanoes,

and they, too, can be large and dangerous.

Volcanoes in the subduction zones are formed

by magmas that are more silicic, or more viscous,

and they are much more explosive.

So those are the volcanoes that can really be dangerous

and really create trouble.

Subduction volcanoes are Earth's most notorious:

Mount Tungurahua in Ecuador,

mount Vesuvius in Italy,

mount Asama in Japan,

mount Saint Helens in the u.S.

All are known for explosive, destructive eruptions.

They also grow to great heights.

This is chimborazo,

rising 20,700 feet above sea level

in the Andes mountains of Ecuador.

When measured from the center of the Earth's core

to the volcano's summit,

chimborazo is Earth's tallest mountain,

soaring over 7,000 feet closer to the sun than mount Everest.

That's because the volcano sits on the equator

where the Earth bulges outward from the force of rotation.

Earth scientists have gotten a good handle

on the forces driving plate tectonics,

like subduction,

but questions remain.

One of the biggest questions is how plate tectonics is linked

to processes operating deep below Earth's crust.

The deeper we look, the less we know.

But new research is turning up big surprises

lurking in the center of the Earth.

Tumble into cascading salt beds,

escape from hissing steam caves,

behold the staggering underground ocean.

You will encounter breathtaking dangers beyond belief

here at the center of the Earth.

The inside of Earth,

an object of great curiosity,

wild conjecture.

At the center of it all is the Earth's core,

which was discovered accidentally only 140 years ago.

A huge earthquake in the 1880s was...

The waves from that earthquake,

traveling through the Earth's core,

bouncing off the Earth's core,

were recognized as arriving at different times

from the seismic waves

that traveled through the outer layers of the Earth.

And with that, the Earth's core was discovered.

It took until the 1920s to recognize

that the Earth had a solid inner core

inside the fluid outer core.

At half the size of the planet,

the Earth's core is peculiar in that it's all made of iron.

But its inner half is solid, and its outer half is liquid.

The Earth's outer core is molten

because it's very high temperature, and it's...

The temperatures are above the melting temperature

of this iron-rich material,

but the Earth's inner core is actually... can be solid

because we have even higher pressures,

so get to over 3 million atmospheres,

so 3 million times the pressure at the surface of the planet.

And at those high pressures,

they cause the material to actually solidify.

Fortunately for US and all living things,

the Earth's outer core is fluid,

for it generates our planet's protective magnetic field.

If we didn't have the Earth's magnetic field today,

we probably couldn't have life on the surface of the planet.

The sun's always throwing all these particles at the planet,

but then the Earth's magnetic field

protects this nice, protective coating

that protects US from these damaging rays

and particle... High-energy particles.

The Earth is unique among planets

in having a magnetic field.

Mars had one early on,

but its small core cooled quickly,

and it was lost and, with it,

the chance for life to take hold on its surface.

Meanwhile, Earth's magnetic field, for all its importance,

is not well understood.

What is getting clearer is what is actually going on

in the Earth's core itself.

Okay. Let's put it on there.

That is because

scientists have finally re-created

the core's extreme conditions in the lab.

3 million atmospheres and 10,000 degrees fahrenheit,

the same temperature as the surface of the sun.

So you have to reach millions of atmospheres,

and you have to reach thousands of degrees,

so that's a very challenging technological problem.

Okay, great.

Looks good.

So one way to reach the very high pressures

and temperatures in the planet is to compress materials

between the tips of two diamonds,

and since diamond is the hardest material on the planet,

we can generate very high pressures.

And then, at the same time,

we can use lasers to heat the sample

and to reach the same conditions

we think are inside the planet.

The core's sun-hot heat is crucial to understand.

It affects everything at the surface,

1,800 miles above.

To get up here,

it travels through two layers of solid rock.

The first is the mantle, Earth's biggest layer by far.

And then the crust,

really just a thin veneer on the Earth.

Scientists believe that the immense heat of the core

makes the mantle so hot that it acts like a fluid

and rises until it hits the crust.

Some of the heat then circulates along the crust's underside

until subducting slabs of crust take it downward.

It's a giant recycling treadmill called convection.

Through experiments,

scientists have calculated that heat's round trip

from the core to the crust and back to the core again

takes 300 million years.

This circulation of heat is kept alive

by the sinking of cold, heavy plates

into the hot mantle below.

In other words,

those slabs cause quite a stir as they descend.

We can track those slabs with tomography

down at least into the middle mantle.

And probably those slabs reach all the way

to the core-mantle boundary,

and there's a slab graveyard that sits above the core.

That slab graveyard may in turn help drive

the convection cells in Earth's mantle

that in turn then reach up

and drive the pattern of plate tectonics

and the shifting plates at Earth's surface.

The sinking slabs not only drive heat in the mantle;

they have radically changed the makeup of the mantle

over the course of geologic time.

The mantle is a very complicated mixture of things

that have been stirred internally

and things that have come up to the surface,

been ocean floor for a while, and then taken back down.

In the 4.5 billion-year history of the Earth,

the entire mantle has been circulated about once.

Virtually nothing at the Earth's surface

or in the interior is permanent.

Things are changing all the time.

Heat rising up from the core

does not always recycle back down.

Sometimes it escapes its earthly confines

as volcanic eruptions.

Earth is by far

the solar system's most volcanically active planet.

Volcanoes are nature's most violent and destructive force.

But ironically,

this force emanating from the Earth's mantle deep below

is at the very root of the creation and maintenance

of life on Earth's surface.

Volcanoes have certainly influenced life on Earth.

And that's not just when they have a big eruption

and people die.

But they also may have created life on Earth.

In the last few decades,

hydrothermal vents were discovered deep in the ocean,

and minerals are coming out that can actually feed life

around those vents,

and a very surprising find was that life could exist

at such depth in the ocean.

Volcanoes were the original architects

of our atmosphere,

unlocking co2 and water from the mantle's rocks

and taking them to the surface.

One of the largest concentrations

of active volcanoes on Earth is in Japan...

More than 100.

Mount fuji is Japan's largest volcano,

rising 12,400 feet high.

This graceful peak is a massive subduction volcano,

but the largest volcanoes on Earth are 100 times bigger.

They belong to a rare class of volcano

known as a mantle plume volcano,

and the largest of all are on Hawaii.

Lava has been erupting from this spot in the pacific

for at least 75 million years.

It built Mauna Loa, Earth's largest volcano by volume.

But Mauna Loa is not the biggest volcano in the solar system.

The largest volcano in the solar system

is Olympus mons on Mars.

It's about 26 kilometers high

and around 600 kilometers across.

Compared to Hawaii,

Olympus mons is vastly bigger in all respects.

Why are Earth's volcanoes so much smaller?

On Mars, there's no plate tectonics,

so the same, let's say, hot spot can build up a volcano

in the same location for very, very long periods of time.

On the Earth, we can't have that situation

because the plates are moving, so you get, for example,

hot spot like the one that formed the Hawaiian volcanoes,

but the plate is moving over it, so it punches one volcano,

but then the plate moves,

and the volcano moves somewhere else.

So on Earth, we don't really build those sizes of volcanoes.

The source that feeds Earth's giant volcanoes

is thought to be a narrow cylinder of hot rock

that reaches deep into the mantle.

How deep is a matter of debate.

But recent seismic studies of the Hawaiian chain

imaged for the first time

a hot plume reaching halfway to the Earth's core.

Chemical analysis of lavas retrieved in drill cores

traces the plume even deeper,

actually extending to the Earth's core itself.

The deepest reaches of our planet, then,

are intimately connected

with geological action at the surface.

Without these deep-seated forces,

life would have not gained a foothold on our planet,

and it wouldn't have flourished

and evolved into countless wondrous forms

without something else:

A remarkably stable, precisely tuned climate.

For most of Earth's existence,

the temperature at the surface has rarely wavered.

Why this is is a great mystery.

The global mean temperature is a comfortable 59 degrees.

Increase that by just 18 degrees,

and ice at the Poles would melt.

Decrease it by 18 degrees,

and the entire world would freeze over.

Other planets are far from habitable.

Venus, with 200,000 times more co2

in its atmosphere than Earth,

is ravaged by temperatures reaching 900 degrees fahrenheit.

Mars, long ago stripped of its atmosphere and magnetic shield,

is deeply frozen to an average of 70 degrees below zero.

Earth's climate is kept moderate by a little understood interplay

of geologic and biologic forces.

Volcanic eruptions spew co2 into the air.

Plate tectonics builds mountains

and rifts apart ocean basins.

Then weather erodes the land,

capturing CO2

and locking it away in sediments.

Living organisms add their alterations into the mix.

All is built into a system of global checks and balances.

There have been only a few times

when Earth's climate tipped well beyond its comfort zone.

Earth's coldest extreme happened about 750 million years ago

and probably several times before that.

It was called snowball Earth.

Snowball Earth refers to a time in the Earth's history

when the Earth was entirely covered with ice.

That means glaciers on the continents

and sea ice covering most of the oceans

all the way down to the equator.

During snowball Earth,

the temperature at the equator plunged

to minus 4 degrees fahrenheit.

Life, still in its primitive bacterial stage,

survived the deep freeze as it does today:

Below ice shelves and even below glaciers.

Scientists think that the trigger for snowball Earth

was the bunching of continents at the equator.

The giant land mass attracted moisture,

and great storms ensued.

Rain stripped co2 from the atmosphere

and locked it in eroding sediments.

As co2 levels dropped, so did the temperature.

Volcanic eruptions eventually overpowered snowball Earth

by injecting co2 back into the atmosphere.

As the climate restabilized,

the surviving bacterial life forms multiplied,

photosynthesized,

and filled the air with oxygen,

a requirement for more complex life.

Finally, 550 million years ago,

life branched out and began its extraordinary journey

of diversification and evolution.

Why this happened is still a mystery,

but the extreme hit that life took

during snowball Earth

may have actually been part of the reason.

Once life was established on the Earth,

it continued to show evidence that it was extremely adaptable,

that whatever harsh conditions were thrown at it,

that it was able to adapt to find a way to survive.

So even through periods like snowball Earth,

life would adapt to those conditions and reemerge.

And in fact, those sorts of conditions were major factors

driving the evolution of life on Earth.

The next time Earth's climate was knocked off balance

was at a time when Earth experienced its biggest upheaval

since the early days of its formation.

About 250 million years ago,

95% of life on Earth disappeared.

We think that's because of very large eruptions

in the siberian traps.

Earth scientists have mapped 800,000 square miles

of Siberia that were covered by lava flows

up to 12,000 feet thick.

They call this province the siberian traps.

Those eruptions would have put a lot of dust,

a lot of sulfur in the atmosphere,

cooled the temperature.

So this killed a lot of life on Earth.

The climate recovered eventually, and so did life.

But 90 million years ago,

the hottest period ever known enveloped the Earth.

Dinosaurs roamed what are now the icy realms

of the arctic circle.

Around them were fellow swamp-dwellers like alligators.

What were they doing at the Earth's Poles?

Scientists figured out that plate tectonics was the culprit.

100 million years ago,

the continents had wrenched apart,

and sea level rose.

Mountain building slowed down,

and so did rainfall and erosion.

But co2-spewing volcanic eruptions,

especially in the mid-ocean rift zones, did not.

When continents are splitting apart

and you have more sea floor spreading,

you're adding more co2 to the atmosphere,

and it's not being removed as quickly

by weathering of the continents,

so it's warmer during those periods.

Temperatures rose to a balmy 54 degrees

at the north and south Poles.

The climate so agreed with the dinosaurs

that they dominated life on Earth for millions of years.

Then, 65 million years ago,

a massive meteorite impact suddenly ended their reign.

The meteorite gouged out a huge crater

over 100 miles across

with the town of Chicxulub, Mexico, at its center.

The city of Merida also lies within the crater,

which was buried by sediment millions of years ago

and only discovered by chance as geologists explored for oil

in the 1970s.

The crater and its surroundings are also the site

of the great cities built by the ancient Maya.

But none of the human drama playing out in this region

would have been possible

if it weren't for the killer meteorite

that struck here,

wiping out the dinosaurs.

Their extinction opened an evolutionary door

for mammals like US to evolve.

But 65 million years ago,

the earthly stage was far from set

for the entrance of people.

Co2 levels and temperatures were much too high.

Luckily for US, plate tectonics built the most massive structure

on Earth to make the climate human-friendly:

The great Himalaya range.

About 60 million years ago, India, once a massive island,

rammed Asia, and the building of the Himalaya began.

As the himalayas grew,

a great block of land rose up behind the range.

It formed the Tibetan plateau

and even raised the land as far to the north as Mongolia,

more than 1,000 miles away.

Since then, the Earth's climate has never been the same.

There's a real belief amongst geologists

that the growth of the Himalaya in Tibet

over the last 60 million years

has led to major changes in the global climate.

The Indian monsoon is created

when warm air from the Indian ocean

meets the mountains of the Himalaya

and dumps the rain on that mountain front.

Prior to the formation of the Himalaya in Tibet,

there was no monsoon.

There was no barrier to the northward progress

of the moist Indian ocean air.

The new monsoon began dumping rain and snow

on the growing mountains of Asia.

This eroded the rocks and changed the chemical balance

in the atmosphere.

Massive amounts of co2 were removed from the air,

so much that the global climate began to cool.

It was the first time in hundreds of millions of years

that the Earth's climate cooled so deeply.

But before we humans made our appearance,

plate tectonics had a few more modifications to make.

All right, here. Here we go.

In Antarctica,

scientists made the startling discovery

that this lonely continent at the bottom of the world

was once connected to Australia.

Then 40 million years ago,

the land masses rifted apart,

completely changing ocean currents in the region.

Over the last 40 million years,

Antarctica became an isolated continent down there,

and so the circum-antarctic current formed,

which isolated that continent so it could cool

and start to grow an ice sheet.

For the first time,

Antarctica became frozen with ice,

up to 15,000 feet thick,

dropping global temperatures even more.

And then, 3 million years ago,

a new land bridge welded south and North America together.

Before that, the two continents were separated

by the ancient Panama seaway.

As plate tectonics joined them together,

ocean currents were radically rerouted once again.

The Gulf stream, instead of going into the pacific ocean,

it actually was then redirected upward in the Atlantic,

bringing more moisture up into the north Atlantic,

and that's a source of moisture for ice

on the continents up there.

By 2 1/2 million years ago,

temperatures plunged so much that the pleistocene ice ages

were triggered,

and in this ice age world,

when freezing glacial periods alternated

with much warmer interglacials,

the story of human evolution unfolded

in the east African rift.

Humans have lived through eight major ice ages

over the past million years.

The last glacial period ended 10,000 years ago,

and we are now at the tail end of an interglacial.

The climate has been warm on average,

but with enough variation to profoundly affect people

and even civilizations.

Once we entered the warm period we're in now

that started about 10,000 years ago,

the changes in climate have been relatively smaller

than the ice ages,

but they've been big enough

that there were droughts and floods and cold periods

and little mini ice ages.

And that people were still migrating

and still having to adapt.

Researchers studying past climate changes

in Guatemala found that long droughts occurred

at the height of the ancient Maya civilization,

from 3,000 to 1,000 years ago.

This may have been a big factor

in the Maya civilization's mysterious collapse.

Because global warming is again creating serious problems

for people and ecosystems,

scientists are engaged in an urgent effort

to study past climate changes

in order to predict what might happen in the future.

One of their best tools is the study of gases

that cause global warming,

especially carbon dioxide, or co2.

These gases are detected in ancient glacial ice

hundreds of thousands of years old in Antarctica.

Atmospheric gases get trapped in the ice,

and you get these trapped bubbles of atmosphere

that are preserved,

and so when you take the ice core,

you have this sort of ancient atmosphere.

And so they can release that and measure the amount of co2

that was around.

And also how it changed since the last ice age.

So we know from the ice cores that the natural level of co2

should be about 280 parts per million,

and today it's about 390 parts per million,

as humans have been burning fossil fuels and releasing co2.

That's a really big rise.

We've never seen this rapid a rise of co2

in the atmosphere

over, you know, 100 million years.

So it appears that one natural Earth system,

the climate,

may indeed be entering a period of intense upheaval.

What about the other natural Earth systems,

like those that cause earthquakes and volcanoes?

Iceland's most active volcano has started erupting.

Have any of these

begun a period of increased activity?

Has been spewing ash and smoke 12 miles into the air.

Looking back 100 years,

the number of volcanic eruptions each year

has actually held steady.

In general, the public only hears about volcanic eruptions

when they're particularly big, when they kill people,

but, in fact, there are usually couple dozen volcanic eruptions

going on every month.

And some volcanoes have been erupting for dozens of years,

like kilauea...

Or even hundreds or thousands of years

like Aetna in Italy.

So there is a lot of variation

in terms of sometimes we have really big volcanic eruptions,

and then we might go for months or years without one of those.

But, in fact, volcanoes are erupting

around the Earth all the time.

A similar scenario holds for earthquakes.

The number of big quakes greater than magnitude 7

averages 18 per year.

At least 250 people have died

after a major earthquake hit southwestern Pakistan.

One of these quakes

is higher than magnitude 8 each year.

This number has held steady since the year 1900.

But it may seem that earthquakes have become more frequent

for several reasons.

Yo!

Excuse me!

First, more are being recorded and pinpointed.

More than 8,000 stations listen for quakes worldwide.

In 1931, there were just 350.

Earthquakes also seem more numerous

because of the soaring population

and near-instant reporting of news worldwide.

The earthquake that killed 1/4 million people in Haiti

would only have killed 1/10 that number 100 years ago,

and we wouldn't have known about it.

This is some of what I saw on the streets.

Now we hear about these things instantly.

We have the live footage of the tsunamis

arriving on the Japanese coast,

and the whole world knows about it.

The recorded number of tsunamis has increased

in the past 100 years,

but scientists attribute this to ever-improving detection.

And with better warning systems being installed,

the loss of life is decreasing.

So do scientists see an uptick

in increased geological activity?

No, the Earth's doing the same thing

it's been doing for billions of years.

We notice the events more

because our more complex society

and our far more populated planet

are much more vulnerable to these disasters.

There's no real evidence that we're living through a period

of enhanced activity.

In any one area,

there may be regions where we have a few more earthquakes

for a few hundred years, and then after that clustering,

seismicity will die away.

But on a planet-wide scale, there's no evidence for that.

Looking deeper into Earth's geologic past,

far bigger upheavals have occurred

than anything known by humans.

A supervolcano thousands of times bigger

than the huge mount Saint Helens eruption of 1980

went off at yellowstone 640,000 years ago.

Supervolcanoes are exceedingly rare in human terms.

None have erupted for 26,000 years.

Earth's history book chronicles other massive upheavals

like the siberian traps.

There was snowball Earth, when the whole world froze over.

In more recent times,

our distant ancestors experienced the bitter cold

of the ice ages,

when Chicago lay under a mile-thick sheet of ice.

If planet Earth were acting up more than usual right now,

it would barely register

in the annals of major earthly events.

The exception is the climate,

which is changing in ways never before seen

in the geologic record.

People may have some influence over this process,

but deep Earth processes

like earthquakes and volcanic eruptions

are beyond anyone's control.

So scientists are focusing on

ways to live with these powerful forces

and reduce their threats.

The public would love it if we could provide

short-term predictions of hours to days

before an earthquake, and we can't do that.

But what we can do is provide long-term estimates...

Decadal estimates, perhaps.

That should be important to US, to our society,

to plan for these events.

In my lifetime, there's a 3/4 chance

there will be a major earthquake in the bay area where I live,

and that should be enough to encourage me

to plan wisely for that earthquake

that I need to expect.

With earthquake predictions still in the future,

earthquake-prone Japan has developed

the world's first earthquake and Tsunami early warning system.

Its first major test came with the massive earthquake in 2011.

Residents of Tokyo were warned

eight to nine seconds before the quake hit.

At 2:46 in the afternoon.

The shaking went on for more than 2 minutes.

The closest major city, sendai...

Residents of the nearest coastal city of sendai

heard Tsunami warnings eight minutes before

the 33-foot Tsunami came ashore.

Japan's advanced state of preparedness

and strong building codes were the critical factors

in saving countless lives during this disaster.

And the early warning system

saved an untold number of lives as well.

The Japan meteorological agencies early warning system

also doubles as Japan's volcanic eruption warning system.

Eruptions are easier to predict than earthquakes.

We are getting better all the time

at predicting volcanic eruptions,

but it's by no means perfect.

And it's still a very difficult thing to do.

We have much better instrumentation.

We can measure the inflation of the ground.

Seismographs see how magma is moving up.

Even temperatures of fumaroles, changes in the gases.

So we are getting better all the time.

Meanwhile Earth's inner and outer forces

will keep reworking the planet,

from the center of the inner core

to the highest peaks on the surface.

No other planet known to US

has so many different natural forces at work.

The Earth we see today

has been 4 1/2 billion years in the making

and has never looked quite this way before,

and it will never look the same again.

I think the way the Earth looks now as a planet

is unique in history,

as is almost every period in geologic history.

The Earth is constantly undergoing change.

What are the Greens?

The Greens are country roads,

and the yellows are local roads.

From a geologist's point of view,

the Earth is really, really interesting

because, on Earth, we see all the major processes

that form the surface of a planet actually going on.

We have volcanism.

We have impact craters formed not that long ago.

We have an atmosphere,

which means we have liquid water.

We have wind. We have erosion.

We have plate tectonics on Earth

which appears to be unique in the solar system.

So it's very geologically diverse and very dynamic.

So I think we are lucky, actually, to live here.