All language subtitles for Killer Hurricanes PBS Nova 720p HDTV MVGroup

NARRATOR:
Our planet is capable of
unleashing extreme chaos.

Volcanoes,

earthquakes,

hurricanes,

and floods can cause
untold devastation.

We may think we've seen
the worst Mother Nature

can throw at us,

but scientists struggling
to understand these disasters

are discovering evidence
that even more extreme events

have struck in the past.

JEAN CHRISTOPHE KOMOROWSKI:
So this is about 13 times

more powerful
than the Pompeii eruption.

NARRATOR:
They're uncovering clues
that the worst catastrophes

in history could strike again.

More than 200 years ago,

a disaster tore through
the Caribbean.

(thunder)

WAYNE NEELY:
This is by far the deadliest
hurricane to ever impact

the entire Caribbean.

NARRATOR:
The Great Hurricane of 1780

was both huge and powerful.

But can we trace its impact
using today's tools

because it appears storms
are becoming more powerful.

MARSHALL SHEPHERD:
We believe that there should be
more intense storms,

perhaps not as many of them are
happening, but when they do,

they'll be stronger.

NARRATOR:
The clues are here,
buried and hidden

beneath the waves
and beneath the ground.

AMY FRAPPIER:
There's this whole
history in here

about what's happened over time.

NARRATOR:
Now scientists and
historians come together,

to scour our volatile earth,
to find out if, in the future,

there will be more
"Killer Hurricanes."

Right now on
NOVA.

¶ ¶

nder)

NARRATOR:
Hurricanes,

some of the most powerful
natural forces on earth.

Called typhoons or cyclones
in the Pacific,

these storms can build up over
warm waters and can reach

hundreds of miles across.

Storms like Harvey, Irma, Maria,

and Typhoon Haiyan
are synonymous

with death and destruction.

(siren blaring)

KERRY EMANUEL:
Hurricanes kill on average

10,000 people a year.

SHEPHERD:
They can cause death.

They can actually destroy
infrastructures, entire cities.

WOMAN:
It's just complete devastation,

it's unbelievable.

NARRATOR:
In 2017, a series of
catastrophic hurricanes--

Harvey, Irma, and Maria--

impacted millions of people
in the U.S. and the Caribbean.

Entire islands and the city
of Houston were devastated.

Is this the worst that nature
can throw at us?

To find out,

scientists are hunting for clues
in the distant past.

SHEPHERD:
The more we understand
about them,

the better we can predict them.

NARRATOR:
They're unearthing evidence
of the deadliest

hurricane in Atlantic history--

the Great Hurricane of 1780.

It destroyed every house
in Barbados.

And it devastated
this massive fort.

That storm was
truly exceptional.

(drill whirring)

NARRATOR:
And looking even further back
in time,

scientists are discovering that,
a thousand years ago,

major Atlantic hurricanes
may have struck more often.

And then you get this
really big layer here

that dates to
about a thousand years ago.

NARRATOR:
So were monster hurricanes
more frequent or more powerful

in the past?

And what does this bode
for the future?

Two record-breaking hurricanes
struck the Caribbean

in September 2017.

The first, Irma,
leaves a trail of destruction

across the islands of Barbuda,
St. Maarten, and St. John.

Its 185 mile an hour winds
blow for a record 37 hours.

¶ ¶

Then, less than two weeks later,
comes Maria.

Its sustained winds blast the
entire island of Puerto Rico,

destroying the power grid, water
supply, roads, and cell towers.

For weeks, millions are stranded
in the dark without clean water,

food, gas, or phones.

The terrible toll
of these storms

puts them in the record books.

But there is growing evidence
that hurricanes just as powerful

could once have been
more common.

¶ ¶

Wayne Neely, a meteorologist
and historian, has come

to the National Archives
on the island of Barbados,

the most easterly
of all the Caribbean islands.

He's here to investigate

the deadliest Atlantic
hurricanes since records began:

The Great Hurricane of 1780.

According to historical
documents,

this storm was so destructive
it killed

an estimated 22,000 people
across the Caribbean.

And on October 10, 1780,
the first place it strikes

is the island of Barbados,
then under British rule.

In the records from the time,

Wayne finds evidence
of the human cost.

NEELY:
Here, in these death records,
we have 4,500 names

of persons who died in
the Great Hurricane of 1780.

Elizabeth Finlay, Thomas Fisher,
Nehimah Fish, William Folks,

Richard Foster, Elizabeth Forte.

These are a few names of persons
who died in that storm

in Barbados.

NARRATOR:
And in the archives,
Wayne also finds

a harrowing eyewitness account
that describes the fate

that likely befell
many of these people.

It's a letter written
by the commander-in-chief

of the British Forces
in the Leeward Islands.

This is a letter written to the
king by Major General Warren.

"I do not believe that ten
houses are saved

"in the whole island.

"Whole families were buried in
ruins of the inhabitations.

"Many in attempting to escape
were maimed and disabled,

"the ground covered
with mangled bodies

of their friends and relations."

NARRATOR:
Almost all buildings, whether
stone or wood, were torn apart.

NEELY:
Well, it must have been
a really great storm to destroy

all the buildings on the island,
so you can imagine the horror

and the misery that the
residents of Barbados felt

during the peak of the storm;
it was horrifying and horrific.

MIKE CHENOWETH:
You have no home to go to;
you have no food.

The fresh water supply
is now destroyed.

NARRATOR:
In 1780, the people of Barbados
are far from the only victims.

In the following days, the storm
kills thousands more on islands

to the north and west.

The whole region was devastated.

Islands such as Martinique,
St. Lucia, St. Vincent,

and Grenada all reported
death tolls from the storms.

The wind is going to destroy
your plantations.

It's going to destroy
everything.

You'll lose all your supplies.

NARRATOR:
In 1780, it's estimated that
22,000 people

are killed immediately.

Most of the victims are slaves,
brought over from Africa

to work on the sugar
plantations.

Then, thousands more die
in the aftermath

from starvation and disease.

It's always the lowest end
of the economic chain

that feels the effects
of these things first.

The slaves are going
to die off first.

And most of them did.

EMANUEL:
Certainly the 1780 hurricane

was the deadliest hurricane
we know about

in the western hemisphere.

NARRATOR:
The records also reveal,
just as in 2017,

this storm was one of
three devastating hurricanes

that struck in quick succession.

1780 was a terrible year
for hurricanes.

There were three
incredibly destructive ones.

NARRATOR:
Only a week earlier, another
hurricane had struck the region.

And then,
just over a week later,

another struck,
also with catastrophic effect.

But the death toll from
the Great Hurricane

overshadows them all.

(wind roaring)

Even today, storms like

Hurricane Katrina that struck
the Gulf Coast in 2005;

and Hurricane Mitch,
the deadliest Atlantic hurricane

in living memory, which hit
Central America in 1998,

can leave thousands dead
in their wake.

So what makes these
killer storms so unusual?

And will they become
more frequent?

Modern hurricanes, like Harvey,
Irma, and Maria,

are tracked by aircraft
and weather satellites.

What we've learned about them
gives scientists an idea

of how the 1780 storm
likely formed and developed.

On August 27, 2017,

nine days before it makes
landfall in the Caribbean,

Irma begins as a storm off
the west coast of Africa.

As it travels west across
the Atlantic Ocean,

it quickly gathers strength.

Heat rising from
the warm ocean waters

feeds energy
into the storm system.

Hurricanes are like
heat engines.

They actually convert the energy
from the ocean

into motion in the hurricane.

NARRATOR:
Warm, moist air cools as it
rises, and then condenses.

That releases that energy that
was carried away from the ocean

and that powers the storm.

EMANUEL:
And one of the reasons
hurricanes are so intense is

the ocean is very warm and the
air at the top of the hurricane

is very cold.

NARRATOR:
These winds can then spin
into a powerful cyclone,

partly a product
of Earth's rotation,

with rain bands stretching
hundreds of miles across.

August 30-- as wind speeds
reach 50 miles an hour,

Irma becomes a named hurricane.

Over the next six days,
it intensifies rapidly.

By the time it slams into
tiny exposed Barbuda

on September 6,
with sustained wind speeds

of a 185 miles an hour,
Irma is a Category 5 hurricane.

It's likely the Great Hurricane
of 1780 formed in a similar way

to this recent superstorm.

But how strong did it get?

Historians are able to piece
together some details

of the storm's strength,

thanks in large part
to ships' records

kept by competing naval powers.

In 1780, the Caribbean
is a battleground

during the
American Revolutionary War.

CHENOWETH:
Four years earlier,

the Americans had announced
their independence,

and the British decided,
"No, you don't.

"We're going to send a bunch
of our troops

over there to stop this."

(soldiers shouting)

NARRATOR:
In October, the area is full of
British and European warships.

There was a lot of European
ships in the area.

You had the French; you had
the Dutch and the Spanish

fighting against
British warships

for control of the Caribbean
and the region.

NARRATOR:
Like the communities
on the islands,

these ships were vulnerable
to violent hurricanes,

as the British Naval Records
in Barbados reveal.

Looking in the naval records,
you can see the loss

of so many ships--

HMS Laurel, HMS Andromeda,
and HMS Deal Castle,

all lost
in the Great Hurricane of 1780.

NARRATOR:
The fate of HMS Deal Castle is
depicted in paintings

from the period.

And historical documents reveal
that thousands of sailors

from all sides perished at sea.

So given its devastating scale,
was the Great Hurricane of 1780

more powerful than
more recent killer storms?

EMANUEL:
Well, we just don't know whether

it was a uniquely powerful
hurricane,

or whether it was just an
ordinarily powerful hurricane

that took a very
destructive path.

NARRATOR:
Given the lack
of scientific data,

how can experts today track
this megastorm from the past?

Climate historian Mike Chenoweth
has taken up the challenge.

He's scouring old ships' logs
hunting for hidden clues.

And he's uncovered an entry
from October 9,

the day before the hurricane
strikes Barbados.

One thing I found in doing
the research was

the first detection
of the hurricane

by the Spanish frigate Diana.

NARRATOR:
This is the earliest account
we have of a ship

encountering
the Great Hurricane.

And so they were about 300 miles
east of the island of Trinidad,

off to the southeast
of Barbados,

at 10 degrees of latitude.

NARRATOR:
"There bore down upon us
a violent hurricane

"which lasted 48 hours
without intermission.

"The wind ran the entire compass
with violent gusts

and the frigate moved heavily
in the swells."

CHENOWETH:
So in several days ahead

of reaching Barbados,

it had already
become a hurricane.

NARRATOR:
But to quantify the strength
of the hurricane,

Mike faces a major problem.

In 1780, there were no
instruments for measuring

the actual speed of the wind.

The only clues are time of day,

wind direction, and estimates
of wind intensity.

Mike continues with logs
dated to October 10.

CHENOWETH:
So, this is at 6:00
in the morning--

the hurricane has just finished
devastating

the island of Barbados.

Everything is
a complete wreck now.

This ship plot here is for
HMS Albemarle

off the west coast
of the island of Barbados.

They have a hurricane blowing
from the south on this map.

That's the approximate center
of the hurricane at this point

in time, and then we have
other ships off the coast

of St. Lucia reporting
northeast winds,

and another ship
approaching Martinique

with an east-northeast wind.

NARRATOR:
Mike draws together records
from all the ships

and plots the position of the
storm in six-hour intervals.

In this way, he tracks the storm
as it moves

across the Caribbean.

So from the 11th to the 15th,
this storm is out

in the eastern Caribbean
stirring up the water

with this huge wind field

that just continues to batter
the islands all in the region.

NARRATOR:
For nine days, the Great
Hurricane ravages the Caribbean

before passing
into the North Atlantic.

And Mike spots two features that
made this storm so destructive.

First, with hurricane force
winds that he estimates

extended up to 75 miles out

from the hurricane's eye,
the storm was vast.

CHENOWETH:
75 miles is about triple
what we usually see

for strong hurricanes
in that area,

the gale force winds extending
out 220 miles to the north.

NARRATOR:
That means its strongest winds
may have blasted

an even greater area
than Irma's.

The storm was moving forwards
unusually slowly,

ravaging Barbados
for eight hours.

CHENOWETH:
Because the storm is moving
slower than average,

it just had
more opportunity to keep

knocking more buildings down.

You've got thousands
of people dead.

NARRATOR:
From the ships' logs,

Mike is building up
a more accurate picture

of the path and size
of the 1780 megastorm.

But the logs can't answer
the most pressing question:

how powerful was this storm

by comparison
with hurricanes today?

To answer this question,

scientists are trying
to understand how

different conditions contribute
to creating monster hurricanes.

SHEPHERD:
You have a complex

web of conditions that have
to come together

perfectly to produce this
intensifying storm,

and that's really a challenge.

NARRATOR:
Scientists agree there
are three key ingredients

for the perfect storm.

First, for a hurricane to grow
in strength and power,

there needs to be a layer of
warm ocean water reaching down

to at least 200 feet.

The hurricanes churn up
cold water

from beneath the warm water
on the surface,

so they cool the surface.

It's like throwing cold water
on a fire.

NARRATOR:
Second, crosswinds near the top
of the hurricane,

called wind shear,
can't be too strong

or they disrupt
the formation of the storm.

EMANUEL:
That tends to blow dry air into
the core of a hurricane.

That also is like throwing
cold water on the fire.

NARRATOR:
And, third,
hurricanes need moisture

throughout the atmosphere.

If all these conditions
come together,

wind speeds within the eyewall
of the hurricane

can rapidly rise.

JEFF DONNELLY:
They become the most powerful
storms on the planet.

NARRATOR:
On a scale of one to five,
a major hurricane is

a Category 3 storm or above,
with wind speeds

of 111 miles an hour
or greater.

You could probably go outside
and walk around

in 75 mile an hour winds...
I wouldn't recommend it.

There's going to be things
flying through the air.

If it was
a Cat 3 or 4 hurricane,

you're not even going
to be able to walk or stand.

CHENOWETH:
Generally, if over half
the buildings

are knocked down, that's usually
a good sign you've got at least

a Category 3 storm.

NARRATOR:
The deadliest storm ever to hit
the United States

was a Category 4 in Galveston,
Texas, in 1900

with estimates up to
12,000 dead.

Category 5 storms are the most
powerful hurricanes of all,

with wind speeds over
156 miles an hour.

There are very few structures--
boats, buildings, cars--

that can withstand the force
of a Category 5 hurricane.

NARRATOR:
As wind speeds increase,
their destructive power grows

even faster.

DONNELLY:
It's not a sort of linear
change.

It's actually exponential,

so the wind is able to do
a lot more work.

EMANUEL:
If you double the wind speed,

you increase the destruction
by at least a factor of eight.

NARRATOR:
So a Category 5 hurricane
is at least

eight times more destructive
than a Category 1.

And recent history reveals
the extreme destructiveness

of these Category 5 storms.

In 2017, Hurricane Maria wrecked
power and water systems

across most of Puerto Rico.

Estimates put the damage
at over $90 billion.

In 1969,
Camille blasted Mississippi,

flattening hundreds
of miles of coastline.

And in 1935 one of the most
intense hurricanes

ever to hit the United States,
the Great Labor Day Hurricane,

ripped through the Florida Keys,

killing over 400 people.

But according to climatologist
Mike Chenoweth,

that the Great Hurricane of 1780
devastated a wider area

and left thousands
more people dead

than any of these modern
Category 5 storms.

And he's found detailed
eyewitness accounts

that reveal another reason why
this unusually large

and slow-moving storm
was so destructive.

Its power didn't come
from the wind alone.

Another devastating force in
a tropical storm is the water,

an intense surge from the sea...

(water roaring)

...known as storm surge.

This is what killed most people
when Typhoon Haiyan

slammed into
the Philippines in 2013.

In the Philippines,
more than 60 percent

of the population lives
within a mile of the water.

One of the most powerful storms
ever to strike land,

Haiyan's wind speeds reached
a 195 miles per hour

as it moved slowly
across the islands.

You have the wind driving
this water towards the coast.

(wind howling)

As the storm is approaching
landfall,

it's going from deeper water
to more shallow water.

Many people lost their lives in
that storm because of the surge.

NARRATOR:
Along any coastline, tsunami-
like waves pushed onshore

by hurricane winds can threaten
lives and property.

EMANUEL:
What you see is
a wall of water

that just wipes out
the neighboring houses and trees

and everything before it.

SHEPHERD:
And in some cases, this storm
surge can be ten to 20 feet high

and anything
within several feet to

up to a mile or so
of the coastline

can essentially be wiped out.

EMANUEL:
Anybody that's told to evacuate

because of the danger
of a storm surge,

they should not think twice
about it; just go.

NARRATOR:
Every year, millions of people
in the U.S., Central America,

and the Caribbean face the
threat of hurricane storm surge.

And it's storm surge that likely
caused much of the destruction

in Barbados in 1780.

CHENOWETH:
Suddenly,

these massive storm surges came

which carry this
incredible force with them

and devastated the city.

¶ ¶

NARRATOR:
Along the shore on Barbados,
Wayne Neely

is hunting for physical evidence
of the storm surge in 1780.

¶ ¶

This is Fort Charles,

just outside the capital,
Bridge Town.

It was entirely rebuilt
after the 1780 hurricane

completely destroyed
the original.

In 1780, before the hurricane,

the fort itself was
a massive structure.

The walls were much higher and
were three to four feet thick.

The size was immense.

For the walls to be destroyed
from that hurricane,

it had to be
a catastrophic storm.

NARRATOR:
The records describe
an unusually high storm surge,

over 25 feet.

Eyewitness accounts tell us the
hurricane swept heavy cannons

140 yards inland.

NEELY:
Nothing stood in place;
it devastated this fort.

It destroyed every house
in Barbados,

and it devastated the Caribbean.

CHENOWETH:
We had entire fortresses

and batteries of war,

you know, just totally
swept away.

NARRATOR:
The scale of destruction
reveals the extreme impact

of the 1780 hurricane.

And now, drawing together
all the clues,

it's possible to recreate
what happened.

On the night of October 9, 1780,

the Great Hurricane approaches
Barbados from the southeast.

The storm is
over 300 miles across.

On October 10, winds and rains
lashed the island.

Finally, as the eyewall
approaches land,

the huge winds push up a storm
surge 25 feet high.

It slams into the coast...

and inundates the land.

Ravaging the island
for over eight hours,

it leaves more than 4,000
dead...

And goes on to claim a total
of 22,000 lives

across the Caribbean.

The Great Hurricane of 1780
shows how devastating

a hurricane can be in an exposed
low-lying landscape.

Mike Chenoweth believes
its destructiveness

was unparalleled.

CHENOWETH:
We haven't had a storm like
that since,

so we're talking about something
that's happened only once

in 400 years.

NARRATOR:
But what are the chances
of it happening again?

CHENOWETH:
We don't know how far back

something similar
might've happened,

and we certainly don't know what
that possibility in future is.

NARRATOR:
Major hurricanes of Category 3
and above are not common events.

On average, one makes landfall
in the United States

every two years,
and Category 4 and 5 hurricanes

are even less frequent.

EMANUEL:
Category 5 hurricanes
are very rare.

There are far, far more
Category 1s than Category 5s.

NARRATOR:
But now, with the buildup
of greenhouse gases

in our atmosphere from
burning fossil fuels,

our climate is changing.

So many scientists like
Kerry Emanuel are predicting

that intense hurricanes
will become more frequent.

But it's a challenging problem.

Today, scientists rely on
complex weather data

from satellites and aircraft
to create computer simulations

that can help them
make predictions.

But this kind of precise data
only goes back a few decades.

We have reasonably accurate data
going back only to about 1970.

And if you go back in time,
we don't have satellites.

And then if you go back before
the 1940s,

we don't have aircraft.

NARRATOR:
There just isn't enough modern
data to discern patterns

in hurricane behavior.

Better data about hurricanes
in the past would be valuable.

SHEPHERD:
If we have understanding of
the activity, the intensity,

the frequency of storms

a thousand, 2,000 years ago,

then that gives us better
confidence

in our ability to assess
what hurricanes are doing now

and in the future.

NARRATOR:
But extracting clues about
what hurricanes were doing

thousands of years ago--

well before detailed
historic records--

requires some
very clever detective work,

and some of it needs
to be done at sea.

Oceanographer Jeff Donnelly
is looking for evidence

of ancient hurricanes
on the sea floor.

His search has brought him over
1,200 miles west of Barbados,

to Jamaica.

This is Discovery Bay,
on the northern coast.

Jeff and his team want to expand
our understanding of hurricanes

by exploring the deep past.

DONNELLY:
We're looking into
the sediments to sort of

find these long-term records

to extend our knowledge
back thousands of years.

NARRATOR:
Jeff focuses on
one part of the bay,

next to a reef too shallow for
the research vessel Atlantis.

The only way to get there is
by building a raft...

and towing it out to position.

Their target: a blue hole...

A natural sinkhole created
in the limestone rock.

Jeff is interested
in the sediments trapped

at the bottom of this hole.

DONNELLY:
The blue hole is really
a good recorder of hurricanes,

so it's sort of
a nice time capsule.

The sediment can go in,
but it never can come back out.

NARRATOR:
On calm days,
fine grain silt and sand

drift into the hole
and accumulate over time.

But the violent force
of a major hurricane

propels a different kind
of debris into the hole.

DONNELLY:
When a hurricane will hit,
you'll get really strong winds,

big waves, storm surge.

That all sort of comes up
over the reef here,

and there are really strong
currents associated with that

that will tear up pieces
of coral that transport sand

into the blue hole.

NARRATOR:
The more intense the hurricane,

the larger the pieces of coral
and sand the waves transport.

And over thousands of years,
this sediment

builds up as layers
inside the blue hole.

The deeper the layers are,
the further back in time

they were laid down.

The team lowers a hollow tube,

vibrates it into
the sediments,

and retrieves the sample
trapped inside.

Back on board Atlantis,
Donnelly cuts into the core.

(whirring)

So, we basically can
start at the top,

you know, this might be
what's depositing today,

and then you go back further
in time as you go down the core.

NARRATOR:
It's mostly fine, silty sand,

but Jeff spots one sediment
layer that's different.

When you get
down to the bottom,

this interval here...

and there's really big bits of
shell and coral fragments

in there.

NARRATOR:
Washing and sieving the sample
reveals larger pieces of coral

mixed up in the sand.

DONNELLY:
It's quite coarse,
compared to the rest

of the core, but this was all
that material that was ripped up

and washed into this basin.

NARRATOR:
To Jeff, the coarseness of
the sediments

is clear evidence
of powerful waves,

most likely driven
by a major hurricane,

striking here
sometime in the past.

DONNELLY:
You'd have to have a quite
a high energy event

to be moving this kind of
sediment from the barrier reef

into that blue hole.

15-foot waves.

It's something like
an intense hurricane strike.

NARRATOR:
By retrieving organic materials

washed in with the storm,

like twigs and leaves
that contain carbon,

Jeff is able to radiocarbon
date these coarser layers.

It'll take months to know for
sure when this hurricane struck,

but he's dated layers
from cores taken from sites

all across the Caribbean.

DONNELLY:
So, this is a piece
of a sediment core that we took

in the Bahamas.

This particular section dates
to the 18th century.

See these light bands
here, here, and here

are these hurricane event beds.

They're much coarser than
the sediment around them.

You can really feel the grit
between your fingers.

NARRATOR:
And he's finding that the most
recent layers exactly match up

with the dates
of modern hurricanes.

DONNELLY:
We started coming into it, you
know, with a healthy level

of skepticism,

but all the storms you expect
to find end up being there.

NARRATOR:
That gives him confidence

that his technique is valid.

And now he's finding evidence of
hurricanes long before

historic records began.

At present, we've been able
to go back about 2,000 years

at most sites.

Every time we find a layer that
dates to before 1600, 1700 AD,

we're finding an event
that we never knew occurred.

NARRATOR:
By plotting the dates
of major hurricanes

back 1,400 years into the past,
Jeff sees a pattern emerge.

For the first 700 years,
during the height

of Mayan civilization,
and as the Vikings

were colonizing Greenland,
it appears powerful hurricanes

were more frequent than today.

DONNELLY:
Not necessarily any more intense

than the ones we've
experienced today,

they just occurred
much more frequently.

NARRATOR:
Then, over the next 700 years,

during the Renaissance
in Europe,

and as European settlers
were arriving in the Americas,

the record shows
a marked decrease.

And then, suddenly,
it shuts down.

NARRATOR:
The Great Hurricane of 1780
falls in the period

where there appears to have been
far fewer major hurricanes,

making it even more unusual.

So, what caused this decline
in hurricane activity

beginning about 700 years ago?

Jeff suspects it might be
partly due

to trends in
sea-surface temperature.

(wave crashing)

And it turns out
there's a way to recover

ocean temperature data

from the remains
of tiny animals.

DONNELLY:
We can find out about sea
surface temperatures in the past

by looking at these
fossil corals.

NARRATOR:
Corals build up
giant colonies

that can last
for thousands of years.

Cores drilled out from deep
inside their structures

reveal layers of growth.

DONNELLY:
These corals grow almost like
trees, so each year,

it puts on a new band
of growth,

and you can actually
count back in time.

NARRATOR:
While forming their skeletons,
corals absorb oxygen

from the seawater.

Oxygen comes in two forms--

oxygen 16 has eight protons and
eight neutrons in its nucleus.

But oxygen 18
has two extra neutrons

and is more abundant
in the ocean

when temperatures are colder.

By measuring the ratio
of these two forms of oxygen

in the layers
of a coral skeleton,

scientists can calculate
relative ocean temperatures

over many thousands of years.

Using corals to reconstruct
sea surface temperatures

is really precise.

We're able to reconstruct
it right down to the year.

NARRATOR:
Plotting sea surface temperature
for the Caribbean

over the last 1,400 years
also reveals a trend.

The first 700 years
look generally warmer

than the next 700 years.

The results match
known historical

and scientific records that
chart a changing climate,

from an era known today as
the Medieval Warm Period

to a cooler period
known as the Little Ice Age.

And when Jeff compares
sea-surface temperatures

to his graph of hurricane
activity,

it appears that, in the past,
whenever the sea surface

was warmer, there were more
major hurricanes.

There are a whole variety
of factors that can influence

tropical cyclone activity
or hurricane activity,

but it's clear that there
is this interaction

between sea surface
temperatures and hurricanes.

NARRATOR:
The pattern seems to fit what is
known about hurricane formation.

The more energy available to
power a storm, it makes sense

that the storms are going
to potentially get stronger.

NARRATOR:
But these findings
present a puzzle.

1780, the year of
the Great Hurricane,

falls in the generally colder
period of the Little Ice Age.

DONNELLY:
It appears at first blush
to be quite an anomaly

that this 1780 season
and, actually, that

a couple of decades around it
are actually one of

the most active intervals.

NARRATOR:
But as Jeff looks more closely
at the sea surface temperatures,

he was able to detect a brief
but noticeable spike

in the decades around 1780.

And cool temperatures
in the atmosphere above

with warm ocean water below

is a known ingredient
for hurricane formation.

EMANUEL:
And whenever you have two bodies
of very different temperature,

you can create a lot
of energy that way.

DONNELLY:
So that might explain why in the
middle of the Little Ice Age,

we see, you know, an increase
in hurricane activity.

NARRATOR:
Jeff's work linking hurricane
events to ocean temperatures

could provide an explanation
for the intensity

of the 1780 hurricane.

And this research could sound
a warning for our future...

...because modern data reveals
that the sea surface temperature

of the Atlantic is now higher
than it was

a thousand years ago,
and is still rising.

DONNELLY:
We're actually warmer than any
point of the last millennium,

just a fraction of a degree
at this stage,

but the projections are that
that's going to continue.

But not only is it warmer
but it's increasing faster,

at a faster rate than we've seen
over the entire record.

NARRATOR:
As a result,
will we see major hurricanes

becoming more frequent?

DONNELLY:
We're likely to go back

into one of those
sort of intervals

where we're getting lots more
intense hurricane strikes.

NARRATOR:
The temperature
of our oceans is warming.

Greenhouse gases like
carbon dioxide,

generated by burning
fossil fuels,

are building up
in our atmosphere.

These insulate our planet,

holding in more
of the sun's heat,

causing the oceans
to warm more quickly

than we've detected in the past.

EMANUEL:
The problem right now
is that the rate at which

we're pushing
the climate system

is very fast compared to
anything that's happened

in a very, very long time.

NARRATOR:
Whether this warming climate
means there will be

more Atlantic hurricanes
is still an open question,

but there is a growing agreement
that the hurricanes that do hit

will be stronger.

EMANUEL:
There is a pretty strong
consensus that,

as the planet continues to warm,
we're going to see

a greater incidence of
the high category hurricanes

in most places.

We believe that there should be
more intense storms.

Perhaps not as many of them
are happening,

but when they do,
they'll be stronger.

NARRATOR:
So, can scientists predict
where these strong hurricanes

will strike?

¶ ¶

Amy Frappier, who studies
ancient climates,

has found an ingenious way to
look into the past for answers.

Compared to the last
four centuries,

she's finding that Atlantic
hurricanes are on the move,

likely heading towards
the big coastal cities

of the eastern United States.

¶ ¶

The evidence comes
not from beneath the waves,

but from caves under the ground.

(dripping)

Locked inside stalagmites like
these are the chemical traces

of hurricanes from thousands
of years ago.

So here's a stalagmite
that we collected from Belize,

where we know hurricanes have
been part of the weather.

NARRATOR:
Rainwater that seeps through
the ground above a cave

dissolves minerals
from the surrounding rock.

Then, as it falls drop by drop
from the ceiling, it leaves

a little bit of this mineral
behind and a chemical signature

of the rainwater itself.

This forms stalagmites
that over the years grow upwards

from the cave floor.

Back in her lab,
Amy slices stalagmites open

and polishes the surfaces.

This reveals a series
of distinct layers.

FRAPPIER:
You can see that there's
this whole history

in here about what's
happened over time.

In this one, it's got
lots of different changes

in color and texture
as we go from the older part

to the younger part at the top.

NARRATOR:
Hunting for the chemical traces

of a hurricane
in these layers is possible

because rain from hurricanes
is chemically different

from rain
during ordinary storms.

Again, a result thanks
to the difference between

oxygen 16 and oxygen 18,

which has two extra
neutrons in its nucleus.

In an ordinary storm, raindrops
evaporate slightly as they fall.

The lighter oxygen 16
evaporates more readily,

changing the proportions in rain
hitting the ground.

But in a hurricane,
the air is so humid

that there is very little
evaporation.

This means that in hurricane
rain, there's more oxygen 16

than in weaker, short-lived
rainstorms.

FRAPPIER:
Whenever we see that
light oxygen signature,

we know that that
is a fingerprint

for a past hurricane.

NARRATOR:
To find this chemical signature,
Amy isolates individual layers

in the polished stalagmite,

then drills out
a minute sample as dust.

A mass spectrometer can read out
the chemical traces

locked inside the dust,
and it reveals which layers

are richer in light oxygen,
the signature of hurricane rain.

The results are so accurate
they allow Amy to tell

if a hurricane has hit
in any given year

thousands of years in the past.

FRAPPIER:
The level of detail
is just unprecedented.

We can see
the difference between

years with a storm strike
in Belize

and years without
a storm strike in Belize.

NARRATOR:
With data from caves
across the Caribbean,

the evidence suggests that the
paths of Atlantic hurricanes

appear to be changing over time.

FRAPPIER:
We're starting to be able
to have enough data

that we can see

not only overall patterns
of storm activity,

but also changes
in storm tracks.

NARRATOR:
Over a 450-year period,
the average track of hurricanes

has been moving ever closer to
the continental United States.

FRAPPIER:
400 years ago,
the storm strikes were clustered

in the western Caribbean,
around Central America,

and now the storm strikes seem
to be happening

much more frequently
around the U.S. east coast.

NARRATOR:
It's a trend that Amy
is still exploring.

But based on satellite data,

Kerry Emanuel has come
to a similar conclusion.

EMANUEL:
We've discovered that
over the last 35 years,

the latitude at which
tropical cyclones

reach their peak intensity
has been shifting away

from the equator at a rate
of about 35 miles per decade.

NARRATOR:
Kerry thinks that climate change
is driving the recent shift.

What we see when we look
at global warming is that

the fastest warming
is occurring in the Arctic.

Hurricanes like warm waters
and so they're shifting

toward the poles.

NARRATOR:
In just 30 years, that's over
100 miles closer

to densely populated areas
of the United States

along the Eastern Seaboard.

The last major storm to hit
the northeast coast was

Megastorm Sandy in October 2012.

After leaving a trail of
destruction in the Caribbean,

Sandy approaches the New York-
New Jersey coastline.

Though most of the Atlantic
gets colder as you move north,

Sandy feeds off a ribbon of warm
water that keeps it alive:

the Gulf Stream.

This is a circulating current
that pumps warm water

from the Gulf of Mexico up
and across the Atlantic.

By the time Sandy strikes
on October 29,

it's been downgraded from
a Category 2 hurricane

to a Category 1 storm.

But as Sandy combines

with another North Atlantic
storm system,

it hits with
catastrophic impact.

A 14-foot storm surge
races into New York City,

flooding streets,
tunnels, and subways,

and shorting out
electrical transmission lines.

Across New York and New Jersey,
the storm leaves 60 people dead,

damages 650,000 homes and causes
over $70 billion in damage.

WOMAN:
It's just complete devastation,

and, you know, my parents have
lived here for 40 years,

and it's unbelievable.

NARRATOR:
Superstorm Sandy
was massively large,

and hit
a densely populated area.

Its powerful storm surge
did most of the damage,

but its winds were
not especially strong.

SHEPHERD:
I think many people would be
surprised to know that

Hurricane Sandy, at the time
it was impacting

New York and New Jersey, was not
considered a major hurricane.

NARRATOR:
Imagine if it had been
a Category 3 or higher.

It has happened before.

The strongest recorded hurricane
to strike this coast

hit nearly 200 years ago.

(roaring)

It's known
in historical records as

the 1821 Norfolk, Long Island,
Hurricane, and is now thought

to have come ashore as
a Category 4 storm,

far more intense and extensive
than Sandy.

Experts estimate that
if this storm struck today,

it would cause over
$100 billion in damage.

SHEPHERD:
Imagine a Category 4 storm

impacting New York--
the flooding, the storm surge,

the winds associated with that.

Cities such as New York

need to be prepared
for this type of threat.

NARRATOR:
Many factors can affect
hurricane formation,

but as the climate warms,
the threat of major hurricanes

heading up from the Caribbean,
impacting the southern states

and striking
the northeast coast,

is one that climate scientists
are taking seriously.

And they're highlighting
a less well-known danger

of global warming,

one that will make hurricanes
even more destructive.

Warmer oceans cause
water volume to expand.

At the same time,

glaciers are melting.

The result: sea level
around the world is rising.

SHEPHERD:
If you look at some of
the more recent data,

not only is it rising,
in more recent decades,

it's rising at a faster rate.

EMANUEL:
At the best guess now is that,
if we don't curb emissions,

we'll be up a meter or three
feet by the end of the century.

NARRATOR:
The rising sea level
increases the risk

of damaging storm surge.

When that storm surge rides up
on top of higher sea levels,

then it causes
a lot more destruction.

NARRATOR:
Today, millions of people in
coastal and island communities

are at risk.

EMANUEL:
With all of this infrastructure
very close to sea level,

we're much more vulnerable
to much smaller changes.

NARRATOR:
To better understand our future,
scientists are looking

into the past, and what they're
finding leads some to predict

that a hurricane as deadly as
the Great Storm of 1780,

with its huge storm surges,
will likely strike again.

MASTERS:
We're going to see another
hurricane like

the Great Hurricane of 1780
again on our shores,

and it's going to hit land
at full intensity.

CHENOWETH:
It's happened once before,
so there's no reason

to not expect it
to happen again.

NARRATOR:
While there is no way to predict
what lies ahead with certainty,

the evidence suggests we need

to be prepared to face more
killer storms in the future.

¶ ¶

This NOVA program
is available on DVD.

To order, visit shopPBS.org
or call 1-800-PLAY-PBS.

NOVA is also available
for download on iTunes.

¶ ¶