Orbit: Earth's Extraordinary Journey (2012–…): Season 1, Episode 2 - Episode #1.2 - full transcript
All of us, every day of our lives,
are on the move.
And we don't mean the morning commute
or taking the kids to school,
but a journey of epic proportions.
Even now, as you're watching this,
you're hurtling through space
at 100,000 kilometres an hour.
Every year, our planet,
the Earth, travels around the sun
and we go with it.
I'm Kate Humble.
This is it.
The sun is directly overhead.
My shadow is directly below me.
In this series, we are going
to follow the Earth's voyage
through space for one whole year
to witness the astonishing
consequences this journey has for us all.
I'm Dr Helen Czerski and I study
the physics of the natural world.
Wow, look at that!
SHOUTING: I'll be investigating
how our orbit powers
the most spectacular weather
and how it's also shaped
and reshaped our planet.
But our planet's journey isn't quite
as smooth as you might think
and its orbit changes over time
with significant consequences.
The bottom here is 120 metres down.
And full of sharks.
Wow!
In this episode,
we explore what it means
to live on a planet locked in a
never-ending voyage around the sun.
Join us on the most remarkable
journey of your life.
Since our journey began,
we've travelled almost 500 million kilometres
around the sun to the end of December.
In this episode,
we continue our journey,
travelling from the beginning
of January
to the spring equinox in March.
In the northern hemisphere,
that means we're in winter,
the harshest season.
Whilst in the southern hemisphere,
it's summer,
although it's a little different
to the one in the north.
I'm starting
in the Scottish Highlands
on a particularly significant day
in our journey around the sun.
It's the third of January,
it's minus five...
..and the winds are gusting
to over 60 kilometres an hour.
I'm walking... up Aonach Mor...
..one of the highest mountains
in Scotland.
If it was a beautiful,
clear, sunny day,
you'd be able to see Ben Nevis
over there.
And if I was going
to be very British
and stiff upper lip about this,
I'd say it was a little bit chilly.
But I'm not.
It's absolutely freezing.
So, why, one might ask,
am I going to the effort of climbing
over 1,000 metres
in these conditions?
Well, by being here on Aonach Mor,
I'm about as close to the sun
as I'll ever be
and it's actually not
because of where
but when I'm making this climb.
Today,
we're physically closer to the sun
than on any other day of the year.
It's a day with a special name.
It's called perihelion,
and although it's impossible to
believe in conditions like this,
the Earth is five million kilometres
closer to the sun today
than it will be in July.
At perihelion, being on top
of this mountain on this day,
brings me one more kilometre
closer to the sun.
It may seem strange
that on some days
we can be much closer
to the sun than on others,
but it's the consequence
of a particular feature
of the Earth's orbit.
The Earth's journey through space
is controlled by the sun's gravity.
But it isn't quite
the orbit you might expect.
Now, say that this stone is our sun.
Now, the Earth doesn't orbit the sun
in a perfect circle.
Instead, we go around, on an ellipse.
Not only is the Earth's
orbit elliptical,
the sun isn't in the centre of it.
That means that our distance from the sun
varies continuously throughout the year.
And, today, on our orbit,
January the 3rd, we're there -
closer to the sun than we will be
for the whole of the rest of the year.
The Earth's elliptical orbit
means that in January, at perihelion,
the Earth receives about 7% more
solar energy than it does in July,
when the Earth is at
its furthest point from the sun.
You might think that this extra energy
would mean that January would be warm
and July would be cold.
Well, it turns out that proximity
to the sun doesn't guarantee warmth.
The reason for this apparent anomaly
is that there's a second,
more powerful factor at work.
As it orbits the sun,
the Earth is tilted on its axis
at an angle of just over 23 degrees.
Because of this 23.4 degree tilt,
in January, the northern hemisphere
is pointing away from the sun.
The Earth's tilt reduces the amount of
solar radiation in the northern hemisphere
by up to 50%, far more
than perihelion increases it,
which is why it's winter in Britain,
even though this is when
we're at our closest to the sun.
But perihelion in the southern
hemisphere coincides with summer,
so, in theory, the relative proximity of
the sun and the extra energy this brings
should mean this part of the world
has particularly hot summers.
I've come to Chile
to discover whether this holds true.
This is Puerto Williams. It's not
just the southernmost town in Chile,
it's the southernmost town
in the world.
The next significant land mass
from here is Antarctica.
Puerto Williams
is a good place for us to be
because it's at an equivalent
latitude to the UK.
So we can find out how summers here,
in the southern hemisphere,
compare to ours.
I'm heading into a stretch of water
called the Beagle Channel
that crosses the bottom
of the continent.
It's named after HMS Beagle,
the boat that carried Charles Darwin
here almost 200 years ago.
Truth be told, so far, conditions are not
hugely different to summers back home.
But there is a difference and it's
something you'd never see in the UK.
A glacier.
I am absolutely... blown away
by where we are.
It's just the scale of it
that takes your breath away.
It comes right down into the water,
and, as you can see, there are just
great chunks of ice everywhere you look
that have broken off the glacier.
It's like floating
in a giant gin and tonic.
But look at that!
Ooh, there's ice falling off it now!
And what's so astonishing
about this is its location.
This isn't the only glacier
in this region, not by a long way.
And yet, we're at
55 degrees latitude south.
If you go to the equivalent latitude
in the north, 55 degrees north,
you get to the Lake District
in England.
Now, we all know that
the Lake District is very pretty.
But it hasn't got one of those.
The presence of this glacier is
evidence that, rather than being hotter,
summers in the southern hemisphere
are actually cooler
than in the northern hemisphere.
In fact, on average, they're
a full four degrees Celsius cooler,
despite the added boost that perihelion
gives to the southern hemisphere summer.
So something else is at work here,
counteracting the effects of perihelion.
To discover what it is,
I'm heading back out to sea.
Well, we're now out
in the open ocean
and, I have to say,
if you're not a sailor,
and I'm not...
..it makes you feel very small...
Whoo! ..a little bit scared
and quite sick.
We're sailing
in the Southern Ocean...
..where strong winds and icebergs
have made these waters notorious
as a sailor's graveyard.
This is a very exposed stretch
of water.
To the west is the Pacific Ocean,
whilst to the east is the Atlantic.
To the south, the nearest land mass
is Antarctica.
It's the very vastness
of this expanse of water
that's the reason why summers in
the southern hemisphere are so cool.
If you look at the whole
of the southern hemisphere,
over 80% of it is covered by oceans
and these huge expanses of water
have a powerful effect on the climate.
That's because water
has an important characteristic.
It takes a lot more of the sun's energy
to warm up the sea than it does the land.
In other words,
water has a high heat capacity.
This means that, even in midsummer,
and even with the added warmth
provided by perihelion,
the oceans in the southern
hemisphere are still cool.
And this keeps the air cool too.
Even at this time of year, when the
Earth is physically closest to the sun,
and the southern hemisphere
is tilted towards it,
the influence of the oceans
keep it much cooler.
It's a sobering thought
that without perihelion,
southern hemisphere summers
would be even cooler than they are now.
The Earth's slightly off-centre orbit
is a reminder
that we live on a planet
that's hurtling
through space around the sun.
This journey is controlled by the
immense power of the sun's gravity.
But the sun's gravity is also
responsible for significant dangers.
I've travelled to a place
where you can see these dangers
written into the Earth's surface.
This is the Barringer Crater
in Arizona.
50,000 years ago,
a meteorite struck this site
and excavated this dramatic hole.
That impact spread debris over tens
of thousands of square kilometres.
This crater itself
is more than a kilometre,
or three-quarters of a mile, across,
so as you can imagine,
it was an incredibly violent event.
To get an idea of the force
involved in that impact,
we can look at two types of rock
that you find round here.
Now, this,
this is Coconino sandstone.
This is what was present
before the meteorite hit.
Now, down here, we can see what happened
to this kind of stone after the impact.
So, this rock here,
it's chemically exactly the same,
but what the impact did to it
was just pulverise it. Look at this.
It's just breaking apart
in my fingers.
And the reason for that is that the
shock that went through from this impact
just fractured
all the tiny grains of quartz.
Incredibly, all this was done
by a meteorite just 50 metres across.
There are thousands of objects
circling the sun,
trapped by
its immense gravitational field.
Every now and then,
we collide with one.
But not all of them are as small as the
one that created the Barringer Crater.
Hidden underneath what is today
a place called Chicxulub in Mexico
is a huge crater.
The impact of the Chicxulub meteorite
was cataclysmic.
It blasted so much hot debris
into the atmosphere
that almost the whole planet
caught fire.
The overall impact was so great
it eventually contributed
to the extinction of the dinosaurs.
ROARING
Our orbit regularly takes us into
the path of asteroids and comets.
And it's a sobering thought
that our voyage through space
could deliver a random disaster
to the whole planet.
The good news is that the bigger
the potential disaster, the rarer it is.
But there's another potential danger
that comes from our orbit around the sun.
And the best time to see it
is at this time of year,
in the middle of winter.
The long nights mean that this is the
peak season for an extraordinary spectacle.
For thousands of years, people have
marvelled at the spectacular light displays
that sometimes appear
in the night sky
and they've wondered what on earth
they could possibly mean.
The Vikings believed them to be
the reflections of dead maidens.
The Cree Native Americans called
them the Dance of the Spirits,
and, in Europe in the Middle Ages,
they believed the lights
meant that God was angry.
But the truth
is actually even more extraordinary.
This celestial light show,
or aurora, as it's known,
is the front line in the battle between
the sun and the Earth's atmosphere.
Every second, the sun blasts out a
million tonnes of radioactive particles
and the Earth is in the firing line.
The sun emits a continuous flow
of charged particles,
known as the solar wind.
This streams outward,
in a wash of radiation.
But when it reaches the Earth,
it encounters a barrier.
The Earth's magnetic field
deflects the particles
and funnels them towards the poles.
Here, they collide with atoms of
nitrogen and oxygen in the atmosphere.
These collisions emit energy in the
form of light, giving us the aurora.
From
the International Space Station,
you get a better sense
of the awesome scale of the aurora.
We don't often think of it this way,
but the aurora is graphic evidence that
we live inside the atmosphere of the sun.
This is the sun's atmosphere
colliding with the Earth's atmosphere.
So our orbit, close to the sun,
is full of risk.
But it's also vital
for our survival.
Almost all life on our planet
depends on the energy
we receive from the sun.
Our location close to the sun
provides one critical benefit -
it allows the presence
of liquid water.
If our planet
was much closer to the sun,
it would be too hot
and the water would boil away.
Too far away, and it would freeze.
Our planet is in
what's known as the habitable zone.
The zone where water can exist
and life can flourish.
Earth may be dangerously close
to the sun,
but this is the price
that has to be paid to sustain life.
And our location close to the sun is
even more favourable than it first appears.
Earth orbits the sun at just the
right distance to allow water to exist
in all three states -
as a solid, a liquid and a gas.
And it's switching between those
states all the time.
But water in each of those states
behaves very differently,
and it's those differences that generate
the climate system as we know it on Earth.
It's now the middle of January.
This time of year
gives us a great opportunity
to see two ways
in which water changes state,
with very different consequences.
I'm back in the southern hemisphere,
in the foothills
of the Andes in Argentina.
Here, you can see water
moving between states
and how this process
transforms our planet.
This is the cloud forest of Calilegua,
2,000 metres above sea level.
And, as you can see, clouds
are definitely a feature here.
There's a wonderful thick wisp
of cloud down in the valley there
and then this great bank
over the trees on the horizon.
And then you've got these ghostly
wisps climbing up above the trees.
It really is a magical place.
This is a classic summer's day
in the cloud forest.
It's hot, it's humid...
It's like being in
a giant steam room.
BIRDS CAW
The humidity I'm feeling is because
the heat has evaporated water,
so the air is laden with vapour,
the gaseous form of water.
As the day progresses,
some of this warm, moist air
will change state again.
It's mid-afternoon and it's getting
increasingly hot and steamy.
In fact, if feels like this heat
is about to trigger something
absolutely spectacular.
Throughout the day, the land has
been absorbing more and more heat.
That heat warms up the moist air
and forces it to rise
high into the atmosphere,
forming towering cumulus clouds.
These clouds are the transformation
of water made visible.
The rising water vapour
has cooled and changed state
and become liquid again.
What's incredible is watching this cumulus
cloud growing in front of my very eyes.
It has to be eight,
ten kilometres tall already
and you can almost feel the energy
crackling away inside it.
There's a tremendous sense of
build-up and anticipation in the air.
Powerful updraughts push the cloud
so high, the top spreads out
to form
a characteristic anvil shape.
An approaching storm like this
could last half an hour.
It could last 10 or 12 hours.
Sometimes they even join up with other
storms to create destructive megastorms
that can devastate the whole region.
These tropical storms are an extreme
version of a familiar phenomenon.
Rain.
RAIN SPATTERING
Rain is so familiar
that it's easy to forget
what a critical role
it plays on Earth.
It's the way in which water is transported
from the oceans and deposited over land.
Without the ability of water to change
from liquid to gas, and back again,
the land would be
a dry and barren desert.
Meanwhile, in the northern hemisphere
at this time of year,
a different transformation of water
occurs, from liquid to solid.
I've come to the edge
of Lake Ontario in North America
to see one of the most extreme
examples of this transformation in action.
This area is home to some of
the heaviest snowfalls in the world.
But it's not immediately obvious
why this should be so.
It's so peaceful here.
There's a beautiful blue sky.
It's been a stunning day.
But tomorrow,
from across the lake over there,
there's a huge storm coming our way,
although you'd never know that
to look at it now.
The snowstorm
is likely to be particularly heavy
because of a unique set
of conditions.
The air outside is cold and dry.
It's come straight from the Arctic.
But this frigid air
is about to be transformed.
You can see
what does it right below me.
Water.
Warm water.
Even though it looks
pretty chilly down there,
the water's significantly warmer
than the land around it.
And there's lots of it.
Even though
it's frozen round the edges,
there's plenty of open water
in the middle.
Lake Ontario is one of the Great
Lakes, so it's a huge body of water.
Water's high heat capacity
means it's held onto much of the heat
it absorbed during the summer.
As the cold, dry air passes over
this relatively warm lake,
water evaporates.
As it rises over Upstate New York,
it forms clouds.
Those clouds are the start
of a special type of snowstorm,
which leads to some of the biggest
and fastest accumulations of snow
anywhere in the world.
And it's called
a lake-effect snowstorm.
These snowstorms
are particularly intense
because the cold air can keep on
blowing across the lake for days.
It's like a conveyor belt
of cloud formation.
Within these clouds, the cold air
means that water turns from liquid
to its solid, crystalline state...
..a snowflake.
And they start because there are
tiny grains of dust, way up in the clouds
and the warm lake air provides moisture,
which condenses onto those droplets.
And as they're carried up and up
into the cloud,
the temperature goes down
and so they freeze into a crystal.
And that crystal is a snowflake.
Here, conditions produce
a very particular type of snowflake.
Because the air is so cold, it
produces crystals with sharper tips.
These grow more branches,
called dendrites,
which make the snowflakes fluffier.
It's the kind of snow we all love -
as long as there isn't too much of it!
It's now approaching nightfall
and the snowstorm is almost upon us.
How much snow falls
will depend on one final factor.
The wind direction.
If the wind comes from the north, it
passes over the narrow part of the lake
and so picks up
only a small amount of moisture,
making just a light shower of snow.
But if the wind comes from the west,
it passes over almost
the full length of the lake
and picks up a lot of moisture,
producing much more snow.
At night, the storm finally arrives.
I'm here in the middle
of the snowstorm
and the winds are really strong.
The thing is that powerful winds like this
are exactly what you get up in the clouds
where snowflakes form.
So next time you see
a peaceful snow scene,
remember that all
of those delicate snowflakes
are formed in a violent,
windy environment, just like this.
WIND HOWLS
Next morning, the town beside the
lake wakes up to a heavy coating of snow.
But because it's a regular event,
people here are prepared.
Across the northern hemisphere,
the same interaction of cold land
and relatively warm moisture
produces many other spectacular
weather phenomena.
In January 2005, these
remarkable ice sculptures formed
when spray from Lake Geneva in
Switzerland was thrown up by strong winds
and froze as soon as it landed.
In Canada in 1998,
rain falling on frozen ground
turned to ice as it landed,
a phenomenon known as an ice storm.
It continued for 80 hours.
The sheer weight of ice
crushed over 1,000 steel pylons,
leaving four million people
without electricity.
Closer to home, frost forms
when air saturated with moisture
touches surfaces
that are already frozen.
Our orbit around the sun exposes
our planet to potentially deadly radiation.
But the payoff is a big one...
..a planet where water can be
distributed across the whole Earth,
providing spectacular weather
and making it habitable.
It's now late January and the
northern hemisphere is locked in winter.
And yet there is a paradox
about our winter,
because in January,
winter is still getting colder,
even though the northern hemisphere
is receiving more energy from the sun.
I've come to Northern Canada,
to the best - or perhaps the worst -
place to explore this paradox.
Whoo!
Cor! This...
..is Yellowknife.
It has the dubious distinction
of being the coldest city
in the whole of North America.
Today is January the 19th.
On average, this is the coldest day of
the year across the northern hemisphere.
It's minus 35 degrees Celsius,
which certainly qualifies as cold to me.
It's pretty hard to describe to you
just how it feels to be at minus 35,
but I'm going to give it a go.
When you breathe, it hurts.
It kind of gets you
at the back of the throat.
Your nose feels
like it's permanently frozen solid.
And despite the fact that I've got
the feathers of about 25 geese
stuffed into this jacket,
and more thermal underwear
than I thought possible to wear
at exactly the same time,
I still feel cold.
In these conditions, even familiar
things behave in unfamiliar ways.
You can take a lovely, hot,
steaming cup of coffee,
throw it in the air, and the steam
from that coffee will freeze instantly.
Well, you've got to give it a go,
haven't you?
Right...
Here goes.
Wow!
That is amazing!
Oh, my word!
There's something curious about the way
winter peaks towards the end of January.
The winter solstice falls
on December the 21st
and this marks the day
when the northern hemisphere
receives the least amount
of solar energy from the sun.
So you might expect the December
solstice to be the coldest day of the year.
But it's not.
On average, temperatures
on the 19th of January are colder
than they are in mid-December.
But, you say,
the days are getting longer.
The northern hemisphere
is getting more sun.
It should be warming up.
In Yellowknife, there are people
whose livelihoods depend
on the way winter's peak is delayed.
In the driving seat
is Blair Weatherby.
His family have been driving through
the bitter cold of this region
for three generations.
He's not an ordinary trucker.
He's an ice road trucker.
And this is his highway.
In the summer, what happens here?
We'd be in a boat!
That's because we're not driving
on land, but on a frozen lake.
So really to appreciate
Yellowknife's splendid isolation,
you have to look at a map.
And here it is,
right on Great Slave Lake.
But it's surrounded
by water and tundra.
So at this time of year, of course,
it freezes, and Yellowknife,
and all these tiny, little,
incredibly remote communities
can get linked up by the ice roads.
So what time of year
can you start driving on the lake,
as opposed to boating on the lake?
The season starts
towards the end of January.
It's about 30 inches thick at this point.
It just keeps getting thicker and thicker.
So whilst the northern hemisphere's
coldest day is the 19th of January,
here in Yellowknife, it's still
bitterly cold for many weeks to come.
For the truckers, this delayed
winter means their work season
runs from late January
well into March.
Here, you can go for hours with your
hands off the steering wheel sometimes.
There's lakes that take two and a half
hours to drive across. People watch movies.
You put a DVD player on your dash and watch
a movie when you're going across the ice.
So why is the worst of winter
delayed so long
after the solstice
on December the 21st?
It's all about the balance
between the heat coming in
and the heat going out.
Throughout early winter,
the northern hemisphere
receives declining amounts
of the sun's energy,
so it starts to cool down.
But there's a lag in this cooling,
because the Earth's surface
loses heat relatively slowly.
So well into January, the Earth's
surface is still losing heat,
even though solar energy
is slowly increasing.
It isn't until around the 19th of
January that a tipping point is reached.
From this day onwards,
the increase in solar radiation
will overwhelm the effects
of the heat loss
and the northern hemisphere
will begin to warm up.
But it'll still
be a few more weeks yet
before the ice here is too thin
to support the weight of the trucks.
We've seen how the Earth's journey
through space is critical for life
and how the Earth's angle of tilt
defines our seasons.
But you only really understand just
how important our orbit is for our planet
when you look into the Earth's past.
There's evidence
in the most unexpected places.
A few miles out there is one of the
most spectacular wonders of the world,
but I can't see it from here
because it's underwater.
I'm in Belize in Central America
and what I'm going to see
is known as the Blue Hole.
It's not often
that nature produces something
as beautifully symmetrical as this.
It's almost a perfect circle.
But it's more than just a stunning
piece of natural architecture,
because deep down there are clues
to some of the most dramatic events
in Earth's history.
This wall seems to go down for ever
and I'm told that the bottom here
is 120 metres down,
which sounds like
a very, very long way just now.
And I'm just dropping into the abyss.
That shark just swam
past in front of me.
I've never, ever been
so close to a reef shark.
And there's another two
just behind me.
Finally, I've reached my goal.
So down here at 40 metres...
..it's really eerie.
Gloomy.
And this is what I've come to see.
And they're stalactites.
But there's only one way I know
of for stalactites to form.
And it isn't down here,
in 40 metres of water,
with sharks swimming about nearby.
Stalactites are created when mineral-rich
water drips from the roof of a cave,
over hundreds
or even thousands of years,
leaving behind mineral deposits.
In other words,
they didn't form in the ocean.
Stalactites like this
can only ever form above ground.
And that means that when these grew,
the sea level was much, much lower
than it is today.
Scientists have precisely dated
stalactites from the Blue Hole
and, by comparing these and other
sea level indicators from around the world,
they've built up a picture
of changing sea levels
dating back
hundreds of thousands of years.
It reveals a striking pattern.
Sea levels across the world
have risen and fallen over time.
Genuinely one of the eeriest things
I've ever seen, that.
20,00 years ago, the entire surface
of the world's oceans
was 120 metres below
where it is today.
And that means if I was standing here
20,000 years ago,
all of this, including the Blue Hole
cave system, would be dry land.
So where did the ocean go?
The answer is that it was on land.
But it wasn't liquid water,
it was ice,
because 20,000 years ago, our planet
was in the middle of an ice age.
The Earth has experienced
regular ice ages
in a cycle going back
several million years.
These dramatic changes
to the state of our planet
are triggered by small changes
in the Earth's orbit.
I've travelled back to Britain
to uncover the relationship between
the Earth's orbit and an ice age.
Snowdonia's peaks and valleys
were carved out in the last ice age.
It's in mountainous locations like
this that an ice age would have begun
as snow gradually built up.
When we think of ice ages, we think
of extreme cold during the winter.
But, counterintuitively,
it's summer temperatures which are
important in starting ice ages.
And the reason for that is, now, ice
will build up here during the winter,
but it will all melt away
in the summer.
But if the summer is a little bit
cooler, a layer of ice will be left behind.
And a series of cool summers
will leave layer after layer,
one on top of the other, building up.
And here, the ice could have
been hundreds of metres high.
Ice ages always start
in the northern hemisphere
because there's so much more
land surface on which ice can build up.
So the question is, what causes cooler
summers in the northern hemisphere?
The answer comes from small changes
in the Earth's orbit,
caused by the gravitational pull
of other planets.
Our orbit
isn't exactly the same every time.
Aspects of it change just slightly,
in cycles lasting thousands of years.
And when all of those cycles
reach their most extreme point
all at the same time,
that can change our summer temperatures
just enough to tip us into an ice age.
There are three cycles
to do with the Earth's orbit
that must all coincide
to trigger an ice age.
The first of these cyclical changes
affects the time of year
when perihelion occurs.
This is the day when the Earth
is closest to the sun.
Today, perihelion is in January,
but over thousands of years,
the date of perihelion changes.
When perihelion occurs
in the northern hemisphere summer,
it makes summers particularly hot.
But when it occurs in winter,
as it does today,
then northern hemisphere summers
are cooler.
So at the moment, the perihelion cycle is
at the right point to generate an ice age.
But two other cycles
are not in an ice age phase.
The first is the angle
of the Earth's tilt.
The Earth's tilt is currently at an
angle to the vertical of 23.4 degrees.
But that angle changes
between 22 and 24.5 degrees.
It's only when the angle
is at its shallowest - 22 degrees -
that the seasons become less extreme
and the summers cooler.
Today, the angle of tilt
is too great for an ice age.
The final cycle affecting an ice age
is the shape of the Earth's orbit.
The Earth's orbit is an ellipse,
but over time, it becomes slightly
more, and then slightly less, elliptical.
When the orbit is at its most elliptical,
the result is lower summer temperatures.
At the moment, the Earth
is midway through this cycle,
so again,
it's not in an ice age phase.
It's only when all three of these changes
to the Earth's cycle line up together
that they produce
the really cool summers
in the northern hemisphere
that result in ice ages.
At the moment, those three cycles
are all at different positions,
and so we're still getting
enough sun during the summer
to melt ice
and keep us out of an ice age.
It'll be around 60,000 years
before the cycles line up again
and the next ice age starts.
It's now the beginning of March and
we're nearing the end of our journey.
In most of the northern hemisphere,
spring is on the way.
But there is still one part of
the world that is locked in winter.
Long after January the 19th,
on average the coldest day
in the northern hemisphere,
winter still clings on
in the Arctic.
I've come to Greenland, where there's
definitely not much sign of spring yet.
This is Kulusuk. It's a tiny
settlement of just 355 people
perched on the edge of an island
in eastern Greenland.
To the north of here is the Arctic
Circle and the Greenland ice cap.
Kulusuk is surrounded
by the Arctic Ocean.
At this time of year, it's frozen,
covered in a thick layer of sea ice.
I've come here to find out
about sea ice - how far it extends
and why it hasn't melted, despite
the fact that it's now March,
the days are getting longer and the
amount of solar energy is increasing.
In fact, not only is the sea ice
not melting, it's still expanding.
Each year, the extent
of the sea ice is different.
To see how far it reaches this year,
I need to travel right
to the edge of the sea ice.
But before I can set off,
a massive snowstorm hits Kulusuk.
We can't go anywhere.
The 140-kilometre-an-hour winds
make a trip to the local shop
a major expedition.
'By the next morning,
the storm has passed.
'I meet up with my guide,
local hunter Gio Utuaq.
'His hunting grounds lie
right at the edge of the sea ice.
'I'm hitching a lift on the only form
of transport that can get us there.'
She's so keen!
How far do we have to go
to get to the hunting grounds?
20, maybe 25 kilometres.
After two hours,
we reach a huge expanse of sea ice.
It's impossible to comprehend that the
snow we're travelling across sits on ice,
which sits on the ocean.
We're travelling across
a frozen sea. And look at this!
This is an iceberg
actually trapped within the sea ice.
It's the most astonishing landscape,
or seascape or ice-scape...
What do you call it?!
..that I've ever seen!
It's like another world.
And then, surprisingly quickly,
the edge of the ice comes into view
and I can see the Arctic Ocean.
Gio tells me that only days ago,
the ice extended out
for several more kilometres,
but it seems the storm
has broken it up.
For obvious reasons, we make the
last stretch of the journey on foot.
Using his traditional spiked tool,
Gio checks the thickness
of the ice at the edge.
Are you sure?
SHE CHUCKLES
There is something
very disconcerting
about walking on sea ice
when the open sea is so close.
Is it safe? No problem? No problem.
Are you sure?
Yeah, it looks pretty solid.
How thick is the ice?
Like this thick?
Oh, you can see it.
Actually, you can, you can see
it's like a great cliff of ice
that goes right down into the water.
It seems strange to be walking
across a frozen sea here in Greenland
when back at home, the daffodils
are beginning to come up.
But what's even stranger
is that measurements of the sea ice
over the last 50 years
show that it only reaches
its full extent now, in early March.
So clearly there's a lag between
the arrival of the warmth of the sun
and the melting of the ice. But why?
It comes down to
the properties of water.
We've already seen that,
well into January,
land continues
to lose more heat than it gains.
Because water radiates heat
even more effectively than land,
the oceans take even longer
to start warming up.
So although the land has been
warming since January the 19th,
the sea is still losing heat
and the ice continues to grow.
Greenland sea ice is at its maximum
extent at this time of year, in March.
But over the next few weeks, the tilt of
the Earth towards the sun as it orbits it
will allow the northern hemisphere to
get an increasing amount of solar energy.
The days will get longer and warmer
and the sea ice
will begin to break up and recede.
Then the hunting season
will be over.
The existence of the sea ice
here in Greenland
is testament to the complex response
our planet has to the sun,
in whose orbit we travel.
But it's a very delicate balance
and no-one is more acutely aware of
that than the people who live here.
Gio tells me that, even before the
storm, this year there was less ice
than in previous years.
It's part of a trend
over the whole of the Arctic.
The area covered by sea ice has
shrunk significantly in the last 20 years.
A series of warm winters
have meant that the seas haven't
cooled down as much as normal
so not as much ice
has been able to form.
There's little doubt that the cause
of the warmer winters is us.
Global warming can feel like a myth
when, back in the UK,
we've endured a string
of very cold winters.
But here on the front line,
it's a reality.
Most predictions suggest that the
Arctic will continue to warm rapidly
over the course of this century.
It could be that we may well prove capable
of generating the kind of climate change
that in the past has been created
by changes in the Earth's orbit.
We've now reached the middle of March
and we're approaching
the spring equinox,
the end of our journey for now.
Next time,
we'll complete our voyage...
Wow!
..travelling from the equinox back to
where we started - the summer solstice.
are on the move.
And we don't mean the morning commute
or taking the kids to school,
but a journey of epic proportions.
Even now, as you're watching this,
you're hurtling through space
at 100,000 kilometres an hour.
Every year, our planet,
the Earth, travels around the sun
and we go with it.
I'm Kate Humble.
This is it.
The sun is directly overhead.
My shadow is directly below me.
In this series, we are going
to follow the Earth's voyage
through space for one whole year
to witness the astonishing
consequences this journey has for us all.
I'm Dr Helen Czerski and I study
the physics of the natural world.
Wow, look at that!
SHOUTING: I'll be investigating
how our orbit powers
the most spectacular weather
and how it's also shaped
and reshaped our planet.
But our planet's journey isn't quite
as smooth as you might think
and its orbit changes over time
with significant consequences.
The bottom here is 120 metres down.
And full of sharks.
Wow!
In this episode,
we explore what it means
to live on a planet locked in a
never-ending voyage around the sun.
Join us on the most remarkable
journey of your life.
Since our journey began,
we've travelled almost 500 million kilometres
around the sun to the end of December.
In this episode,
we continue our journey,
travelling from the beginning
of January
to the spring equinox in March.
In the northern hemisphere,
that means we're in winter,
the harshest season.
Whilst in the southern hemisphere,
it's summer,
although it's a little different
to the one in the north.
I'm starting
in the Scottish Highlands
on a particularly significant day
in our journey around the sun.
It's the third of January,
it's minus five...
..and the winds are gusting
to over 60 kilometres an hour.
I'm walking... up Aonach Mor...
..one of the highest mountains
in Scotland.
If it was a beautiful,
clear, sunny day,
you'd be able to see Ben Nevis
over there.
And if I was going
to be very British
and stiff upper lip about this,
I'd say it was a little bit chilly.
But I'm not.
It's absolutely freezing.
So, why, one might ask,
am I going to the effort of climbing
over 1,000 metres
in these conditions?
Well, by being here on Aonach Mor,
I'm about as close to the sun
as I'll ever be
and it's actually not
because of where
but when I'm making this climb.
Today,
we're physically closer to the sun
than on any other day of the year.
It's a day with a special name.
It's called perihelion,
and although it's impossible to
believe in conditions like this,
the Earth is five million kilometres
closer to the sun today
than it will be in July.
At perihelion, being on top
of this mountain on this day,
brings me one more kilometre
closer to the sun.
It may seem strange
that on some days
we can be much closer
to the sun than on others,
but it's the consequence
of a particular feature
of the Earth's orbit.
The Earth's journey through space
is controlled by the sun's gravity.
But it isn't quite
the orbit you might expect.
Now, say that this stone is our sun.
Now, the Earth doesn't orbit the sun
in a perfect circle.
Instead, we go around, on an ellipse.
Not only is the Earth's
orbit elliptical,
the sun isn't in the centre of it.
That means that our distance from the sun
varies continuously throughout the year.
And, today, on our orbit,
January the 3rd, we're there -
closer to the sun than we will be
for the whole of the rest of the year.
The Earth's elliptical orbit
means that in January, at perihelion,
the Earth receives about 7% more
solar energy than it does in July,
when the Earth is at
its furthest point from the sun.
You might think that this extra energy
would mean that January would be warm
and July would be cold.
Well, it turns out that proximity
to the sun doesn't guarantee warmth.
The reason for this apparent anomaly
is that there's a second,
more powerful factor at work.
As it orbits the sun,
the Earth is tilted on its axis
at an angle of just over 23 degrees.
Because of this 23.4 degree tilt,
in January, the northern hemisphere
is pointing away from the sun.
The Earth's tilt reduces the amount of
solar radiation in the northern hemisphere
by up to 50%, far more
than perihelion increases it,
which is why it's winter in Britain,
even though this is when
we're at our closest to the sun.
But perihelion in the southern
hemisphere coincides with summer,
so, in theory, the relative proximity of
the sun and the extra energy this brings
should mean this part of the world
has particularly hot summers.
I've come to Chile
to discover whether this holds true.
This is Puerto Williams. It's not
just the southernmost town in Chile,
it's the southernmost town
in the world.
The next significant land mass
from here is Antarctica.
Puerto Williams
is a good place for us to be
because it's at an equivalent
latitude to the UK.
So we can find out how summers here,
in the southern hemisphere,
compare to ours.
I'm heading into a stretch of water
called the Beagle Channel
that crosses the bottom
of the continent.
It's named after HMS Beagle,
the boat that carried Charles Darwin
here almost 200 years ago.
Truth be told, so far, conditions are not
hugely different to summers back home.
But there is a difference and it's
something you'd never see in the UK.
A glacier.
I am absolutely... blown away
by where we are.
It's just the scale of it
that takes your breath away.
It comes right down into the water,
and, as you can see, there are just
great chunks of ice everywhere you look
that have broken off the glacier.
It's like floating
in a giant gin and tonic.
But look at that!
Ooh, there's ice falling off it now!
And what's so astonishing
about this is its location.
This isn't the only glacier
in this region, not by a long way.
And yet, we're at
55 degrees latitude south.
If you go to the equivalent latitude
in the north, 55 degrees north,
you get to the Lake District
in England.
Now, we all know that
the Lake District is very pretty.
But it hasn't got one of those.
The presence of this glacier is
evidence that, rather than being hotter,
summers in the southern hemisphere
are actually cooler
than in the northern hemisphere.
In fact, on average, they're
a full four degrees Celsius cooler,
despite the added boost that perihelion
gives to the southern hemisphere summer.
So something else is at work here,
counteracting the effects of perihelion.
To discover what it is,
I'm heading back out to sea.
Well, we're now out
in the open ocean
and, I have to say,
if you're not a sailor,
and I'm not...
..it makes you feel very small...
Whoo! ..a little bit scared
and quite sick.
We're sailing
in the Southern Ocean...
..where strong winds and icebergs
have made these waters notorious
as a sailor's graveyard.
This is a very exposed stretch
of water.
To the west is the Pacific Ocean,
whilst to the east is the Atlantic.
To the south, the nearest land mass
is Antarctica.
It's the very vastness
of this expanse of water
that's the reason why summers in
the southern hemisphere are so cool.
If you look at the whole
of the southern hemisphere,
over 80% of it is covered by oceans
and these huge expanses of water
have a powerful effect on the climate.
That's because water
has an important characteristic.
It takes a lot more of the sun's energy
to warm up the sea than it does the land.
In other words,
water has a high heat capacity.
This means that, even in midsummer,
and even with the added warmth
provided by perihelion,
the oceans in the southern
hemisphere are still cool.
And this keeps the air cool too.
Even at this time of year, when the
Earth is physically closest to the sun,
and the southern hemisphere
is tilted towards it,
the influence of the oceans
keep it much cooler.
It's a sobering thought
that without perihelion,
southern hemisphere summers
would be even cooler than they are now.
The Earth's slightly off-centre orbit
is a reminder
that we live on a planet
that's hurtling
through space around the sun.
This journey is controlled by the
immense power of the sun's gravity.
But the sun's gravity is also
responsible for significant dangers.
I've travelled to a place
where you can see these dangers
written into the Earth's surface.
This is the Barringer Crater
in Arizona.
50,000 years ago,
a meteorite struck this site
and excavated this dramatic hole.
That impact spread debris over tens
of thousands of square kilometres.
This crater itself
is more than a kilometre,
or three-quarters of a mile, across,
so as you can imagine,
it was an incredibly violent event.
To get an idea of the force
involved in that impact,
we can look at two types of rock
that you find round here.
Now, this,
this is Coconino sandstone.
This is what was present
before the meteorite hit.
Now, down here, we can see what happened
to this kind of stone after the impact.
So, this rock here,
it's chemically exactly the same,
but what the impact did to it
was just pulverise it. Look at this.
It's just breaking apart
in my fingers.
And the reason for that is that the
shock that went through from this impact
just fractured
all the tiny grains of quartz.
Incredibly, all this was done
by a meteorite just 50 metres across.
There are thousands of objects
circling the sun,
trapped by
its immense gravitational field.
Every now and then,
we collide with one.
But not all of them are as small as the
one that created the Barringer Crater.
Hidden underneath what is today
a place called Chicxulub in Mexico
is a huge crater.
The impact of the Chicxulub meteorite
was cataclysmic.
It blasted so much hot debris
into the atmosphere
that almost the whole planet
caught fire.
The overall impact was so great
it eventually contributed
to the extinction of the dinosaurs.
ROARING
Our orbit regularly takes us into
the path of asteroids and comets.
And it's a sobering thought
that our voyage through space
could deliver a random disaster
to the whole planet.
The good news is that the bigger
the potential disaster, the rarer it is.
But there's another potential danger
that comes from our orbit around the sun.
And the best time to see it
is at this time of year,
in the middle of winter.
The long nights mean that this is the
peak season for an extraordinary spectacle.
For thousands of years, people have
marvelled at the spectacular light displays
that sometimes appear
in the night sky
and they've wondered what on earth
they could possibly mean.
The Vikings believed them to be
the reflections of dead maidens.
The Cree Native Americans called
them the Dance of the Spirits,
and, in Europe in the Middle Ages,
they believed the lights
meant that God was angry.
But the truth
is actually even more extraordinary.
This celestial light show,
or aurora, as it's known,
is the front line in the battle between
the sun and the Earth's atmosphere.
Every second, the sun blasts out a
million tonnes of radioactive particles
and the Earth is in the firing line.
The sun emits a continuous flow
of charged particles,
known as the solar wind.
This streams outward,
in a wash of radiation.
But when it reaches the Earth,
it encounters a barrier.
The Earth's magnetic field
deflects the particles
and funnels them towards the poles.
Here, they collide with atoms of
nitrogen and oxygen in the atmosphere.
These collisions emit energy in the
form of light, giving us the aurora.
From
the International Space Station,
you get a better sense
of the awesome scale of the aurora.
We don't often think of it this way,
but the aurora is graphic evidence that
we live inside the atmosphere of the sun.
This is the sun's atmosphere
colliding with the Earth's atmosphere.
So our orbit, close to the sun,
is full of risk.
But it's also vital
for our survival.
Almost all life on our planet
depends on the energy
we receive from the sun.
Our location close to the sun
provides one critical benefit -
it allows the presence
of liquid water.
If our planet
was much closer to the sun,
it would be too hot
and the water would boil away.
Too far away, and it would freeze.
Our planet is in
what's known as the habitable zone.
The zone where water can exist
and life can flourish.
Earth may be dangerously close
to the sun,
but this is the price
that has to be paid to sustain life.
And our location close to the sun is
even more favourable than it first appears.
Earth orbits the sun at just the
right distance to allow water to exist
in all three states -
as a solid, a liquid and a gas.
And it's switching between those
states all the time.
But water in each of those states
behaves very differently,
and it's those differences that generate
the climate system as we know it on Earth.
It's now the middle of January.
This time of year
gives us a great opportunity
to see two ways
in which water changes state,
with very different consequences.
I'm back in the southern hemisphere,
in the foothills
of the Andes in Argentina.
Here, you can see water
moving between states
and how this process
transforms our planet.
This is the cloud forest of Calilegua,
2,000 metres above sea level.
And, as you can see, clouds
are definitely a feature here.
There's a wonderful thick wisp
of cloud down in the valley there
and then this great bank
over the trees on the horizon.
And then you've got these ghostly
wisps climbing up above the trees.
It really is a magical place.
This is a classic summer's day
in the cloud forest.
It's hot, it's humid...
It's like being in
a giant steam room.
BIRDS CAW
The humidity I'm feeling is because
the heat has evaporated water,
so the air is laden with vapour,
the gaseous form of water.
As the day progresses,
some of this warm, moist air
will change state again.
It's mid-afternoon and it's getting
increasingly hot and steamy.
In fact, if feels like this heat
is about to trigger something
absolutely spectacular.
Throughout the day, the land has
been absorbing more and more heat.
That heat warms up the moist air
and forces it to rise
high into the atmosphere,
forming towering cumulus clouds.
These clouds are the transformation
of water made visible.
The rising water vapour
has cooled and changed state
and become liquid again.
What's incredible is watching this cumulus
cloud growing in front of my very eyes.
It has to be eight,
ten kilometres tall already
and you can almost feel the energy
crackling away inside it.
There's a tremendous sense of
build-up and anticipation in the air.
Powerful updraughts push the cloud
so high, the top spreads out
to form
a characteristic anvil shape.
An approaching storm like this
could last half an hour.
It could last 10 or 12 hours.
Sometimes they even join up with other
storms to create destructive megastorms
that can devastate the whole region.
These tropical storms are an extreme
version of a familiar phenomenon.
Rain.
RAIN SPATTERING
Rain is so familiar
that it's easy to forget
what a critical role
it plays on Earth.
It's the way in which water is transported
from the oceans and deposited over land.
Without the ability of water to change
from liquid to gas, and back again,
the land would be
a dry and barren desert.
Meanwhile, in the northern hemisphere
at this time of year,
a different transformation of water
occurs, from liquid to solid.
I've come to the edge
of Lake Ontario in North America
to see one of the most extreme
examples of this transformation in action.
This area is home to some of
the heaviest snowfalls in the world.
But it's not immediately obvious
why this should be so.
It's so peaceful here.
There's a beautiful blue sky.
It's been a stunning day.
But tomorrow,
from across the lake over there,
there's a huge storm coming our way,
although you'd never know that
to look at it now.
The snowstorm
is likely to be particularly heavy
because of a unique set
of conditions.
The air outside is cold and dry.
It's come straight from the Arctic.
But this frigid air
is about to be transformed.
You can see
what does it right below me.
Water.
Warm water.
Even though it looks
pretty chilly down there,
the water's significantly warmer
than the land around it.
And there's lots of it.
Even though
it's frozen round the edges,
there's plenty of open water
in the middle.
Lake Ontario is one of the Great
Lakes, so it's a huge body of water.
Water's high heat capacity
means it's held onto much of the heat
it absorbed during the summer.
As the cold, dry air passes over
this relatively warm lake,
water evaporates.
As it rises over Upstate New York,
it forms clouds.
Those clouds are the start
of a special type of snowstorm,
which leads to some of the biggest
and fastest accumulations of snow
anywhere in the world.
And it's called
a lake-effect snowstorm.
These snowstorms
are particularly intense
because the cold air can keep on
blowing across the lake for days.
It's like a conveyor belt
of cloud formation.
Within these clouds, the cold air
means that water turns from liquid
to its solid, crystalline state...
..a snowflake.
And they start because there are
tiny grains of dust, way up in the clouds
and the warm lake air provides moisture,
which condenses onto those droplets.
And as they're carried up and up
into the cloud,
the temperature goes down
and so they freeze into a crystal.
And that crystal is a snowflake.
Here, conditions produce
a very particular type of snowflake.
Because the air is so cold, it
produces crystals with sharper tips.
These grow more branches,
called dendrites,
which make the snowflakes fluffier.
It's the kind of snow we all love -
as long as there isn't too much of it!
It's now approaching nightfall
and the snowstorm is almost upon us.
How much snow falls
will depend on one final factor.
The wind direction.
If the wind comes from the north, it
passes over the narrow part of the lake
and so picks up
only a small amount of moisture,
making just a light shower of snow.
But if the wind comes from the west,
it passes over almost
the full length of the lake
and picks up a lot of moisture,
producing much more snow.
At night, the storm finally arrives.
I'm here in the middle
of the snowstorm
and the winds are really strong.
The thing is that powerful winds like this
are exactly what you get up in the clouds
where snowflakes form.
So next time you see
a peaceful snow scene,
remember that all
of those delicate snowflakes
are formed in a violent,
windy environment, just like this.
WIND HOWLS
Next morning, the town beside the
lake wakes up to a heavy coating of snow.
But because it's a regular event,
people here are prepared.
Across the northern hemisphere,
the same interaction of cold land
and relatively warm moisture
produces many other spectacular
weather phenomena.
In January 2005, these
remarkable ice sculptures formed
when spray from Lake Geneva in
Switzerland was thrown up by strong winds
and froze as soon as it landed.
In Canada in 1998,
rain falling on frozen ground
turned to ice as it landed,
a phenomenon known as an ice storm.
It continued for 80 hours.
The sheer weight of ice
crushed over 1,000 steel pylons,
leaving four million people
without electricity.
Closer to home, frost forms
when air saturated with moisture
touches surfaces
that are already frozen.
Our orbit around the sun exposes
our planet to potentially deadly radiation.
But the payoff is a big one...
..a planet where water can be
distributed across the whole Earth,
providing spectacular weather
and making it habitable.
It's now late January and the
northern hemisphere is locked in winter.
And yet there is a paradox
about our winter,
because in January,
winter is still getting colder,
even though the northern hemisphere
is receiving more energy from the sun.
I've come to Northern Canada,
to the best - or perhaps the worst -
place to explore this paradox.
Whoo!
Cor! This...
..is Yellowknife.
It has the dubious distinction
of being the coldest city
in the whole of North America.
Today is January the 19th.
On average, this is the coldest day of
the year across the northern hemisphere.
It's minus 35 degrees Celsius,
which certainly qualifies as cold to me.
It's pretty hard to describe to you
just how it feels to be at minus 35,
but I'm going to give it a go.
When you breathe, it hurts.
It kind of gets you
at the back of the throat.
Your nose feels
like it's permanently frozen solid.
And despite the fact that I've got
the feathers of about 25 geese
stuffed into this jacket,
and more thermal underwear
than I thought possible to wear
at exactly the same time,
I still feel cold.
In these conditions, even familiar
things behave in unfamiliar ways.
You can take a lovely, hot,
steaming cup of coffee,
throw it in the air, and the steam
from that coffee will freeze instantly.
Well, you've got to give it a go,
haven't you?
Right...
Here goes.
Wow!
That is amazing!
Oh, my word!
There's something curious about the way
winter peaks towards the end of January.
The winter solstice falls
on December the 21st
and this marks the day
when the northern hemisphere
receives the least amount
of solar energy from the sun.
So you might expect the December
solstice to be the coldest day of the year.
But it's not.
On average, temperatures
on the 19th of January are colder
than they are in mid-December.
But, you say,
the days are getting longer.
The northern hemisphere
is getting more sun.
It should be warming up.
In Yellowknife, there are people
whose livelihoods depend
on the way winter's peak is delayed.
In the driving seat
is Blair Weatherby.
His family have been driving through
the bitter cold of this region
for three generations.
He's not an ordinary trucker.
He's an ice road trucker.
And this is his highway.
In the summer, what happens here?
We'd be in a boat!
That's because we're not driving
on land, but on a frozen lake.
So really to appreciate
Yellowknife's splendid isolation,
you have to look at a map.
And here it is,
right on Great Slave Lake.
But it's surrounded
by water and tundra.
So at this time of year, of course,
it freezes, and Yellowknife,
and all these tiny, little,
incredibly remote communities
can get linked up by the ice roads.
So what time of year
can you start driving on the lake,
as opposed to boating on the lake?
The season starts
towards the end of January.
It's about 30 inches thick at this point.
It just keeps getting thicker and thicker.
So whilst the northern hemisphere's
coldest day is the 19th of January,
here in Yellowknife, it's still
bitterly cold for many weeks to come.
For the truckers, this delayed
winter means their work season
runs from late January
well into March.
Here, you can go for hours with your
hands off the steering wheel sometimes.
There's lakes that take two and a half
hours to drive across. People watch movies.
You put a DVD player on your dash and watch
a movie when you're going across the ice.
So why is the worst of winter
delayed so long
after the solstice
on December the 21st?
It's all about the balance
between the heat coming in
and the heat going out.
Throughout early winter,
the northern hemisphere
receives declining amounts
of the sun's energy,
so it starts to cool down.
But there's a lag in this cooling,
because the Earth's surface
loses heat relatively slowly.
So well into January, the Earth's
surface is still losing heat,
even though solar energy
is slowly increasing.
It isn't until around the 19th of
January that a tipping point is reached.
From this day onwards,
the increase in solar radiation
will overwhelm the effects
of the heat loss
and the northern hemisphere
will begin to warm up.
But it'll still
be a few more weeks yet
before the ice here is too thin
to support the weight of the trucks.
We've seen how the Earth's journey
through space is critical for life
and how the Earth's angle of tilt
defines our seasons.
But you only really understand just
how important our orbit is for our planet
when you look into the Earth's past.
There's evidence
in the most unexpected places.
A few miles out there is one of the
most spectacular wonders of the world,
but I can't see it from here
because it's underwater.
I'm in Belize in Central America
and what I'm going to see
is known as the Blue Hole.
It's not often
that nature produces something
as beautifully symmetrical as this.
It's almost a perfect circle.
But it's more than just a stunning
piece of natural architecture,
because deep down there are clues
to some of the most dramatic events
in Earth's history.
This wall seems to go down for ever
and I'm told that the bottom here
is 120 metres down,
which sounds like
a very, very long way just now.
And I'm just dropping into the abyss.
That shark just swam
past in front of me.
I've never, ever been
so close to a reef shark.
And there's another two
just behind me.
Finally, I've reached my goal.
So down here at 40 metres...
..it's really eerie.
Gloomy.
And this is what I've come to see.
And they're stalactites.
But there's only one way I know
of for stalactites to form.
And it isn't down here,
in 40 metres of water,
with sharks swimming about nearby.
Stalactites are created when mineral-rich
water drips from the roof of a cave,
over hundreds
or even thousands of years,
leaving behind mineral deposits.
In other words,
they didn't form in the ocean.
Stalactites like this
can only ever form above ground.
And that means that when these grew,
the sea level was much, much lower
than it is today.
Scientists have precisely dated
stalactites from the Blue Hole
and, by comparing these and other
sea level indicators from around the world,
they've built up a picture
of changing sea levels
dating back
hundreds of thousands of years.
It reveals a striking pattern.
Sea levels across the world
have risen and fallen over time.
Genuinely one of the eeriest things
I've ever seen, that.
20,00 years ago, the entire surface
of the world's oceans
was 120 metres below
where it is today.
And that means if I was standing here
20,000 years ago,
all of this, including the Blue Hole
cave system, would be dry land.
So where did the ocean go?
The answer is that it was on land.
But it wasn't liquid water,
it was ice,
because 20,000 years ago, our planet
was in the middle of an ice age.
The Earth has experienced
regular ice ages
in a cycle going back
several million years.
These dramatic changes
to the state of our planet
are triggered by small changes
in the Earth's orbit.
I've travelled back to Britain
to uncover the relationship between
the Earth's orbit and an ice age.
Snowdonia's peaks and valleys
were carved out in the last ice age.
It's in mountainous locations like
this that an ice age would have begun
as snow gradually built up.
When we think of ice ages, we think
of extreme cold during the winter.
But, counterintuitively,
it's summer temperatures which are
important in starting ice ages.
And the reason for that is, now, ice
will build up here during the winter,
but it will all melt away
in the summer.
But if the summer is a little bit
cooler, a layer of ice will be left behind.
And a series of cool summers
will leave layer after layer,
one on top of the other, building up.
And here, the ice could have
been hundreds of metres high.
Ice ages always start
in the northern hemisphere
because there's so much more
land surface on which ice can build up.
So the question is, what causes cooler
summers in the northern hemisphere?
The answer comes from small changes
in the Earth's orbit,
caused by the gravitational pull
of other planets.
Our orbit
isn't exactly the same every time.
Aspects of it change just slightly,
in cycles lasting thousands of years.
And when all of those cycles
reach their most extreme point
all at the same time,
that can change our summer temperatures
just enough to tip us into an ice age.
There are three cycles
to do with the Earth's orbit
that must all coincide
to trigger an ice age.
The first of these cyclical changes
affects the time of year
when perihelion occurs.
This is the day when the Earth
is closest to the sun.
Today, perihelion is in January,
but over thousands of years,
the date of perihelion changes.
When perihelion occurs
in the northern hemisphere summer,
it makes summers particularly hot.
But when it occurs in winter,
as it does today,
then northern hemisphere summers
are cooler.
So at the moment, the perihelion cycle is
at the right point to generate an ice age.
But two other cycles
are not in an ice age phase.
The first is the angle
of the Earth's tilt.
The Earth's tilt is currently at an
angle to the vertical of 23.4 degrees.
But that angle changes
between 22 and 24.5 degrees.
It's only when the angle
is at its shallowest - 22 degrees -
that the seasons become less extreme
and the summers cooler.
Today, the angle of tilt
is too great for an ice age.
The final cycle affecting an ice age
is the shape of the Earth's orbit.
The Earth's orbit is an ellipse,
but over time, it becomes slightly
more, and then slightly less, elliptical.
When the orbit is at its most elliptical,
the result is lower summer temperatures.
At the moment, the Earth
is midway through this cycle,
so again,
it's not in an ice age phase.
It's only when all three of these changes
to the Earth's cycle line up together
that they produce
the really cool summers
in the northern hemisphere
that result in ice ages.
At the moment, those three cycles
are all at different positions,
and so we're still getting
enough sun during the summer
to melt ice
and keep us out of an ice age.
It'll be around 60,000 years
before the cycles line up again
and the next ice age starts.
It's now the beginning of March and
we're nearing the end of our journey.
In most of the northern hemisphere,
spring is on the way.
But there is still one part of
the world that is locked in winter.
Long after January the 19th,
on average the coldest day
in the northern hemisphere,
winter still clings on
in the Arctic.
I've come to Greenland, where there's
definitely not much sign of spring yet.
This is Kulusuk. It's a tiny
settlement of just 355 people
perched on the edge of an island
in eastern Greenland.
To the north of here is the Arctic
Circle and the Greenland ice cap.
Kulusuk is surrounded
by the Arctic Ocean.
At this time of year, it's frozen,
covered in a thick layer of sea ice.
I've come here to find out
about sea ice - how far it extends
and why it hasn't melted, despite
the fact that it's now March,
the days are getting longer and the
amount of solar energy is increasing.
In fact, not only is the sea ice
not melting, it's still expanding.
Each year, the extent
of the sea ice is different.
To see how far it reaches this year,
I need to travel right
to the edge of the sea ice.
But before I can set off,
a massive snowstorm hits Kulusuk.
We can't go anywhere.
The 140-kilometre-an-hour winds
make a trip to the local shop
a major expedition.
'By the next morning,
the storm has passed.
'I meet up with my guide,
local hunter Gio Utuaq.
'His hunting grounds lie
right at the edge of the sea ice.
'I'm hitching a lift on the only form
of transport that can get us there.'
She's so keen!
How far do we have to go
to get to the hunting grounds?
20, maybe 25 kilometres.
After two hours,
we reach a huge expanse of sea ice.
It's impossible to comprehend that the
snow we're travelling across sits on ice,
which sits on the ocean.
We're travelling across
a frozen sea. And look at this!
This is an iceberg
actually trapped within the sea ice.
It's the most astonishing landscape,
or seascape or ice-scape...
What do you call it?!
..that I've ever seen!
It's like another world.
And then, surprisingly quickly,
the edge of the ice comes into view
and I can see the Arctic Ocean.
Gio tells me that only days ago,
the ice extended out
for several more kilometres,
but it seems the storm
has broken it up.
For obvious reasons, we make the
last stretch of the journey on foot.
Using his traditional spiked tool,
Gio checks the thickness
of the ice at the edge.
Are you sure?
SHE CHUCKLES
There is something
very disconcerting
about walking on sea ice
when the open sea is so close.
Is it safe? No problem? No problem.
Are you sure?
Yeah, it looks pretty solid.
How thick is the ice?
Like this thick?
Oh, you can see it.
Actually, you can, you can see
it's like a great cliff of ice
that goes right down into the water.
It seems strange to be walking
across a frozen sea here in Greenland
when back at home, the daffodils
are beginning to come up.
But what's even stranger
is that measurements of the sea ice
over the last 50 years
show that it only reaches
its full extent now, in early March.
So clearly there's a lag between
the arrival of the warmth of the sun
and the melting of the ice. But why?
It comes down to
the properties of water.
We've already seen that,
well into January,
land continues
to lose more heat than it gains.
Because water radiates heat
even more effectively than land,
the oceans take even longer
to start warming up.
So although the land has been
warming since January the 19th,
the sea is still losing heat
and the ice continues to grow.
Greenland sea ice is at its maximum
extent at this time of year, in March.
But over the next few weeks, the tilt of
the Earth towards the sun as it orbits it
will allow the northern hemisphere to
get an increasing amount of solar energy.
The days will get longer and warmer
and the sea ice
will begin to break up and recede.
Then the hunting season
will be over.
The existence of the sea ice
here in Greenland
is testament to the complex response
our planet has to the sun,
in whose orbit we travel.
But it's a very delicate balance
and no-one is more acutely aware of
that than the people who live here.
Gio tells me that, even before the
storm, this year there was less ice
than in previous years.
It's part of a trend
over the whole of the Arctic.
The area covered by sea ice has
shrunk significantly in the last 20 years.
A series of warm winters
have meant that the seas haven't
cooled down as much as normal
so not as much ice
has been able to form.
There's little doubt that the cause
of the warmer winters is us.
Global warming can feel like a myth
when, back in the UK,
we've endured a string
of very cold winters.
But here on the front line,
it's a reality.
Most predictions suggest that the
Arctic will continue to warm rapidly
over the course of this century.
It could be that we may well prove capable
of generating the kind of climate change
that in the past has been created
by changes in the Earth's orbit.
We've now reached the middle of March
and we're approaching
the spring equinox,
the end of our journey for now.
Next time,
we'll complete our voyage...
Wow!
..travelling from the equinox back to
where we started - the summer solstice.