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We're all on an amazing journey.
A 940 million kilometre voyage through space.
Even though we can't feel it,
we're travelling at over 100,000 kilometres an hour,
circling a star we call the Sun.
Every year, our planet, the Earth,
travels around the Sun - and we go with it.
We're looking at the huge impact this journey has on our world.
Wow, look at that!
We'll see how the Earth's tilt gives us our seasons
and leads to monsoon rains.
How the planet's spin stirs the atmosphere
into giant, rotating hurricanes.
And how small changes in the Earth's movement
can cover the planet with ice.
Our yearly orbit around the Sun has created
and shaped the world we live in.
We start our circumnavigation of the Sun
at a very special place,
and on a very special day.
It's March the 20th, the spring equinox.
This is the great pyramid in Chichen Itza -
an ancient Mayan city in Mexico.
Built 1,500 years ago,
the city is one of the world's great archaeological sites.
And it contains a remarkable insight
into our journey through space.
The ancient Maya had developed a deep understanding
of the Earth's movement around the sun,
and they built it into the very fabric of this city.
But it's something that can only be seen at two very precise
and magical times of the year.
One of those is today, March the 20th.
As afternoon approaches, the city fills
with followers of Mayan beliefs...
..and those curious to see an ancient wonder.
There is a unique and particular feature of our planet
as it orbits the sun,
and it's encoded in the way that light and stone
interact at the great pyramid.
This is the moment that all these thousands of people
have been waiting for, they've all stood up
and there are hands raised to welcome in the sun,
and it's now aligned perfectly on the edge of the steps here,
creating this very specific pattern of light and shade which resembles
the body of a snake. And that's no coincidence
because it joins up with the carved snake's head
at the bottom of the pyramid.
The Maya believed the snake, known as Kukulcan,
was a messenger between gods and man.
This is a remarkable display of Mayan architectural design.
The appearance of this snake isn't an accident,
they absolutely planned it
and it happens on the same day every year.
This is the spring equinox.
So, more than 1000 years ago,
the Maya recognised the equinox as a pivotal moment in the year.
Here on Earth, there are a few moments that we all share,
because we're all on the same journey around the Sun.
And one of those moments is the equinox,
when day and night are equal.
'It's a time of balance we can all experience,
'wherever we are on the planet.'
So whether you are here in Britain,
amongst the fitful showers and overcast skies,
'or in the bright spring sunshine of Mexico,'
on the March equinox
you'll get 12 hours of daylight and 12 hours of night time.
That's if the sun ever comes through the clouds!
But it's more than just a time of balance.
It's also a turning point in our year.
From the March equinox onwards,
the days get longer in the northern hemisphere,
'while in the southern hemisphere, the opposite occurs.
'This is because of a special feature of our planet
'as it journeys through space.'
Let's say this rock is the sun.
This is going to be our Earth,
and as the Earth travels around its orbit
spinning like this, it travels around on a flat plane.
So you would think that its axis would point upwards
but it isn't, it's tilted over at 23.4 degrees.
'This means that the North Pole, the stem of the apple,
'isn't vertical, it's at an angle.'
And that tilt stays pointing in the same direction
as the Earth travels around on its orbit.
Because of this tilt for part of our orbit,
the hemisphere north of the equator leans towards the sun.
This brings with it extra solar energy,
which fuels spring and then summer.
Six months later, the situation is reversed.
The southern hemisphere now leans towards the sun,
while the northern hemisphere experiences declining energy,
ushering in winter.
Tilt creates the Earth's seasons.
But there's a moment, twice a year as we orbit,
when the sun favours neither hemisphere.
At this point, both experience 12 hours of daylight and night time.
This is the equinox.
If the Earth wasn't tilted,
every day would be like the equinox,
with the 24 hours equally split between day and night.
And that would mean no seasons.
But the Earth's tilt means as we travel from the equinox,
seasonal changes do occur.
On the equinox,
the Sun's energy is felt most strongly directly on the Equator.
As we pass through spring,
this focused energy moves slowly northward.
All over the northern hemisphere, this solar shift means the Sun
arcs higher in the sky, and the hours of daylight increase.
The Earth orbits the Sun not standing up straight,
but tilted over at an angle of 23.4 degrees.
Our 23 degree tilt is just right.
It's enough to provide a relatively benign seasonal shift.
It makes our planet habitable.
However, it may have been the result of a cosmic accident,
and here in America, we can gain an insight into what happened.
This is the Barringer Crater in Arizona.
50,000 years ago, a meteorite struck this site
and just look what it left behind - this enormous hole in the ground.
'This impact would have thrown debris out
'over tens of thousands of square kilometres.'
And all the rock around here, like this,
is what's left after that explosive event.
This enormous crater is like a lesson in how size isn't everything,
because the crater itself is a kilometre across,
but the thing that caused it was only about 50 metres in diameter,
which is really quite small.
And the reason that such a small thing could cause such a big hole
is because it was travelling so fast.
'Impacts like these are extremely rare,
'but in the Earth's past, they were far more common
'and a lot bigger.'
Around four and a half billion years ago,
the solar system was still in the process of formation.
The Earth was just one of many of protoplanets that orbited the sun.
Amongst these protoplanets
was a small Mars-sized planet that's been named Theia.
Its orbit put it on a collision course with the Earth.
Theia smashed into the larger Earth and was obliterated.
The impact very nearly destroyed our planet too.
The collision knocked the planet over,
tilting the Earth's axis of rotation.
This tilted Earth might still be oscillating madly,
were it not for another consequence of Theia's impact.
A huge amount of debris was blasted into space.
Gradually, this debris coalesced,
captured by the Earth's gravity...
..and it formed the moon.
Billions of years later, the gravity of the sun and the moon together
act as a sort of counterweight, stabilising our tilt.
It's extraordinary to think that the moon is both evidence
of what caused Earth's 23 degree tilt
and the celestial object that helps maintain it.
Without this stabilising effect, the planet would wobble in space.
There would be no seasons, and the weather would be chaotic.
Spring triggers a seasonal transformation on land.
But the rising temperatures also transform our weather,
in some places with dramatic effect.
A tornado is the most volatile of these seasonal weather events.
They occur most frequently in the spring
and especially in the Midwest of America -
a region known as Tornado Alley.
MAN: 'Did you see that? The whole house came apart!
'Oh, my God!'
But despite its violence,
at the core of a tornado is a very simple process.
This goes on like a backpack.
'To experience it, I'm taking to the air,
'over the Midwestern state of Colorado.'
One, two, three, go. Run!
Paragliding pilots like Honza Rejmanek,
love this time of year.
Spring provides the perfect conditions for soaring...
..because the increasing temperatures generate thermals.
So right now we are in a thermal.
These are basically almost like invisible smokestacks of rising air.
Right now we've found one, I'm going to take a turn in it
and circle around and try to gain height.
'Thermals form when the sun warms the ground,
'and the ground, in turn, warms the air above it.'
What I'm experiencing
is one of the most fundamental principles of atmospheric physics -
warmer air rises.
'When air warms, it expands and becomes less dense.
'So this air has a lower atmospheric pressure
'than the cooler air that surrounds it.'
So it floats upwards, forming this rising thermal column.
The atmosphere tries to even out differences
in air temperature and pressure,
attempting to return to equilibrium.
So the rising thermal will mix with the cooler air above.
This basic process of moving towards equilibrium
lies at the heart of every significant weather event
on the planet.
'But in the springtime air over Tornado Alley,
'particularly powerful storms can develop.
'This is due to the unusual conditions here
'that intensify this basic atmospheric process.'
There's a stable layer of dry air that acts as a barrier
between the warm air down below and the cooler air higher up.
So the warm air is trapped,
and what's more, the ground keeps heating it as the day goes on.
The thermals get more and more powerful until, by late afternoon,
they finally punch through the barrier layer at colossal speed.
These rapid updraughts of less dense, lower pressure air
are so strong that they generate huge thunderstorms.
It's from these thunderstorms that, in certain conditions,
tornadoes can emerge.
'I'm going to investigate how this happens...'
Not as bad as north of us.
...with the help of atmospheric scientist, Josh Wurman.
I don't know what to make of these stringy little features.
The first step in our quest for a tornado
is locating a promising storm.
After a couple of days on the road, we manage to intercept
one moving north through Colorado.
So what's happening behind me is the storm is building
and in the middle of that storm over there,
there's an updraught with low pressure at the centre of it.
And all the air around the outside has higher pressure,
and that high pressure is pushing air into the centre
and up into the storm, and that's what building the storm.
The atmosphere tries to even out
the extreme differences in temperature
that have been generated.
So the air movements at the core of the storm
become exceptionally powerful.
'Hail is one characteristic product of this atmospheric violence.'
'The hail formed when an updraught cooled rapidly,
'so that water condensed out of the air, and turned immediately to ice.'
SHOUTING: This is what was carried from the south,
and it was pushed up into the storm
and it gave the storm its energy.
And now it's falling back down on me!
CAMERAMAN: That's it. Let's get inside. This is too hard now.
And even though this is chaotic and messy,
what this is, is a demonstration that the atmosphere is an unstable place,
and there are all these differences in temperatures and pressures.
And this is what happens when the atmosphere moves around
to even everything out, and make it all the same.
When tornadoes do form, they are often preceded by hail.
But this time, there's no twister.
So we're back on the road,
still trying to see a storm spawn a tornado.
Josh's specialist radar detects one
which shows a revealing swirl of clouds.
JOSH: Going out ahead, this big dark area's the core.
So we're basically going to penetrate through the core
and see what's interesting.
Tornadoes form when powerful rotating cylinders of air
within the storm
get caught by an updraught and are knocked on their side
by a powerful atmospheric wind called the jet stream.
Right now, we're kind of in the centre of the coiled part.
When that column of rotating air touches the ground,
a tornado is born.
At the tornado's core is an area of intense low pressure,
which draws high pressure air towards it.
The dust and debris picked up by the tornado
reveal the swirling pattern of winds.
Just 15 minutes after it first touched down, the tornado dissipates.
There's still so much that we don't understand about storms.
We don't understand when they're going to produce hail,
when they're going to produce rain,
when they are going to produce tornadoes.
But what we do understand is that a storm like this
is a manifestation of something happening round us all the time.
Our planet's atmosphere is a mosaic of warmer and cooler air masses,
constantly in motion.
The air is rising, falling and swirling around
as it seeks to balance differences in temperature and pressure.
During April and May,
the effect of the Earth's tilt is to enhance those differences.
So all over the northern hemisphere,
spring is the season for volatile storms.
Tornadoes are only one consequence.
The heavy and sudden downpours from storms can result in flash floods,
like the one that hit the town of Barranquilla in Colombia
in May 2011.
These occur when the rain inundates densely saturated ground.
The water isn't fully absorbed, but instead flows rapidly downhill
in a near-instantaneous torrent.
Thunderstorms can also give birth to an unexpected phenomenon...
..massive dust storms called haboobs.
This one blew into Phoenix, Arizona in 2011.
Haboobs are produced in normally arid regions,
when the leading edge of a storm collapses,
generating a super-fast downdraught
that kicks up a wall of dust and sand in front of it.
As May turns to June, more solar energy
is reaching the northern hemisphere,
and it drives the biggest single weather event on the planet.
An event centred on the Indian subcontinent.
CAR HORNS TOOT
This is the city of Udaipur in Rajasthan.
It's in the northwestern corner of India.
Since March, temperatures here
have been steadily rising as the Earth's tilt
has warmed the northern hemisphere.
But by June, everything is on the brink of an exhilarating change.
I'm here at the time of an epic weather event of huge importance
not just to Rajasthan but to the whole subcontinent
and the over billion people who live here.
'There's a wonderful place to appreciate the event's significance,
'on one of the hills that overlook the city,
'here, at this cliff-top palace.'
It was built at the end of the 19th century
by the 72nd Maharana of Udaipur
and it's known as Sajjan Garh.
He built this palace to get a pure, unadulterated view of the sky
and the clouds that start to build at this time of year.
Sajjan Garh is the monsoon palace.
When the rains do eventually arrive,
they'll be an essential relief from the heat of the Indian summer.
But what's intriguing is that the monsoon is actually a consequence
of the rising seasonal temperatures that precede it.
To reveal why this is, we need to travel 2,000 kilometres...
I'm in the coastal state where the monsoon first arrives in India -
The key to understanding the monsoon is here, on the beach.
The monsoon is powered by a simple,
but incredibly significant difference -
the difference between land and sea.
And in particular, the differing ways in which they respond to the sun.
Take this sand as an example.
The sun's energy is heating all of this surface,
but if I dig down just a little way...
..the sand underneath is quite cool, and that's quite familiar,
we see that on sunny beaches all the time.
And here, where it gets really hot,
the surface can reach 40 degrees Celsius.
Just 15 centimetres down into the sand,
it can be only 7 degrees Celsius.
So, all the sun's energy is going into a really thin surface layer,
and that layer heats up really, really, quickly.
The sun is also beating down on the ocean,
and that responds very, very differently.
This water is much warmer than the sea at home
but it's much cooler than the beach,
and the reason for that
is that the ocean takes much more of the sun's energy to heat it up.
So a kilogram of water will take three times as much energy
as a kilogram of sand to heat by one degree.
The ocean is also relatively cool because to heat the surface
you have to heat much more than just a thin layer.
What happens is that winds that blow across the surface of the ocean
generate turbulence which mixes that top layer.
So as soon as some water's been heated at the top,
it gets mixed down below.
'This means that, unlike the land, the ocean warms up only very slowly,
'as the sun's energy is absorbed.
'So as we enter summer, the land heats up quickly,
'while the ocean lags further and further behind.'
This increasing temperature difference is critical,
because both land and sea heat the air above them.
As the sun has baked the Indian subcontinent,
the land has warmed the air above it.
The warmer air is less dense, so it rises.
This draws in the cooler air from the ocean.
Because of India's particular geography,
this process is magnified.
It's a triangular peninsula, with wide, hot plains
and, crucially, a very long coastline.
This combination sets up a powerful
and sustained movement of cooler ocean air -
the monsoon wind.
Of course when most of us think of a monsoon
we think not of seasonal winds, but of rain.
'By setting up a time-lapse camera,
'I'm hoping to watch the rain clouds forming.'
There is an enormous process on the go here.
When the sun shines down on the ocean surface,
some of the water at the surface will evaporate,
so water and energy are carried up into the atmosphere.
And as the monsoon winds come inland
and they carry that water vapour with them,
the heated land makes that moist air rise,
goes up into the clouds and there droplets condense -
the water condenses out, becomes visible, we see clouds.
When those droplets join together to form droplets which are large enough,
we get rain like this.
And it's really raining hard now!
None of this would be happening if it wasn't for the Earth's tilt.
It's the seasonal heating is what widens the gap in temperature
between the land and the sea, and this drives everything.
And this massive system of rain and wind rushes inland
and that's the monsoon.
80% of all India's rains arrive in this seasonal deluge.
It's not just the volume of the monsoon rains which is impressive.
It's the distance they travel.
As summer progresses in India,
the difference in temperature between land and ocean actually increases.
This makes the whole monsoon system more powerful,
drawing this moisture-laden air further and further inland.
From when the monsoon first arrives on the Kerala coast
around June the 1st,
it spreads more than 2,000 kilometres
until it eventually reaches the far north of the country.
It's now early September.
Although the summer is almost at an end,
in the northern hemisphere, it has a sting in its tail.
Because this is hurricane season.
The development of a hurricane is a wonderful example
of how the Earth's spin controls the weather.
I'm hoping to see one in action.
Tropical Storm Nate. Now, that one looks like it's got potential.
It's trapped in the Gulf, due to grow into a hurricane by tomorrow
and it looks as though it's almost certain to get to the Mexican coast.
24 hours later, I'm in eastern Mexico,
heading towards the Gulf of Mexico and the oncoming storm.
The winds are building up and the normal sunny skies
are replaced with cloud and rain.
At this time of year, the Gulf of Mexico
has the perfect ingredients to make a hurricane.
The sea is relatively shallow and close to the equator,
so the water gets particularly hot.
This water is warm, really warm
and the reason for that is that the ocean out there
has been absorbing the sun's energy, storing it up.
And now, it's that energy which can build tropical storms.
The way the storm is built is that the warm ocean
heats the air above it.
And once the air is warm, it expands and rises.
As the warm air rises, the pressure drops,
sucking in even more moist air, creating powerful winds.
But there's one final ingredient needed to create a hurricane.
It needs to start turning.
And that rotation comes from the spin of the Earth
through a phenomenon known as the Coriolis effect.
Now, let's say this is our planet, the northern hemisphere
and that's the North Pole.
Now, this planet isn't spinning,
so when I throw a ball in a straight line...it travels in a straight line.
But we live on a rotating world.
So, let's take our planet and make it spin,
round anticlockwise, like in the northern hemisphere.
So, now I'm on a spinning planet, things look quite different.
When I try and throw a ball in a straight line,
it bends around to the right.
From my point of view, this ball is always curving to the right,
even though I'm trying really hard to throw it in a straight line.
Now, the reason that this matters
is that this ball represents winds on Earth
and when the wind blows in the northern hemisphere,
the wind is also moved to the right.
In the southern hemisphere, the effect is reversed
and the winds bend to the left.
And that is all the Coriolis effect is.
A hurricane shows the Coriolis effect in action.
Winds are drawn inwards towards the low pressure
at the centre of the hurricane.
But as they head towards the centre,
the Coriolis effect makes them turn to the right.
This creates the hurricane's characteristic
circular swirl of wind.
It also means that the wind never reaches the centre of the storm.
So the eye of the hurricane remains calm.
Out at sea, Nate has the characteristic rotating,
swirling clouds of a hurricane.
but frustratingly, Nate begins to lose power.
Before it can make landfall, the winds die away.
Instead, the 2011 hurricane season
became famous for a different storm.
Unusually for a hurricane,
it travelled far enough up the east coast of the USA
to flood parts of New York city.
It caused billions of dollars worth of damage.
And all this because our planet spins.
In January, 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.
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.
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?
That is amazing!
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.
At this time of year, of course, it freezes.
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.
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.
The bottom here is 120 metres down,
and I'm just dropping into the abyss.
Finally, I've reached my goal.
So down here at 40 metres...
..it's really eerie.
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.
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.
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.
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.
It'll be around 60,000 years before the cycles line up again
and the next ice age starts.
In our journey around the sun, it's now the beginning of March
and the shackles of winter are being loosened in Britain
as we move into spring.
The land starts greening as longer days bring more energy from the sun.
But in some parts of the world,
the effect of that warming has yet to be felt.
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.
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.
-Want me to bring this?
'I meet up with my guide, local hunter Gio Utuaq.
'His hunting grounds lie right at the edge of the sea ice.
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.
For obvious reasons, we make the last stretch of the journey on foot.
-Are you sure?
There is something
about walking on sea ice
when the open sea is so close.
Yeah, it looks pretty solid. How thick is the ice?
Like this thick?
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 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.
And there are many scientists who argue
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.
Subtitles by Red Bee Media Ltd
Right now you're hurtling around the sun at 64,000 miles an hour (100,000 kms an hour). In the next year you'll travel 584 million miles, to end up back where you started.
Presenters Kate Humble and Dr Helen Czerski follow the Earth's voyage around the sun for one complete orbit, to witness the astonishing consequences this journey has for us all.
They see how the planet's tilt is the result of a huge cosmic collision, and how this tilt gives us the single largest weather event on Earth - the monsoon. Helen tries to put herself in the path of a hurricane, a giant rotating storm that only exists because the planet spins. They also discover that small changes in the planet's movement can give us ice ages. It has happened many times in the past, and it is going to happen again.