Orbit: Earth's Extraordinary Journey (2012) s01e02 Episode Script
Episode 2
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 .
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and the winds are gusting to over 60 kilometres an hour.
I'm walkingup Aonach Mor 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 absolutelyblown 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 .
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it makes you feel very small Whoo! .
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a little bit scared and quite sick.
We're sailing in the Southern Ocean .
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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 .
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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 .
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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 .
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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 .
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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?! .
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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.
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 .
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and the winds are gusting to over 60 kilometres an hour.
I'm walkingup Aonach Mor 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 absolutelyblown 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?! .
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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! .
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travelling from the equinox back to where we started - the summer solstice.