Original subtitles

At this moment, you're speeding around the Sun at 67,000 miles an hour.

In the next year, you will travel 584 million miles, to end up exactly where you started.

Three things: the orbit, spin and tilt of the Earth...

...have created and continue to shape the planet, and us, every minute of each day of every year.

Since the beginning, the Earth has made this trip around the Sun over four billion times.

The orbital path around the Sun is the ultimate Earth environment.

It's a dangerous place, bathed in solar radiation, deathly cold and crossed by asteroids and comets.

But despite the dangers, the orbit of the Earth is in just the right place.

Just right for life.

For most of human history, the idea of Earth orbiting anything at all was a bizarre concept.

The Earth, it was thought, sat at the centre of everything,

with everything else revolving around it.

Discovering that the Earth actually orbits the sun, spinning as it goes, solved some mysteries.

Day and night and the year suddenly made a lot more sense.

Today we know a lot more about the planet's annual trip around the Sun.

We've discovered that the orbital journey influences almost everything on the Earth...

...in ways that our ancestors could never have imagined.

The ancient Egyptians knew there was a special relationship between the Earth and the Sun,

and they got some of the science right -

but not everything.

Part shrine to the Sun god Amun Ra, and part stone calendar,

the architects built it specifically with December 21st in mind.

This day is the winter solstice.

It's the shortest day of the year in the northern hemisphere.

The people who built this temple didn't know why it was the shortest day.

But they did know that it happened on the same day every year.

So the temple was designed to line up perfectly with the Sun on this one special morning.

We know now that the regularity of this phenomenon is not the gift of the gods,

but because of the Earth's orbit.

Our planet will be here in the same place at the same time every year.

Planet Earth was born over four billion years ago,

from the planet-forming cloud of debris orbiting the new-born Sun.

It was a violent birth, for both the planet and its orbit.

As it grew, the young Earth was battered by other debris orbiting the Sun.

It even collided with a smaller planet which knocked Earth over,

leaving it spinning on an axis 23 degrees off the vertical.

Today, Earth's epic annual trip round the Sun may seem serene.

In reality it's still fraught with danger.

Though our planet's course is constant, the swirling soup of space is ever-changing.

Its path is littered with asteroids, meteorites and comets that could spell disaster.

This hole in the ground is Barringer crater in Arizona.

NASA impact specialist Dan Durda has come here to see what happens...

...when our planet comes into contact with something else.

This is what happens when we cross orbits with another object in the solar system.

About 50,000 years ago,

a relatively large meteorite landed on Earth, about maybe 40-45 metres across or so.

Slammed out of the sky at 17 or 20 kilometres a second.

The explosive energy that excavated this crater five or ten megatons,

something like that, is -

it's a little less than a thousand Hiroshima's worth of explosive energy.

So it's - it's far larger than anything human beings have ever witnessed.

You could put the Washington Monument down there in the bottom...

...and the top of it wouldn't quite make it to the - to the height of the rim of the crater.

So when - when objects like that hit the Earth they unleash a lot of energy.

The crater is more than three quarters of a mile across, and is considered a small one.

To get an idea of the force that created it, you only have to look at the rock it left behind.

So here I've got a - pretty much a pretty pristine piece of the Coconino sandstone,

a piece of rock from the basement of the crater.

In my other hand, I'm holding effectively the same stone, the same rock,

but this material has been pulverised,

crushed by the immense pressures produced in the impact even itself.

This stuff here is - is crushed to the point where I can break it apart in my hand,

and it gives you an idea of the damage done to even solid rock by an impact like this.

Scientists have calculated that the temperature in this crater reached 6,000 degrees Celsius,

hotter than the surface of the Sun.

Barringer crater is a little over a kilometre wide.

Incredibly, all this was done by an asteroid just 45 metres long.

Here's a nice piece of ejecta, a few meters across.

It came to rest here on the - on the rim of the crater,

but there are rocks just like it that were hurled for -

for miles in every direction, landing across the desert.

I can see block out there the size of small buildings.

And those weren't the only ones.

There were ones deposited out in the desert.

You can see the remains of them eroding today.

So it's - it's an immense amount of energy...

that can hurl rocks the size of buildings across the desert for miles in every direction.

Incredibly, all this was done by an asteroid just 45 metres across.

But not all are as small as the one that created Barringer crater.

65 million years ago, an asteroid six kilometres long...

...hit shallow seas just off the coast of the Yucatan peninsular in Mexico.

So put ourselves near the point of impact 65 million years ago.

It's just a few seconds to impact.

The asteroid is glowing far brighter then the sun in the sky.

Disappears over the horizon.

And then we would have seen the immense flash of the impact itself.

Followed 10 or 20 seconds later.

Boom! We're blasted flat by this shock wave of the blast from the impact itself,

which was strong enough to have actually laid flat forests for,

you know, 800 miles or so in every direction.

This is an immense impact.

It made a crater well over 180 kilometres wide,

and blasted so much hot debris into the atmosphere that almost the whole planet caught fire.

The impact was so catastrophic, it helped wipe out the dinosaurs.

Earth's orbit regularly takes it into the path of asteroids and comets.

But Earth has a secret weapon to defend itself.

It has an atmosphere.

Most objects along the Earth's orbit are small.

The atmosphere is very effective at burning up anything under 35 metres wide.

When we see shooting stars in the night sky,

they are actually small asteroids or bits of comet burning up as they hit the atmosphere.

Still, over the ages many large objects have struck the Earth - and will again.

Asteroids and comets are obvious, if occasional, dangers along the orbital path of the Earth.

But the Sun itself poses the greatest danger.

In the northern sky during winter you can sometimes see the Earth protecting itself.

For thousands of years, people have marvelled at the spectacular light displays...

...that sometimes appear in the night sky.

They've wondered what on Earth they could possibly mean.

The Vikings believed them to be the reflections of dead maidens.

Native Americans called them 'the dance of the spirit.'

What we see as the northern lights and the southern lights,

the aurora borealis and australis, are manifestations here on the Earth of living...

...within kind of the outer extension of the Sun's atmosphere.

Every second, the Earth flies through millions of tons of radioactive particles...

...blasted out from the Sun's surface.

The auroras are caused by those particles smashing into our atmosphere.

The Sun emits a continuous flow of charged particles known as the 'solar wind'.

This streams outwards in a blizzard of radiation.

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.

You're watching the Sun's atmosphere colliding with Earth's.

Without our protective atmosphere and magnetic field,

we'd be living on a much more harsh and alien and inhospitable planet.

We'd - we'd look like maybe the surface of Mercury,

baked by UV radiation and x-rays and blasted by solar flares.

So we live in this lovely zone with our protective atmosphere and magnetic field.

Earth's orbit is full of risk, not just from asteroids, but also from the Sun's harmful radiation.

But for all the danger from the Sun, being close to it provides us with one critical benefit.

Our planet enjoys a precise and very narrow band of temperatures...

that means water can exist in all three of its states.

As a solid, as a liquid, and as gas.

Each of those states behaves very differently.

And it's those differences that drives the climate system on Earth.

This is the cloud forest of Calilegua in northern Argentina.

Sitting in the foothills of the Andes, it's a ghostly place.

The trees and hills are shrouded in clouds.

The heat of summer has evaporated water,

so the air is laden with vapour, the gaseous form of water.

Dr Kristen Rasmussen has come hereto see...

...how this water vapour creates some of the most dynamic storms on the planet.

The exciting part about these storms is we've identified this region as...

...being a hot spot of very, very intense thunderstorms.

The thunderstorms in this part of the world bring a lot of hail, lightning, tornadoes, flooding,

a lot of things that can be very hazardous to the local population.

Throughout the day the land has absorbed more and more heat from the Sun.

It's approximately three o'clock right now.

The - the land surface has been heating all day, since the sun rose.

And all of that warm buoyant energy from the surface is now rising very quickly.

You kind of see a bubbling effect of that:

the cloud parcel rising up and condensing water as it forms the cloud.

As heat evaporates water and forces the wet vapour high into the atmosphere,

there it hits colder air and condenses back into liquid water,

forming towering cumulus clouds.

The huge mat of clouds pushes up into the atmosphere.

These storms are growing very, very quickly.

You can actually sit here for about five minutes and watch almost the whole thunderstorm develop.

The updrafts keep the storm growing up to ten miles high,

and at this point high winds spread the clouds out sideways to form the anvil shaped thunderhead.

By early evening, the storm has grown to epic proportions,

enveloping the entire mountain range of Calilegra.

For Dr Rasmussen it's a chance to experience one of the great forces of nature at first hand.

A phenomenon that can only happen...

...because Earth's orbit means its temperature is rising for liquid water.

This is actually the cloud base where the storm - where the storm initiates.

You cans see some lightning off here in the distance.

This means that there's ice the clouds.

They're very, very tall, they're very intense,

and they have the capability of producing large amounts of rain, and hail as well.

The extraordinary fact, the Earth orbits the Sun at the just the right distance,

gives rise to the very ordinary phenomenon of rain.

Water evaporates from the surface as a gas, condenses into a liquid, as it rises,

and eventually falls back to earth, where the cycle starts again.

It's this very cycle, unique to the Earth in our solar system,

which supports life in all corners of the planet.

This is incredible.

This is the thing I've come here for.

It's - I study these storms all clay long every day, and it's just amazing to see them in person.

If Earth's orbit around the sun were just a little different,

temperatures on our planet might be either too cold or too hot for water as a liquid and a gas.

Life as we know it, and the weather we enjoy, simply couldn't happen.

But within the narrow band of temperatures our planet experiences,

there is room for water's third state.

The edge of Lake Ontario sees...

...one of the most extreme demonstrations of liquid water becoming a solid.

This area is home to some of the heaviest snowfalls in the world.

Professor Scott Steiger has lived and worked here all his life.

He studies how water vapour can be transformed into lake effect snow.

The key thing for the lake effect is...

...the temperature difference between the lake and the air going above it.

So you want really cold air, you know, maybe 10-15 degrees Celsius...

...colder than the lake temperature about a mile above the ground.

And that cold air over the lake makes it very unstable,

and allows a lot of air to start rising,

and it's the rising motion that produces snow clouds.

And the second thing is the lake is a constant moisture source.

You have moisture fluxing off the lake and it keeps feeding the clouds.

Lake effect snowstorms dominate weather conditions here throughout the winter,

depositing huge amounts of snow in a short space of time.

In 2007, an incredible 77 inches of snow fell in just one day.

The biggest difference between lake effect snow and a normal snowstorm is that it's very local.

It's a very local snowstorm.

The band is maybe 10 miles wide, extends about maybe 5O miles inland.

So basically you think about - about it as a rectangle,

and just in that rectangle it can be snowing very heavy,

up to two, three, four, sometimes five-six inches an hour.

And then outside that band it can be sunny.

Can be completely sunny.

To forecast exactly where these narrow storms will hit,

Professor Steiger's team use sophisticated computer models.

To refine these models,

they need to keep taking data directly from lake effect storms as they develop.

The models have been indicating the lake effect snow band is going to form...

...over central Lake Ontario, going on the eastern part of the lake.

To make an accurate forecast, Professor Steiger needs to know two things:

how much snow a storm can produce, and where that snow will fall.

He's tracking the course of a storm as it heads towards upper New York State.

There are only five hours until nightfall.

The leading front of the snow is just reaching the south-eastern shores of the lake.

The first step is to put instruments directly into the clouds,

to analyse the conditions in which the snowflakes are growing.

As the snow starts to intensify, Scott and the team send up a weather balloon.

It will record the direction of the storm and the conditions inside the developing snow cloud.

Hopefully what this is going to measure is...

...the temperature, humidity and wind at multiple levels throughout the atmosphere,

up to maybe 10 miles above the ground.

Let her go.

The huge amount of water vapour in the cloud turns from a gas to liquid,

and then into its solid, crystalline state as snowflakes.

But not just any snowflakes.

The most destructive snowstorms are those that can accumulate snow fast,

like, two, three, four, five inches an hour.

And the type of snowflake that does that is a dendritic snowflake.

So your classic six-sided snowflakes.

Kind of like what's falling right now.

You know, if these kept falling all clay long we'd pile up several inches of snow.

As night falls, the snow is getting even heavier,

and the team heads straight for the centre of the storm.

You can see right now, the band is continually reforming out over the lake...

...and heading right towards the radar location.

The radar shows that the core of the storm has been sitting directly over the lake for several hours.

This means that it's picked up lots of water vapour,

so the snow cloud is super-saturated with moisture.

As the clouds rise high into the air,

they hit temperatures of around minus 15 Celsius.

Perfect conditions for fluffy, disruptive, dendritic snow.

You cans see the storm is headed right toward the radar.

So we're in the core of the storm right now.

So we're going to be in heavy snow, one to three inches per hour,

for the next several hours.

You may get upwards of a foot of snow here by the end of the night.

Across the northern hemisphere,

this same interaction of cold land and relatively warm moisture...

...produce many other spectacular weather phenomena.

In 2005, these phenomenal ice sculptures formed when spray from Lake Geneva in Switzerland...

...was thrown up by strong winds and froze instantly as it landed.

In Canada in 1998, rain falling on frozen ground turned to ice as it landed.

Not a snowstorm, but an ice storm.

It continued for 80 hours.

...Though at its extremes living on our planet isn't always easy,

it's orbit around the sun means that life itself flourishes.

It's easy to assume that our world's range of climates have always been as they are now,

because day to day, year to year, things on Earth don't change very much.

The Earth's orbit is constant and reliable.

Or so it seems.

Physicist Dr Helen Czerski is off the coast of Belize in Central America.

She's about to dive into what is known here as the 'great blue hole',

to look for evidence of an ancient world that was very different from the one we take for granted now.

This wall seems to go down forever.

And I'm told that the bottom here is 120 metres down, which sounds a very, very long way just now.

And I'm just dropping into the abyss.

That shark's just swum past in front of me.

L've never, ever been so frightened.

Go away, shark.

And there's another two just behind me.

So down here at 40 metres, it's really eerie, really.

But this is what I've come to see.

And they're stalactites.

But there's only way I know of that stalactites are formed.

And it isn't down here, in 40 metres of water, with sharks swimming around 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.

Stalactites like this only ever form above ground.

That means 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 discovered that over the last few hundred thousand years sea levels have risen and fallen.

20,000 years ago, the entire surface of the world's oceans...

...was 120 metres below where it is today.

That's because 20,000 years ago,

a good part of Earth's water was frozen and trapped on land in continental glaciers.

The Earth has experienced regular ice ages in a pattern going back millions of years.

What could cause such a radical change in the Earth's climate'?

But what could drop global sea level by almost 130 metres, and create such glaciers?

These mountains in north Wales were carved by Ice Age glaciers.

Professor Peter Nienow is convinced the dramatic shifts in the Earth's climate...

...are triggered by tiny changes in its orbit.

The key thing here are these things called orbital cycles.

Basically the - they way the Earth goes around the Sun actually varies on a long time scale,

so there's this - there's this thing called eccentricity,

which is the shape of the orbit, and if it's very circular,

then we get the same amount of sunshine or energy in the different seasons.

But it can go to a sort of elliptical orbit,

and what was crucial is in the northern hemisphere it's getting slightly cooler summers,

basically because it was slightly further away from the Sun...

...during this period of time, on this sort of 100,000-year cycle.

But variation in the shape of the elliptical orbit isn't the only thing.

There are two other orbital cycles that change the amount of solar energy hitting the Earth.

The axis of the Earth tilts between about 21 and 24 degrees and -

and just the way the - the Earth is tilted towards the Sun just affects the amount of - of radiation...

...and then there's another strange cycle,

the procession of the equinoxes, which is the - the axis actually wobbles a little bit.

And all of these things interact in a complex way,

but the net effect is that over about 100,000-year cycles...

...you get periods of warmer or cooler weather...

...because of the amount of - basically energy or radiation coming from the Sun.

And it's a - it's very small variations,

but it's enough to just mean that - that you can start to build up ice sheets.

Ice age conditions are triggered only once every 100,000 years,

when the three cycles coincide.

Right now, the Earth is getting enough solar energy in summer...

...to melt ice and keep us out of an ice age.

But another one is on the way.

You could be expecting another ice age to start in the next sort of well,

five to ten thousand years at most.

Because over the last six or seven hundred thousand years

they've occurred on these 100,000-year intervals.

So we'd be expecting another one to happen,

you know, really in the - in the - in the next sort of few thousand years.

Tiny changes in the Earth's passage around the Sun have a huge impact on our planet.

But they happen over such long periods of time,

it's impossible for us to observe them directly.

In fact, it's only recently that we've known about them at all.

And although the cyclical nature of our life on Earth,

the waxing and waning of the days with the passing seasons...

has been observed and even celebrated for thousands of years,

the mechanism of the Earth's orbit has only been known for a few hundred.

The ancient Egyptians who built this temple...

...did so in part to celebrate the shortest day in the northern hemisphere - the winter solstice.

They knew it was the shortest day,

but they didn't realise that this was because of the way our plant orbits the Sun,

and that it spins tilted at 23 degrees.

But even when you understand all that, the Earth's orbit holds other mysteries.

The winter solstice is the shortest day of the year north of the equator.

This is because the northern hemisphere is angled away from the Sun at this time of year.

And in the far north,

this shortest day of the year means one thing:

extreme cold.

Yellow Knife in northern Canada...

...holds the dubious honour of being the coldest city in all of North America.

During the winter, the temperature rarely gets above 30 below.

Which makes life very tough for truckers like Blair Weatherby.

It was minus 32 or minus 34 last night overnight, and so we've put a Herman Nelson on it.

This blows heat up the tube and put it wherever you want,

so it's on the bottom of the engine.

So it just brings it up a little bit where it'll actually start.

The extreme cold in the far north coincides with a point in the Earth's orbit called perihelion.

Oddly enough, it's the point at which Earth passes closest to the Sun.

It happens because the Earths' orbit around the Sun is not a perfect circle.

It's an ellipse, and the Sun sits off-centre.

You'd think that the seasons might be reversed and Blair wouldn't need his heater,

since the Sun is two and a half million miles closer than in summer.

But because of the planet's tilt,

the winter sun never gets much above the horizon here in the far north.

Even though it's closest to the Sun at this point in the orbit,

and the Earth receives seven per cent more solar radiation,

the Sun's rays have to travel through more atmosphere and their energy is absorbed.

So very little extra warmth reaches out toward the Arctic Circle as this time of year.

And even when the Earth's orbit continues beyond the winter solstice...

...and the days here start to get longer again, the far north doesn't get warmer at all.

It just keeps getting colder.

This is the Great Slave Lake,

and for several weeks now the ice has been getting thicker...

...as the temperatures have continued to drop.

Construction crews have carved out a brand new road over the ice.

The engineers have checked it's thick enough for 40-ton rigs,

so Blair can keep on trucking.

Well, we're driving on a ice road right now.

It's about 30 inches thick at this point.

It'll just keep getting thicker and thicker and you're allowed so much weight per inch.

The season starts towards the end of January, then up to basically April 1st...

...and the ice is still remarkably withstanding at that point.

This doesn't seem to make any sense.

It's weeks after the solstice, the days are getting longer, and yet it isn't getting any warmer.

There's a good reason for this.

Yellow Knife and Great Slave Lake are surrounded by thousands of miles of land.

It's as far from the ocean as it can possibly be.

For a month after the solstice,

the land still loses more heat than it gets from the weak rays from the Sun.

So it slowly keeps on cooling down.

It isn't until around the 19th of January, a month after the shortest day,

that the land begins to absorb more energy than it loses and starts to warm up again.

It may be deep winter in the north, but in the southern hemisphere it's midsummer.

The shortest days in the north mean the longest days down here.

At this point in the orbit,

this part of the Earth is receiving the maximum possible amount of energy from the Sun.

This is Puerto Williams, the southernmost town in Chile,

and in fact the southernmost town in the entire world.

Far to the south lies Antarctica.

You'd think that summer way down here in the south would be pretty similar to summer way up north.

But it's not.

The clues as to why are all around.

There are glaciers here.

Strong evidence that summers in the southern hemisphere are much cooler than in the north.

A full five degrees Celsius cooler.

Though the orbit of the Earth is closest to the Sun now,

it doesn't make the north warmer in winter, nor does it make the south warmer in summer.

Five degrees cooler in summer might not sound like much,

but it has big effects on what lives here.

Resident botanist Christopher Anderson watches how the climate here shapes the plants.

The Moringa tree normally grows to 30 metres.

But here it grows very differently.

Here it's stunted and small leaves,

whereas below it can be a very large tree that's several metres around.

And this tree line is the combination of cold and snow,

but then also the wind, which sculpts the trees and makes them this sort of miniature forest.

Another local species has used a similar trick to survive.

The cushion plants create sort of a cushion - hence the name -

but that cushion is a hard little mat of- of plant matter that the leaves are all stuck together,

they're very compressed,

but what that does is reduces the wind that blows over it,

so they have a adaptation to reduce their surface area...

...so that they're not exposed to the cold temperatures or the wind.

And it's not just here in the far south,

the whole southern hemisphere has a cooler summer than the northern hemisphere.

The gale force winds that buffet southern Chile hold another clue as to why.

We're sailing into the southern ocean beyond Cape Horn.

Strong winds and icebergs have made these waters notorious as a sailors' graveyard.

To the west stretches the Pacific Ocean, to the east the Atlantic, to the south Antarctica.

Vast oceans all around.

It's the very vastness of the surrounding oceans...

...that makes summers in the southern hemisphere so cool.

If you look at the whole of the southern half of the globe,

over 80 per cent of it is covered by oceans.

These huge expanses of water have a powerful effect on the climate down here.

That's because it takes a lot of the Sun's energy to warm it up.

Much more than the land.

Even in midsummer, the warmest time of year,

the oceans in the southern hemisphere are still cool.

And that keeps the air cool too.

During the single year it takes the Earth to orbit the Sun,

the orbital path changes the planet's position relative to the Sun's energy.

This drives changes in climate and weather across the face of the globe in some surprising ways.

But it isn't just the proximity of the Sun that determines the Earth's climate.

The Earth's axial tilt and the distribution of its land masses also have their part to play.

High in the northern hemisphere,

there's now evidence that other factors might be coming into play.

This is the tiny settlement of Kulusuk,

population 350, perched on the edge of an island off eastern Greenland.

At this time of year the Arctic Ocean is covered by a thick layer of sea ice,

blurring the distinction between land and sea.

Local hunter Georg Utuaq and his Danish companion, Lars Anker Mailer

...prepare their sled for a long day's hunting on the ice.

Conditions at this time of year are perfect for...

...the Inuit hunters who travel out onto the ice in search of food.

That's because the sea ice is still expanding.

Lars and Georg set off to their hunting grounds which lie right at the edge of the sea ice.

The ice is thick now because water reacts slowly to solar energy...

...and takes an exceptionally long time to warm up.

And ice sheets reflect a lot of the Sun's energy back into space,

so even though it's already early March here in the northern hemisphere,

the sea isn't warming up yet and the ice sheets continue to grow.

But as Lars and Georg get halfway down the fjord, they get a big surprise.

There's open water where there should be ice.

Only days ago, the sea ice extended out for at least another kilometre.

To see how think the ice is, we use this stick.

And if you hit the ice once and the stick goes through,

you turn around and go somewhere else.

If you hit twice it's safe to walk.

You're safe if you have your stick with you.

That can save your life.

Normally the ice gets thinner and thinner towards the open water,

so it can be dangerous to stand right on the edge.

But here there is thick ice all the way to the sea, telling the hunters that this is a broken edge.

A storm yesterday has snapped off a huge chunk of ice, which has now drifted out to sea.

Before they can go on,

the hunters have the check the thickness of the ice to make sure it's safe.

It has a lot to do with wind, a lot to do with current.

So if you have a crack out here, it can actually just in minutes.

It can be opened up and then you're standing on a piece of ice floating out.

What you're seeing here in the background, that's open water, it's not normal.

Because we had very big storms this year,

it has broken up the ice and now we have open water.

But something else has made the ice more and more fragile in recent years.

Scientific records show that the polar regions have been warming...

...and the ice has been getting thinner.

But what we've seen the last couple of years here...

...is that the dangerous places are getting more dangerous.

Before we maybe had like, 30 centimetres of ice and that was very safe.

Now it's maybe 15.

If we go 30 years back,

you could go straight from here to Tasiilaq, the main town,

just cross the ice here and go straight over.

So the ice is getting thinner, it's not so big any more as it was before.

It's part of a trend over the whole Arctic,

where the sea ice has shrunk significantly over the last 20 years.

A series of warm winters has meant that the seas haven't cooled down...

...as much as normal, resulting in less sea ice.

As global temperatures rise further,

that could spell real trouble for both people and animals in this environment.

There's no ice, we cannot go dog-sledding, we cannot go out hunting.

And if there's no ice, then polar bears cannot go hunting...

...and it's very difficult for them to find food, so it's all a chain.

So it's very important.

For centuries, as we've learned more and more about our place in the universe...

...and got used to the fact that...

our home is a spinning ball of rock orbiting a star we call the Sun,

we've come to appreciate how critical Earth's orbit...

...and its relationship with the sun has been in its history.

Tilts and wobbles, cycles of tiny changes that have precipitated ice ages...

...and radically changed habitats and even changed the course of human history.

Now we've discovered just how precarious our climate is,

the sobering thought that we ourselves have almost certainly become...

...another significant agent for climate change.

It seems that human actions are changing the balance between the Sun and the Earth in its orbit.

How significant these changes are is not yet known, but in just a few years we'll find out.