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At this moment, you're speeding around
the Sun at 67,000 miles an hour.
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In the next year, you will travel 584 million miles,
to end up exactly where you started.
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00:00:20,192 --> 00:00:24,796
Three things: the orbit, spin and tilt of the Earth...
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...have created and continue to shape the planet,
and us, every minute of each day of every year.
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00:00:36,341 --> 00:00:41,980
Since the beginning, the Earth has made this trip
around the Sun over four billion times.
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The orbital path around the Sun
is the ultimate Earth environment.
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It's a dangerous place, bathed in solar radiation,
deathly cold and crossed by asteroids and comets.
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00:01:01,567 --> 00:01:06,638
But despite the dangers,
the orbit of the Earth is in just the right place.
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Just right for life.
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00:01:20,152 --> 00:01:26,792
For most of human history, the idea of Earth orbiting
anything at all was a bizarre concept.
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The Earth, it was thought,
sat at the centre of everything,
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with everything else revolving around it.
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Discovering that the Earth actually orbits the sun,
spinning as it goes, solved some mysteries.
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Day and night and the year
suddenly made a lot more sense.
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Today we know a lot more about
the planet's annual trip around the Sun.
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We've discovered that the orbital journey
influences almost everything on the Earth...
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...in ways that our ancestors
could never have imagined.
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The ancient Egyptians knew there was a special
relationship between the Earth and the Sun,
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and they got some of the science right -
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but not everything.
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Part shrine to the Sun god Amun Ra,
and part stone calendar,
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the architects built it specifically
with December 21st in mind.
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This day is the winter solstice.
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It's the shortest day of the year
in the northern hemisphere.
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The people who built this temple
didn't know why it was the shortest day.
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But they did know that it happened
on the same day every year.
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So the temple was designed to line up perfectly
with the Sun on this one special morning.
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We know now that the regularity of
this phenomenon is not the gift of the gods,
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but because of the Earth's orbit.
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Our planet will be here in the same place
at the same time every year.
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Planet Earth was born over four billion years ago,
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from the planet-forming cloud of
debris orbiting the new-born Sun.
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It was a violent birth,
for both the planet and its orbit.
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As it grew, the young Earth was battered by
other debris orbiting the Sun.
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It even collided with a smaller planet
which knocked Earth over,
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leaving it spinning on
an axis 23 degrees off the vertical.
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Today, Earth's epic annual trip round the Sun
may seem serene.
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In reality it's still fraught with danger.
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Though our planet's course is constant,
the swirling soup of space is ever-changing.
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Its path is littered with asteroids, meteorites
and comets that could spell disaster.
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This hole in the ground is Barringer crater in Arizona.
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NASA impact specialist Dan Durda
has come here to see what happens...
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...when our planet comes into contact
with something else.
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This is what happens when we cross
orbits with another object in the solar system.
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About 50,000 years ago,
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a relatively large meteorite landed on Earth,
about maybe 40-45 metres across or so.
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Slammed out of the sky
at 17 or 20 kilometres a second.
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The explosive energy that excavated
this crater five or ten megatons,
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something like that, is -
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it's a little less than a thousand
Hiroshima's worth of explosive energy.
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So it's - it's far larger than
anything human beings have ever witnessed.
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You could put the Washington Monument
down there in the bottom...
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...and the top of it wouldn't quite make it to the -
to the height of the rim of the crater.
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So when - when objects like that hit the Earth
they unleash a lot of energy.
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The crater is more than three quarters of
a mile across, and is considered a small one.
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To get an idea of the force that created it,
you only have to look at the rock it left behind.
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So here I've got a - pretty much
a pretty pristine piece of the Coconino sandstone,
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a piece of rock from the basement of the crater.
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In my other hand, I'm holding effectively
the same stone, the same rock,
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but this material has been pulverised,
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crushed by the immense pressures
produced in the impact even itself.
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This stuff here is - is crushed to the point
where I can break it apart in my hand,
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and it gives you an idea of the damage done
to even solid rock by an impact like this.
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Scientists have calculated that the temperature
in this crater reached 6,000 degrees Celsius,
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hotter than the surface of the Sun.
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Barringer crater is a little over a kilometre wide.
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Incredibly, all this was done by
an asteroid just 45 metres long.
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Here's a nice piece of ejecta, a few meters across.
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It came to rest here on the - on the rim of the crater,
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but there are rocks just like it
that were hurled for -
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for miles in every direction,
landing across the desert.
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I can see block out there the size of small buildings.
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And those weren't the only ones.
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There were ones deposited out in the desert.
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You can see the remains of them eroding today.
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So it's - it's an immense amount of energy...
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that can hurl rocks the size of buildings
across the desert for miles in every direction.
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Incredibly, all this was done by
an asteroid just 45 metres across.
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But not all are as small as the one
that created Barringer crater.
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65 million years ago,
an asteroid six kilometres long...
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...hit shallow seas just off the coast of
the Yucatan peninsular in Mexico.
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So put ourselves near the point of
impact 65 million years ago.
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It's just a few seconds to impact.
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The asteroid is glowing far
brighter then the sun in the sky.
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Disappears over the horizon.
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And then we would have seen
the immense flash of the impact itself.
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Followed 10 or 20 seconds later.
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Boom! We're blasted flat by
this shock wave of the blast from the impact itself,
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which was strong enough to have actually
laid flat forests for,
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you know, 800 miles or so in every direction.
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This is an immense impact.
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It made a crater well over 180 kilometres wide,
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and blasted so much hot debris into the atmosphere
that almost the whole planet caught fire.
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The impact was so catastrophic,
it helped wipe out the dinosaurs.
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Earth's orbit regularly takes it
into the path of asteroids and comets.
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But Earth has a secret weapon to defend itself.
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It has an atmosphere.
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Most objects along the Earth's orbit are small.
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The atmosphere is very effective at burning up
anything under 35 metres wide.
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When we see shooting stars in the night sky,
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they are actually small asteroids or bits of comet
burning up as they hit the atmosphere.
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Still, over the ages many large objects
have struck the Earth - and will again.
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Asteroids and comets are obvious, if occasional,
dangers along the orbital path of the Earth.
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But the Sun itself poses the greatest danger.
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In the northern sky during winter
you can sometimes see the Earth protecting itself.
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For thousands of years, people have marvelled at
the spectacular light displays...
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...that sometimes appear in the night sky.
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00:10:07,812 --> 00:10:10,849
They've wondered what on Earth
they could possibly mean.
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00:10:12,350 --> 00:10:15,820
The Vikings believed them to be
the reflections of dead maidens.
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Native Americans called them
'the dance of the spirit.'
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What we see as
the northern lights and the southern lights,
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the aurora borealis and australis,
are manifestations here on the Earth of living...
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...within kind of the outer extension of
the Sun's atmosphere.
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Every second, the Earth flies through
millions of tons of radioactive particles...
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...blasted out from the Sun's surface.
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00:10:45,583 --> 00:10:50,488
The auroras are caused by those particles
smashing into our atmosphere.
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00:10:51,623 --> 00:10:57,162
The Sun emits a continuous flow of
charged particles known as the 'solar wind'.
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This streams outwards in a blizzard of radiation.
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00:11:01,766 --> 00:11:05,136
When it reaches the Earth, it encounters a barrier.
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00:11:06,304 --> 00:11:12,210
The Earth's magnetic field deflects the particles
and funnels them towards the poles.
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00:11:12,310 --> 00:11:17,549
Here they collide with atoms of
nitrogen and oxygen in the atmosphere.
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These collisions emit energy in the form of light,
giving us the aurora.
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00:11:25,156 --> 00:11:27,091
From the international space station,
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you get a better sense of
the awesome scale of the aurora.
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You're watching the Sun's atmosphere
colliding with Earth's.
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00:11:39,204 --> 00:11:41,873
Without our protective atmosphere
and magnetic field,
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we'd be living on a much more harsh
and alien and inhospitable planet.
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We'd - we'd look like maybe
the surface of Mercury,
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baked by UV radiation and x-rays
and blasted by solar flares.
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00:11:57,455 --> 00:12:01,492
So we live in this lovely zone with
our protective atmosphere and magnetic field.
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Earth's orbit is full of risk, not just from asteroids,
but also from the Sun's harmful radiation.
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But for all the danger from the Sun,
being close to it provides us with one critical benefit.
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Our planet enjoys a precise
and very narrow band of temperatures...
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that means water can exist in all three of its states.
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As a solid, as a liquid, and as gas.
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Each of those states behaves very differently.
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And it's those differences that
drives the climate system on Earth.
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This is the cloud forest of Calilegua
in northern Argentina.
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Sitting in the foothills of the Andes,
it's a ghostly place.
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The trees and hills are shrouded in clouds.
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The heat of summer has evaporated water,
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so the air is laden with vapour,
the gaseous form of water.
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Dr Kristen Rasmussen has come hereto see...
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...how this water vapour creates some of
the most dynamic storms on the planet.
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The exciting part about these storms
is we've identified this region as...
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...being a hot spot of
very, very intense thunderstorms.
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The thunderstorms in this part of the world bring
a lot of hail, lightning, tornadoes, flooding,
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a lot of things that can be very hazardous
to the local population.
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Throughout the day the land has absorbed
more and more heat from the Sun.
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It's approximately three o'clock right now.
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The - the land surface has been heating all day,
since the sun rose.
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And all of that warm buoyant energy from
the surface is now rising very quickly.
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You kind of see a bubbling effect of that:
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the cloud parcel rising up and
condensing water as it forms the cloud.
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As heat evaporates water and
forces the wet vapour high into the atmosphere,
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there it hits colder air and
condenses back into liquid water,
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forming towering cumulus clouds.
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The huge mat of clouds pushes up
into the atmosphere.
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These storms are growing very, very quickly.
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You can actually sit here for about five minutes and
watch almost the whole thunderstorm develop.
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The updrafts keep the storm
growing up to ten miles high,
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and at this point high winds spread the clouds out
sideways to form the anvil shaped thunderhead.
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By early evening,
the storm has grown to epic proportions,
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enveloping the entire mountain range of Calilegra.
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For Dr Rasmussen it's a chance to experience
one of the great forces of nature at first hand.
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A phenomenon that can only happen...
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...because Earth's orbit means
its temperature is rising for liquid water.
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This is actually the cloud base
where the storm - where the storm initiates.
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You cans see some lightning off
here in the distance.
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This means that there's ice the clouds.
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They're very, very tall,
they're very intense,
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and they have the capability of
producing large amounts of rain, and hail as well.
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The extraordinary fact,
the Earth orbits the Sun at the just the right distance,
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gives rise to the very ordinary phenomenon of rain.
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Water evaporates from the surface as a gas,
condenses into a liquid, as it rises,
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and eventually falls back to earth,
where the cycle starts again.
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It's this very cycle,
unique to the Earth in our solar system,
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which supports life in all corners of the planet.
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This is incredible.
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This is the thing I've come here for.
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It's - I study these storms all clay long every day,
and it's just amazing to see them in person.
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If Earth's orbit around the sun
were just a little different,
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temperatures on our planet might be either
too cold or too hot for water as a liquid and a gas.
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Life as we know it, and the weather we enjoy,
simply couldn't happen.
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But within the narrow band of
temperatures our planet experiences,
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there is room for water's third state.
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The edge of Lake Ontario sees...
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...one of the most extreme demonstrations of
liquid water becoming a solid.
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This area is home to
some of the heaviest snowfalls in the world.
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Professor Scott Steiger has lived
and worked here all his life.
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He studies how water vapour
can be transformed into lake effect snow.
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The key thing for the lake effect is...
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...the temperature difference between
the lake and the air going above it.
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So you want really cold air,
you know, maybe 10-15 degrees Celsius...
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...colder than the lake temperature about a mile
above the ground.
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And that cold air over the lake
makes it very unstable,
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and allows a lot of air to start rising,
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and it's the rising motion
that produces snow clouds.
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And the second thing is the lake
is a constant moisture source.
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You have moisture fluxing off the lake
and it keeps feeding the clouds.
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Lake effect snowstorms dominate
weather conditions here throughout the winter,
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depositing huge amounts of snow
in a short space of time.
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In 2007, an incredible 77 inches of
snow fell in just one day.
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00:18:38,990 --> 00:18:42,827
The biggest difference between lake effect snow
and a normal snowstorm is that it's very local.
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It's a very local snowstorm.
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The band is maybe 10 miles wide,
extends about maybe 5O miles inland.
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00:18:49,567 --> 00:18:51,769
So basically you think about -
about it as a rectangle,
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and just in that rectangle
it can be snowing very heavy,
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up to two, three, four,
sometimes five-six inches an hour.
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And then outside that band it can be sunny.
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Can be completely sunny.
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To forecast exactly where
these narrow storms will hit,
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Professor Steiger's team
use sophisticated computer models.
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00:19:18,763 --> 00:19:20,097
To refine these models,
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00:19:20,364 --> 00:19:25,169
they need to keep taking data directly
from lake effect storms as they develop.
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00:19:27,438 --> 00:19:31,242
The models have been indicating
the lake effect snow band is going to form...
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...over central Lake Ontario,
going on the eastern part of the lake.
218
00:19:38,783 --> 00:19:43,220
To make an accurate forecast,
Professor Steiger needs to know two things:
219
00:19:43,554 --> 00:19:48,125
how much snow a storm can produce,
and where that snow will fall.
220
00:19:48,759 --> 00:19:53,197
He's tracking the course of a storm
as it heads towards upper New York State.
221
00:19:58,135 --> 00:20:00,504
There are only five hours until nightfall.
222
00:20:00,805 --> 00:20:05,076
The leading front of the snow is just
reaching the south-eastern shores of the lake.
223
00:20:07,078 --> 00:20:10,648
The first step is to put instruments
directly into the clouds,
224
00:20:10,881 --> 00:20:14,351
to analyse the conditions
in which the snowflakes are growing.
225
00:20:16,387 --> 00:20:21,425
As the snow starts to intensify,
Scott and the team send up a weather balloon.
226
00:20:22,359 --> 00:20:27,998
It will record the direction of the storm and
the conditions inside the developing snow cloud.
227
00:20:28,599 --> 00:20:30,134
Hopefully what this is going to measure is...
228
00:20:30,367 --> 00:20:34,138
...the temperature, humidity and wind
at multiple levels throughout the atmosphere,
229
00:20:34,505 --> 00:20:36,741
up to maybe 10 miles above the ground.
230
00:20:37,241 --> 00:20:37,975
Let her go.
231
00:20:48,753 --> 00:20:53,524
The huge amount of water vapour in the cloud
turns from a gas to liquid,
232
00:20:53,824 --> 00:20:57,928
and then into its solid,
crystalline state as snowflakes.
233
00:20:59,263 --> 00:21:01,065
But not just any snowflakes.
234
00:21:03,334 --> 00:21:07,772
The most destructive snowstorms
are those that can accumulate snow fast,
235
00:21:08,005 --> 00:21:09,840
like, two, three, four, five inches an hour.
236
00:21:10,174 --> 00:21:13,778
And the type of snowflake that does
that is a dendritic snowflake.
237
00:21:14,078 --> 00:21:16,080
So your classic six-sided snowflakes.
238
00:21:16,347 --> 00:21:17,715
Kind of like what's falling right now.
239
00:21:17,982 --> 00:21:21,886
You know, if these kept falling all clay long
we'd pile up several inches of snow.
240
00:21:28,259 --> 00:21:31,095
As night falls, the snow is getting even heavier,
241
00:21:31,395 --> 00:21:34,365
and the team heads straight
for the centre of the storm.
242
00:21:36,167 --> 00:21:39,570
You can see right now, the band is continually
reforming out over the lake...
243
00:21:39,770 --> 00:21:41,939
...and heading right towards the radar location.
244
00:21:43,774 --> 00:21:49,747
The radar shows that the core of the storm has
been sitting directly over the lake for several hours.
245
00:21:50,347 --> 00:21:52,917
This means that it's picked up lots of water vapour,
246
00:21:53,250 --> 00:21:56,520
so the snow cloud is
super-saturated with moisture.
247
00:21:58,789 --> 00:22:00,991
As the clouds rise high into the air,
248
00:22:01,225 --> 00:22:04,528
they hit temperatures of around minus 15 Celsius.
249
00:22:04,795 --> 00:22:09,433
Perfect conditions for
fluffy, disruptive, dendritic snow.
250
00:22:15,072 --> 00:22:17,408
You cans see the storm is headed
right toward the radar.
251
00:22:17,641 --> 00:22:19,543
So we're in the core of the storm right now.
252
00:22:19,844 --> 00:22:23,047
So we're going to be in heavy snow,
one to three inches per hour,
253
00:22:23,414 --> 00:22:24,181
for the next several hours.
254
00:22:24,515 --> 00:22:27,151
You may get upwards of a foot of snow here
by the end of the night.
255
00:22:39,763 --> 00:22:41,265
Across the northern hemisphere,
256
00:22:41,532 --> 00:22:45,536
this same interaction of cold land
and relatively warm moisture...
257
00:22:45,903 --> 00:22:48,873
...produce many other
spectacular weather phenomena.
258
00:22:51,675 --> 00:22:58,749
In 2005, these phenomenal ice sculptures formed
when spray from Lake Geneva in Switzerland...
259
00:22:59,016 --> 00:23:03,888
...was thrown up by strong winds
and froze instantly as it landed.
260
00:23:09,260 --> 00:23:15,099
In Canada in 1998, rain falling on frozen ground
turned to ice as it landed.
261
00:23:15,666 --> 00:23:18,202
Not a snowstorm, but an ice storm.
262
00:23:19,603 --> 00:23:21,705
It continued for 80 hours.
263
00:23:27,177 --> 00:23:31,215
...Though at its extremes living
on our planet isn't always easy,
264
00:23:31,582 --> 00:23:35,786
it's orbit around the sun
means that life itself flourishes.
265
00:23:54,038 --> 00:23:59,143
It's easy to assume that our world's range of
climates have always been as they are now,
266
00:23:59,443 --> 00:24:04,348
because day to day, year to year,
things on Earth don't change very much.
267
00:24:05,683 --> 00:24:08,252
The Earth's orbit is constant and reliable.
268
00:24:08,886 --> 00:24:10,220
Or so it seems.
269
00:24:19,129 --> 00:24:23,968
Physicist Dr Helen Czerski is off
the coast of Belize in Central America.
270
00:24:26,837 --> 00:24:30,808
She's about to dive into what is known here
as the 'great blue hole',
271
00:24:31,141 --> 00:24:37,281
to look for evidence of an ancient world that was
very different from the one we take for granted now.
272
00:24:59,336 --> 00:25:01,572
This wall seems to go down forever.
273
00:25:02,539 --> 00:25:09,980
And I'm told that the bottom here is 120 metres down,
which sounds a very, very long way just now.
274
00:25:10,681 --> 00:25:12,883
And I'm just dropping into the abyss.
275
00:25:23,193 --> 00:25:25,295
That shark's just swum past in front of me.
276
00:25:26,663 --> 00:25:28,932
L've never, ever been so frightened.
277
00:25:29,900 --> 00:25:30,901
Go away, shark.
278
00:25:32,870 --> 00:25:35,439
And there's another two just behind me.
279
00:26:00,464 --> 00:26:07,371
So down here at 40 metres,
it's really eerie, really.
280
00:26:09,506 --> 00:26:11,175
But this is what I've come to see.
281
00:26:12,676 --> 00:26:14,111
And they're stalactites.
282
00:26:26,757 --> 00:26:30,360
But there's only way I know of
that stalactites are formed.
283
00:26:31,762 --> 00:26:37,734
And it isn't down here, in 40 metres of water,
with sharks swimming around nearby.
284
00:26:46,009 --> 00:26:50,914
Stalactites are created when mineral-rich water
drips from the roof of a cave...
285
00:26:51,215 --> 00:26:56,653
...over hundreds or even thousands of years,
leaving behind mineral deposits.
286
00:27:03,560 --> 00:27:07,464
Stalactites like this only ever form above ground.
287
00:27:08,932 --> 00:27:15,506
That means when these grew the sea level
was much, much lower than it is today.
288
00:27:19,243 --> 00:27:23,213
Scientists have precisely dated stalactites
from the Blue Hole,
289
00:27:23,714 --> 00:27:27,618
and by comparing these and
other sea level indicators from around the world,
290
00:27:28,118 --> 00:27:34,458
they've discovered that over the last few hundred
thousand years sea levels have risen and fallen.
291
00:27:40,497 --> 00:27:44,401
20,000 years ago,
the entire surface of the world's oceans...
292
00:27:44,868 --> 00:27:48,172
...was 120 metres below where it is today.
293
00:27:53,377 --> 00:27:55,579
That's because 20,000 years ago,
294
00:27:55,879 --> 00:28:01,652
a good part of Earth's water was frozen and
trapped on land in continental glaciers.
295
00:28:05,989 --> 00:28:11,495
The Earth has experienced regular ice ages
in a pattern going back millions of years.
296
00:28:15,065 --> 00:28:18,635
What could cause such a radical change
in the Earth's climate'?
297
00:28:20,237 --> 00:28:27,110
But what could drop global sea level
by almost 130 metres, and create such glaciers?
298
00:28:30,814 --> 00:28:34,885
These mountains in north Wales
were carved by Ice Age glaciers.
299
00:28:41,425 --> 00:28:46,029
Professor Peter Nienow is convinced
the dramatic shifts in the Earth's climate...
300
00:28:46,296 --> 00:28:49,366
...are triggered by tiny changes in its orbit.
301
00:28:51,268 --> 00:28:54,071
The key thing here are these things
called orbital cycles.
302
00:28:54,304 --> 00:28:59,243
Basically the - they way the Earth goes around
the Sun actually varies on a long time scale,
303
00:28:59,509 --> 00:29:02,312
so there's this -
there's this thing called eccentricity,
304
00:29:02,579 --> 00:29:05,616
which is the shape of the orbit,
and if it's very circular,
305
00:29:05,849 --> 00:29:10,420
then we get the same amount of
sunshine or energy in the different seasons.
306
00:29:10,687 --> 00:29:13,023
But it can go to a sort of elliptical orbit,
307
00:29:13,257 --> 00:29:18,095
and what was crucial is in the northern hemisphere
it's getting slightly cooler summers,
308
00:29:18,395 --> 00:29:21,331
basically because it was slightly
further away from the Sun...
309
00:29:21,698 --> 00:29:24,468
...during this period of time,
on this sort of 100,000-year cycle.
310
00:29:25,235 --> 00:29:29,773
But variation in the shape of
the elliptical orbit isn't the only thing.
311
00:29:30,974 --> 00:29:36,413
There are two other orbital cycles that change
the amount of solar energy hitting the Earth.
312
00:29:36,847 --> 00:29:40,851
The axis of the Earth tilts
between about 21 and 24 degrees and -
313
00:29:41,218 --> 00:29:46,890
and just the way the - the Earth is tilted towards
the Sun just affects the amount of - of radiation...
314
00:29:47,357 --> 00:29:48,859
...and then there's another strange cycle,
315
00:29:49,092 --> 00:29:52,195
the procession of the equinoxes, which is the -
the axis actually wobbles a little bit.
316
00:29:52,596 --> 00:29:54,865
And all of these things interact in a complex way,
317
00:29:55,065 --> 00:29:58,502
but the net effect is that over
about 100,000-year cycles...
318
00:29:58,935 --> 00:30:02,939
...you get periods of warmer or cooler weather...
319
00:30:03,307 --> 00:30:07,110
...because of the amount of -
basically energy or radiation coming from the Sun.
320
00:30:07,411 --> 00:30:09,713
And it's a - it's very small variations,
321
00:30:09,946 --> 00:30:13,583
but it's enough to just mean that -
that you can start to build up ice sheets.
322
00:30:17,854 --> 00:30:22,125
Ice age conditions are triggered only once
every 100,000 years,
323
00:30:22,359 --> 00:30:24,394
when the three cycles coincide.
324
00:30:25,028 --> 00:30:28,598
Right now, the Earth is getting enough
solar energy in summer...
325
00:30:28,865 --> 00:30:32,636
...to melt ice and keep us out of an ice age.
326
00:30:32,736 --> 00:30:34,671
But another one is on the way.
327
00:30:35,972 --> 00:30:39,843
You could be expecting another ice age
to start in the next sort of well,
328
00:30:40,110 --> 00:30:41,945
five to ten thousand years at most.
329
00:30:42,179 --> 00:30:44,181
Because over the last six or seven hundred
thousand years
330
00:30:44,414 --> 00:30:46,883
they've occurred on these 100,000-year intervals.
331
00:30:47,150 --> 00:30:49,252
So we'd be expecting another one to happen,
332
00:30:49,586 --> 00:30:52,789
you know, really in the - in the -
in the next sort of few thousand years.
333
00:30:57,994 --> 00:31:03,433
Tiny changes in the Earth's passage around
the Sun have a huge impact on our planet.
334
00:31:05,869 --> 00:31:08,305
But they happen over such long periods of time,
335
00:31:08,538 --> 00:31:11,308
it's impossible for us to observe them directly.
336
00:31:12,376 --> 00:31:15,779
In fact, it's only recently that
we've known about them at all.
337
00:31:18,882 --> 00:31:21,718
And although the cyclical nature of
our life on Earth,
338
00:31:22,119 --> 00:31:25,088
the waxing and waning of the days
with the passing seasons...
339
00:31:25,389 --> 00:31:29,126
has been observed and even celebrated for
thousands of years,
340
00:31:29,626 --> 00:31:33,764
the mechanism of the Earth's orbit
has only been known for a few hundred.
341
00:31:41,505 --> 00:31:43,807
The ancient Egyptians who built this temple...
342
00:31:44,074 --> 00:31:49,913
...did so in part to celebrate the shortest day
in the northern hemisphere - the winter solstice.
343
00:31:52,549 --> 00:31:54,518
They knew it was the shortest day,
344
00:31:54,851 --> 00:31:58,989
but they didn't realise that this was because of
the way our plant orbits the Sun,
345
00:31:59,356 --> 00:32:02,692
and that it spins tilted at 23 degrees.
346
00:32:05,162 --> 00:32:10,133
But even when you understand all that,
the Earth's orbit holds other mysteries.
347
00:32:14,204 --> 00:32:18,608
The winter solstice is the shortest day of
the year north of the equator.
348
00:32:19,242 --> 00:32:24,514
This is because the northern hemisphere
is angled away from the Sun at this time of year.
349
00:32:26,283 --> 00:32:27,617
And in the far north,
350
00:32:27,818 --> 00:32:30,387
this shortest day of the year means one thing:
351
00:32:30,921 --> 00:32:32,389
extreme cold.
352
00:32:39,963 --> 00:32:42,032
Yellow Knife in northern Canada...
353
00:32:42,265 --> 00:32:47,037
...holds the dubious honour of
being the coldest city in all of North America.
354
00:32:50,307 --> 00:32:54,411
During the winter,
the temperature rarely gets above 30 below.
355
00:32:58,448 --> 00:33:02,052
Which makes life very tough
for truckers like Blair Weatherby.
356
00:33:06,256 --> 00:33:12,629
It was minus 32 or minus 34 last night overnight,
and so we've put a Herman Nelson on it.
357
00:33:12,896 --> 00:33:15,065
This blows heat up the tube
and put it wherever you want,
358
00:33:15,398 --> 00:33:16,466
so it's on the bottom of the engine.
359
00:33:16,700 --> 00:33:19,369
So it just brings it up a little bit
where it'll actually start.
360
00:33:21,805 --> 00:33:27,978
The extreme cold in the far north coincides
with a point in the Earth's orbit called perihelion.
361
00:33:29,513 --> 00:33:34,050
Oddly enough, it's the point at which
Earth passes closest to the Sun.
362
00:33:34,751 --> 00:33:39,422
It happens because the Earths' orbit
around the Sun is not a perfect circle.
363
00:33:40,357 --> 00:33:43,627
It's an ellipse, and the Sun sits off-centre.
364
00:33:45,028 --> 00:33:49,399
You'd think that the seasons might be reversed
and Blair wouldn't need his heater,
365
00:33:49,699 --> 00:33:53,703
since the Sun is two and a half million miles
closer than in summer.
366
00:33:54,771 --> 00:33:56,439
But because of the planet's tilt,
367
00:33:56,740 --> 00:34:01,611
the winter sun never gets much above
the horizon here in the far north.
368
00:34:03,079 --> 00:34:06,383
Even though it's closest to
the Sun at this point in the orbit,
369
00:34:06,650 --> 00:34:10,220
and the Earth receives seven per cent
more solar radiation,
370
00:34:10,554 --> 00:34:16,192
the Sun's rays have to travel through
more atmosphere and their energy is absorbed.
371
00:34:18,461 --> 00:34:24,134
So very little extra warmth reaches out toward
the Arctic Circle as this time of year.
372
00:34:24,901 --> 00:34:29,105
And even when the Earth's orbit
continues beyond the winter solstice...
373
00:34:29,439 --> 00:34:35,845
...and the days here start to get longer again,
the far north doesn't get warmer at all.
374
00:34:36,379 --> 00:34:37,981
It just keeps getting colder.
375
00:34:48,158 --> 00:34:50,427
This is the Great Slave Lake,
376
00:34:50,627 --> 00:34:53,463
and for several weeks now
the ice has been getting thicker...
377
00:34:53,697 --> 00:34:55,966
...as the temperatures have continued to drop.
378
00:35:08,878 --> 00:35:12,816
Construction crews have carved out
a brand new road over the ice.
379
00:35:16,953 --> 00:35:20,490
The engineers have checked
it's thick enough for 40-ton rigs,
380
00:35:20,690 --> 00:35:22,425
so Blair can keep on trucking.
381
00:35:22,759 --> 00:35:24,594
Well, we're driving on a ice road right now.
382
00:35:25,962 --> 00:35:29,633
It's about 30 inches thick at this point.
383
00:35:30,166 --> 00:35:34,304
It'll just keep getting thicker and thicker
and you're allowed so much weight per inch.
384
00:35:36,306 --> 00:35:43,279
The season starts towards the end of January,
then up to basically April 1st...
385
00:35:43,747 --> 00:35:48,652
...and the ice is still remarkably
withstanding at that point.
386
00:35:49,919 --> 00:35:51,755
This doesn't seem to make any sense.
387
00:35:52,322 --> 00:35:57,594
It's weeks after the solstice, the days are
getting longer, and yet it isn't getting any warmer.
388
00:36:02,699 --> 00:36:04,167
There's a good reason for this.
389
00:36:04,434 --> 00:36:09,272
Yellow Knife and Great Slave Lake
are surrounded by thousands of miles of land.
390
00:36:11,675 --> 00:36:14,978
It's as far from the ocean as it can possibly be.
391
00:36:18,748 --> 00:36:20,383
For a month after the solstice,
392
00:36:20,650 --> 00:36:25,088
the land still loses more heat than it gets from
the weak rays from the Sun.
393
00:36:25,622 --> 00:36:28,925
So it slowly keeps on cooling down.
394
00:36:35,365 --> 00:36:40,203
It isn't until around the 19th of January,
a month after the shortest day,
395
00:36:40,537 --> 00:36:45,875
that the land begins to absorb more energy than
it loses and starts to warm up again.
396
00:36:51,381 --> 00:36:56,519
It may be deep winter in the north,
but in the southern hemisphere it's midsummer.
397
00:36:57,754 --> 00:37:01,758
The shortest days in the north
mean the longest days down here.
398
00:37:02,659 --> 00:37:03,827
At this point in the orbit,
399
00:37:04,127 --> 00:37:09,199
this part of the Earth is receiving the maximum
possible amount of energy from the Sun.
400
00:37:17,607 --> 00:37:21,444
This is Puerto Williams,
the southernmost town in Chile,
401
00:37:21,711 --> 00:37:25,749
and in fact the southernmost town
in the entire world.
402
00:37:31,187 --> 00:37:33,790
Far to the south lies Antarctica.
403
00:37:37,560 --> 00:37:42,966
You'd think that summer way down here in the south
would be pretty similar to summer way up north.
404
00:37:43,433 --> 00:37:44,400
But it's not.
405
00:37:44,968 --> 00:37:47,570
The clues as to why are all around.
406
00:37:49,038 --> 00:37:50,406
There are glaciers here.
407
00:37:51,374 --> 00:37:56,646
Strong evidence that summers in the southern
hemisphere are much cooler than in the north.
408
00:37:58,915 --> 00:38:01,251
A full five degrees Celsius cooler.
409
00:38:03,153 --> 00:38:05,989
Though the orbit of the Earth is closest to the Sun now,
410
00:38:06,322 --> 00:38:11,828
it doesn't make the north warmer in winter,
nor does it make the south warmer in summer.
411
00:38:18,067 --> 00:38:21,437
Five degrees cooler in summer
might not sound like much,
412
00:38:21,738 --> 00:38:23,940
but it has big effects on what lives here.
413
00:38:28,344 --> 00:38:33,516
Resident botanist Christopher Anderson
watches how the climate here shapes the plants.
414
00:38:38,955 --> 00:38:41,958
The Moringa tree normally grows to 30 metres.
415
00:38:42,292 --> 00:38:44,794
But here it grows very differently.
416
00:38:45,929 --> 00:38:47,831
Here it's stunted and small leaves,
417
00:38:48,064 --> 00:38:51,201
whereas below it can be a very large tree
that's several metres around.
418
00:38:51,768 --> 00:38:54,971
And this tree line is the combination of
cold and snow,
419
00:38:55,171 --> 00:39:00,109
but then also the wind, which sculpts the trees
and makes them this sort of miniature forest.
420
00:39:01,978 --> 00:39:05,782
Another local species
has used a similar trick to survive.
421
00:39:06,482 --> 00:39:10,186
The cushion plants create sort of a cushion -
hence the name -
422
00:39:10,620 --> 00:39:16,926
but that cushion is a hard little mat of-
of plant matter that the leaves are all stuck together,
423
00:39:17,260 --> 00:39:18,061
they're very compressed,
424
00:39:18,328 --> 00:39:21,898
but what that does is reduces the wind
that blows over it,
425
00:39:22,165 --> 00:39:25,902
so they have a adaptation to reduce
their surface area...
426
00:39:26,135 --> 00:39:28,638
...so that they're not exposed to
the cold temperatures or the wind.
427
00:39:32,075 --> 00:39:34,110
And it's not just here in the far south,
428
00:39:34,377 --> 00:39:38,748
the whole southern hemisphere has
a cooler summer than the northern hemisphere.
429
00:39:40,116 --> 00:39:44,754
The gale force winds that buffet southern Chile
hold another clue as to why.
430
00:39:49,058 --> 00:39:52,495
We're sailing into the southern ocean
beyond Cape Horn.
431
00:39:52,762 --> 00:39:58,434
Strong winds and icebergs have made
these waters notorious as a sailors' graveyard.
432
00:40:01,104 --> 00:40:07,610
To the west stretches the Pacific Ocean,
to the east the Atlantic, to the south Antarctica.
433
00:40:08,077 --> 00:40:10,146
Vast oceans all around.
434
00:40:13,116 --> 00:40:15,551
It's the very vastness of the surrounding oceans...
435
00:40:15,952 --> 00:40:19,155
...that makes summers
in the southern hemisphere so cool.
436
00:40:25,261 --> 00:40:27,864
If you look at the whole of
the southern half of the globe,
437
00:40:28,131 --> 00:40:31,034
over 80 per cent of it is covered by oceans.
438
00:40:33,269 --> 00:40:37,707
These huge expanses of water have
a powerful effect on the climate down here.
439
00:40:39,175 --> 00:40:42,378
That's because it takes a lot of the Sun's energy
to warm it up.
440
00:40:42,779 --> 00:40:44,480
Much more than the land.
441
00:40:45,815 --> 00:40:48,251
Even in midsummer, the warmest time of year,
442
00:40:48,551 --> 00:40:51,754
the oceans in the southern hemisphere are still cool.
443
00:40:52,422 --> 00:40:54,424
And that keeps the air cool too.
444
00:40:57,727 --> 00:41:01,130
During the single year it takes the Earth to orbit the Sun,
445
00:41:01,464 --> 00:41:06,169
the orbital path changes the planet's position
relative to the Sun's energy.
446
00:41:06,636 --> 00:41:12,208
This drives changes in climate and weather across
the face of the globe in some surprising ways.
447
00:41:15,611 --> 00:41:20,416
But it isn't just the proximity of the Sun
that determines the Earth's climate.
448
00:41:21,951 --> 00:41:28,524
The Earth's axial tilt and the distribution of
its land masses also have their part to play.
449
00:41:35,131 --> 00:41:37,000
High in the northern hemisphere,
450
00:41:37,233 --> 00:41:40,770
there's now evidence
that other factors might be coming into play.
451
00:41:42,705 --> 00:41:45,508
This is the tiny settlement of Kulusuk,
452
00:41:46,042 --> 00:41:51,681
population 350, perched on the edge of
an island off eastern Greenland.
453
00:41:55,251 --> 00:41:59,822
At this time of year the Arctic Ocean
is covered by a thick layer of sea ice,
454
00:42:00,189 --> 00:42:03,259
blurring the distinction between land and sea.
455
00:42:06,095 --> 00:42:11,834
Local hunter Georg Utuaq
and his Danish companion, Lars Anker Mailer
456
00:42:12,068 --> 00:42:15,338
...prepare their sled for
a long day's hunting on the ice.
457
00:42:19,509 --> 00:42:21,978
Conditions at this time of year are perfect for...
458
00:42:22,278 --> 00:42:26,416
...the Inuit hunters who travel out
onto the ice in search of food.
459
00:42:35,024 --> 00:42:37,894
That's because the sea ice is still expanding.
460
00:42:46,069 --> 00:42:51,541
Lars and Georg set off to their hunting grounds
which lie right at the edge of the sea ice.
461
00:43:01,084 --> 00:43:05,321
The ice is thick now because
water reacts slowly to solar energy...
462
00:43:05,621 --> 00:43:08,591
...and takes an exceptionally long time to warm up.
463
00:43:09,192 --> 00:43:13,129
And ice sheets reflect a lot of
the Sun's energy back into space,
464
00:43:13,629 --> 00:43:17,400
so even though it's already early March here
in the northern hemisphere,
465
00:43:17,733 --> 00:43:22,271
the sea isn't warming up yet
and the ice sheets continue to grow.
466
00:43:23,306 --> 00:43:28,344
But as Lars and Georg get halfway down the fjord,
they get a big surprise.
467
00:43:32,482 --> 00:43:36,152
There's open water where there should be ice.
468
00:43:43,526 --> 00:43:48,264
Only days ago, the sea ice extended out for
at least another kilometre.
469
00:43:52,969 --> 00:43:56,572
To see how think the ice is, we use this stick.
470
00:43:57,874 --> 00:44:01,744
And if you hit the ice once
and the stick goes through,
471
00:44:02,044 --> 00:44:04,013
you turn around and go somewhere else.
472
00:44:04,313 --> 00:44:07,116
If you hit twice it's safe to walk.
473
00:44:08,217 --> 00:44:10,119
You're safe if you have your stick with you.
474
00:44:10,319 --> 00:44:11,687
That can save your life.
475
00:44:12,688 --> 00:44:15,892
Normally the ice gets thinner and thinner
towards the open water,
476
00:44:16,159 --> 00:44:18,761
so it can be dangerous to stand right on the edge.
477
00:44:19,162 --> 00:44:24,901
But here there is thick ice all the way to the sea,
telling the hunters that this is a broken edge.
478
00:44:25,968 --> 00:44:30,706
A storm yesterday has snapped off a huge
chunk of ice, which has now drifted out to sea.
479
00:44:31,240 --> 00:44:32,408
Before they can go on,
480
00:44:32,708 --> 00:44:36,779
the hunters have the check
the thickness of the ice to make sure it's safe.
481
00:44:37,146 --> 00:44:40,583
It has a lot to do with wind, a lot to do with current.
482
00:44:40,850 --> 00:44:45,154
So if you have a crack out here,
it can actually just in minutes.
483
00:44:45,421 --> 00:44:50,059
It can be opened up and then
you're standing on a piece of ice floating out.
484
00:44:50,960 --> 00:44:54,497
What you're seeing here in the background,
that's open water, it's not normal.
485
00:44:54,730 --> 00:44:57,900
Because we had very big storms this year,
486
00:44:58,167 --> 00:45:01,404
it has broken up the ice
and now we have open water.
487
00:45:02,872 --> 00:45:07,109
But something else has made the ice
more and more fragile in recent years.
488
00:45:07,910 --> 00:45:11,547
Scientific records show that
the polar regions have been warming...
489
00:45:11,847 --> 00:45:13,649
...and the ice has been getting thinner.
490
00:45:17,286 --> 00:45:19,655
But what we've seen
the last couple of years here...
491
00:45:19,889 --> 00:45:23,192
...is that the dangerous places
are getting more dangerous.
492
00:45:23,526 --> 00:45:28,898
Before we maybe had like, 30 centimetres of ice
and that was very safe.
493
00:45:29,165 --> 00:45:30,499
Now it's maybe 15.
494
00:45:32,435 --> 00:45:34,604
If we go 30 years back,
495
00:45:34,937 --> 00:45:38,474
you could go straight
from here to Tasiilaq, the main town,
496
00:45:38,708 --> 00:45:41,510
just cross the ice here and go straight over.
497
00:45:41,711 --> 00:45:47,083
So the ice is getting thinner,
it's not so big any more as it was before.
498
00:45:50,620 --> 00:45:52,822
It's part of a trend over the whole Arctic,
499
00:45:53,089 --> 00:45:56,926
where the sea ice has shrunk significantly
over the last 20 years.
500
00:45:59,128 --> 00:46:02,498
A series of warm winters has meant
that the seas haven't cooled down...
501
00:46:02,732 --> 00:46:05,601
...as much as normal, resulting in less sea ice.
502
00:46:07,670 --> 00:46:09,639
As global temperatures rise further,
503
00:46:10,006 --> 00:46:15,244
that could spell real trouble for
both people and animals in this environment.
504
00:46:15,878 --> 00:46:20,149
There's no ice, we cannot go dog-sledding,
we cannot go out hunting.
505
00:46:20,816 --> 00:46:24,253
And if there's no ice,
then polar bears cannot go hunting...
506
00:46:24,487 --> 00:46:30,359
...and it's very difficult for them to find food,
so it's all a chain.
507
00:46:30,993 --> 00:46:33,262
So it's very important.
508
00:46:38,401 --> 00:46:43,039
For centuries, as we've learned more and more
about our place in the universe...
509
00:46:43,372 --> 00:46:44,807
...and got used to the fact that...
510
00:46:45,041 --> 00:46:49,111
our home is a spinning ball of
rock orbiting a star we call the Sun,
511
00:46:49,812 --> 00:46:52,782
we've come to appreciate
how critical Earth's orbit...
512
00:46:53,082 --> 00:46:57,119
...and its relationship
with the sun has been in its history.
513
00:46:58,387 --> 00:47:03,693
Tilts and wobbles, cycles of tiny changes
that have precipitated ice ages...
514
00:47:03,959 --> 00:47:09,065
...and radically changed habitats
and even changed the course of human history.
515
00:47:10,166 --> 00:47:14,203
Now we've discovered just
how precarious our climate is,
516
00:47:14,570 --> 00:47:18,974
the sobering thought that
we ourselves have almost certainly become...
517
00:47:19,241 --> 00:47:22,578
...another significant agent for climate change.
518
00:47:23,579 --> 00:47:30,019
It seems that human actions are changing the
balance between the Sun and the Earth in its orbit.
519
00:47:31,087 --> 00:47:38,394
How significant these changes are is not yet known,
but in just a few years we'll find out.
52538
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