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We're all on an amazing journey. | 0:00:03 | 0:00:06 | |
A 940 million kilometre voyage through space. | 0:00:06 | 0:00:11 | |
Even though we can't feel it, | 0:00:15 | 0:00:17 | |
we're travelling at over 100,000 kilometres an hour, | 0:00:17 | 0:00:20 | |
circling a star we call the Sun. | 0:00:20 | 0:00:24 | |
Every year, our planet, the Earth, | 0:00:27 | 0:00:30 | |
travels around the Sun - and we go with it. | 0:00:30 | 0:00:35 | |
We're looking at the huge impact this journey has on our world. | 0:00:35 | 0:00:40 | |
Wow, look at that! | 0:00:40 | 0:00:42 | |
We'll see how the Earth's tilt gives us our seasons | 0:00:42 | 0:00:47 | |
and leads to monsoon rains. | 0:00:47 | 0:00:50 | |
How the planet's spin stirs the atmosphere | 0:00:53 | 0:00:56 | |
into giant, rotating hurricanes. | 0:00:56 | 0:00:58 | |
And how small changes in the Earth's movement | 0:01:01 | 0:01:06 | |
can cover the planet with ice. | 0:01:06 | 0:01:09 | |
Our yearly orbit around the Sun has created | 0:01:12 | 0:01:14 | |
and shaped the world we live in. | 0:01:14 | 0:01:17 | |
We start our circumnavigation of the Sun | 0:01:36 | 0:01:39 | |
at a very special place, | 0:01:39 | 0:01:42 | |
and on a very special day. | 0:01:42 | 0:01:45 | |
It's March the 20th, the spring equinox. | 0:01:45 | 0:01:50 | |
This is the great pyramid in Chichen Itza - | 0:01:50 | 0:01:55 | |
an ancient Mayan city in Mexico. | 0:01:55 | 0:01:58 | |
Built 1,500 years ago, | 0:02:02 | 0:02:06 | |
the city is one of the world's great archaeological sites. | 0:02:06 | 0:02:09 | |
And it contains a remarkable insight | 0:02:09 | 0:02:13 | |
into our journey through space. | 0:02:13 | 0:02:16 | |
The ancient Maya had developed a deep understanding | 0:02:18 | 0:02:21 | |
of the Earth's movement around the sun, | 0:02:21 | 0:02:23 | |
and they built it into the very fabric of this city. | 0:02:23 | 0:02:25 | |
But it's something that can only be seen at two very precise | 0:02:29 | 0:02:34 | |
and magical times of the year. | 0:02:34 | 0:02:38 | |
One of those is today, March the 20th. | 0:02:38 | 0:02:42 | |
As afternoon approaches, the city fills | 0:02:45 | 0:02:47 | |
with followers of Mayan beliefs... | 0:02:47 | 0:02:49 | |
..and those curious to see an ancient wonder. | 0:02:54 | 0:02:57 | |
There is a unique and particular feature of our planet | 0:03:04 | 0:03:07 | |
as it orbits the sun, | 0:03:07 | 0:03:10 | |
and it's encoded in the way that light and stone | 0:03:10 | 0:03:14 | |
interact at the great pyramid. | 0:03:14 | 0:03:17 | |
CHEERING | 0:03:17 | 0:03:19 | |
This is the moment that all these thousands of people | 0:03:21 | 0:03:24 | |
have been waiting for, they've all stood up | 0:03:24 | 0:03:27 | |
and there are hands raised to welcome in the sun, | 0:03:27 | 0:03:30 | |
and it's now aligned perfectly on the edge of the steps here, | 0:03:30 | 0:03:34 | |
creating this very specific pattern of light and shade which resembles | 0:03:34 | 0:03:40 | |
the body of a snake. And that's no coincidence | 0:03:40 | 0:03:43 | |
because it joins up with the carved snake's head | 0:03:43 | 0:03:45 | |
at the bottom of the pyramid. | 0:03:45 | 0:03:47 | |
The Maya believed the snake, known as Kukulcan, | 0:03:53 | 0:03:59 | |
was a messenger between gods and man. | 0:03:59 | 0:04:03 | |
This is a remarkable display of Mayan architectural design. | 0:04:03 | 0:04:08 | |
The appearance of this snake isn't an accident, | 0:04:08 | 0:04:12 | |
they absolutely planned it | 0:04:12 | 0:04:15 | |
and it happens on the same day every year. | 0:04:15 | 0:04:18 | |
This is the spring equinox. | 0:04:18 | 0:04:21 | |
DRUMS BEAT | 0:04:23 | 0:04:27 | |
So, more than 1000 years ago, | 0:04:27 | 0:04:29 | |
the Maya recognised the equinox as a pivotal moment in the year. | 0:04:29 | 0:04:33 | |
Here on Earth, there are a few moments that we all share, | 0:04:47 | 0:04:50 | |
because we're all on the same journey around the Sun. | 0:04:50 | 0:04:54 | |
And one of those moments is the equinox, | 0:04:54 | 0:04:56 | |
when day and night are equal. | 0:04:56 | 0:04:58 | |
'It's a time of balance we can all experience, | 0:05:02 | 0:05:06 | |
'wherever we are on the planet.' | 0:05:06 | 0:05:09 | |
So whether you are here in Britain, | 0:05:12 | 0:05:15 | |
amongst the fitful showers and overcast skies, | 0:05:15 | 0:05:18 | |
'or in the bright spring sunshine of Mexico,' | 0:05:18 | 0:05:20 | |
on the March equinox | 0:05:20 | 0:05:22 | |
you'll get 12 hours of daylight and 12 hours of night time. | 0:05:22 | 0:05:26 | |
That's if the sun ever comes through the clouds! | 0:05:26 | 0:05:32 | |
But it's more than just a time of balance. | 0:05:32 | 0:05:35 | |
It's also a turning point in our year. | 0:05:35 | 0:05:38 | |
From the March equinox onwards, | 0:05:40 | 0:05:41 | |
the days get longer in the northern hemisphere, | 0:05:41 | 0:05:44 | |
'while in the southern hemisphere, the opposite occurs. | 0:05:44 | 0:05:48 | |
'This is because of a special feature of our planet | 0:05:50 | 0:05:53 | |
'as it journeys through space.' | 0:05:53 | 0:05:56 | |
Let's say this rock is the sun. | 0:05:56 | 0:05:59 | |
This is going to be our Earth, | 0:05:59 | 0:06:02 | |
and as the Earth travels around its orbit | 0:06:02 | 0:06:06 | |
spinning like this, it travels around on a flat plane. | 0:06:06 | 0:06:10 | |
So you would think that its axis would point upwards | 0:06:10 | 0:06:13 | |
but it isn't, it's tilted over at 23.4 degrees. | 0:06:13 | 0:06:18 | |
'This means that the North Pole, the stem of the apple, | 0:06:18 | 0:06:22 | |
'isn't vertical, it's at an angle.' | 0:06:22 | 0:06:26 | |
And that tilt stays pointing in the same direction | 0:06:26 | 0:06:29 | |
as the Earth travels around on its orbit. | 0:06:29 | 0:06:31 | |
Because of this tilt for part of our orbit, | 0:06:35 | 0:06:38 | |
the hemisphere north of the equator leans towards the sun. | 0:06:38 | 0:06:42 | |
This brings with it extra solar energy, | 0:06:45 | 0:06:47 | |
which fuels spring and then summer. | 0:06:47 | 0:06:50 | |
Six months later, the situation is reversed. | 0:06:57 | 0:07:00 | |
The southern hemisphere now leans towards the sun, | 0:07:00 | 0:07:03 | |
while the northern hemisphere experiences declining energy, | 0:07:03 | 0:07:07 | |
ushering in winter. | 0:07:07 | 0:07:09 | |
Tilt creates the Earth's seasons. | 0:07:09 | 0:07:12 | |
But there's a moment, twice a year as we orbit, | 0:07:13 | 0:07:16 | |
when the sun favours neither hemisphere. | 0:07:16 | 0:07:19 | |
At this point, both experience 12 hours of daylight and night time. | 0:07:19 | 0:07:24 | |
This is the equinox. | 0:07:26 | 0:07:28 | |
If the Earth wasn't tilted, | 0:07:33 | 0:07:35 | |
every day would be like the equinox, | 0:07:35 | 0:07:38 | |
with the 24 hours equally split between day and night. | 0:07:38 | 0:07:42 | |
And that would mean no seasons. | 0:07:42 | 0:07:45 | |
But the Earth's tilt means as we travel from the equinox, | 0:07:46 | 0:07:50 | |
seasonal changes do occur. | 0:07:50 | 0:07:52 | |
On the equinox, | 0:07:57 | 0:07:58 | |
the Sun's energy is felt most strongly directly on the Equator. | 0:07:58 | 0:08:03 | |
As we pass through spring, | 0:08:03 | 0:08:05 | |
this focused energy moves slowly northward. | 0:08:05 | 0:08:08 | |
All over the northern hemisphere, this solar shift means the Sun | 0:08:10 | 0:08:15 | |
arcs higher in the sky, and the hours of daylight increase. | 0:08:15 | 0:08:18 | |
The Earth orbits the Sun not standing up straight, | 0:08:31 | 0:08:34 | |
but tilted over at an angle of 23.4 degrees. | 0:08:34 | 0:08:38 | |
Our 23 degree tilt is just right. | 0:08:47 | 0:08:51 | |
It's enough to provide a relatively benign seasonal shift. | 0:08:51 | 0:08:54 | |
It makes our planet habitable. | 0:08:54 | 0:08:57 | |
However, it may have been the result of a cosmic accident, | 0:08:59 | 0:09:03 | |
and here in America, we can gain an insight into what happened. | 0:09:03 | 0:09:07 | |
This is the Barringer Crater in Arizona. | 0:09:14 | 0:09:18 | |
50,000 years ago, a meteorite struck this site | 0:09:18 | 0:09:23 | |
and just look what it left behind - this enormous hole in the ground. | 0:09:23 | 0:09:28 | |
'This impact would have thrown debris out | 0:09:32 | 0:09:36 | |
'over tens of thousands of square kilometres.' | 0:09:36 | 0:09:39 | |
And all the rock around here, like this, | 0:09:39 | 0:09:42 | |
is what's left after that explosive event. | 0:09:42 | 0:09:46 | |
This enormous crater is like a lesson in how size isn't everything, | 0:09:47 | 0:09:52 | |
because the crater itself is a kilometre across, | 0:09:52 | 0:09:55 | |
but the thing that caused it was only about 50 metres in diameter, | 0:09:55 | 0:09:59 | |
which is really quite small. | 0:09:59 | 0:10:01 | |
And the reason that such a small thing could cause such a big hole | 0:10:01 | 0:10:05 | |
is because it was travelling so fast. | 0:10:05 | 0:10:08 | |
'Impacts like these are extremely rare, | 0:10:10 | 0:10:12 | |
'but in the Earth's past, they were far more common | 0:10:12 | 0:10:16 | |
'and a lot bigger.' | 0:10:16 | 0:10:18 | |
Around four and a half billion years ago, | 0:10:22 | 0:10:24 | |
the solar system was still in the process of formation. | 0:10:24 | 0:10:28 | |
The Earth was just one of many of protoplanets that orbited the sun. | 0:10:30 | 0:10:37 | |
Amongst these protoplanets | 0:10:37 | 0:10:39 | |
was a small Mars-sized planet that's been named Theia. | 0:10:39 | 0:10:43 | |
Its orbit put it on a collision course with the Earth. | 0:10:45 | 0:10:49 | |
Theia smashed into the larger Earth and was obliterated. | 0:10:53 | 0:10:57 | |
The impact very nearly destroyed our planet too. | 0:11:01 | 0:11:05 | |
The collision knocked the planet over, | 0:11:05 | 0:11:08 | |
tilting the Earth's axis of rotation. | 0:11:08 | 0:11:11 | |
This tilted Earth might still be oscillating madly, | 0:11:14 | 0:11:18 | |
were it not for another consequence of Theia's impact. | 0:11:18 | 0:11:23 | |
A huge amount of debris was blasted into space. | 0:11:23 | 0:11:27 | |
Gradually, this debris coalesced, | 0:11:27 | 0:11:30 | |
captured by the Earth's gravity... | 0:11:30 | 0:11:33 | |
..and it formed the moon. | 0:11:33 | 0:11:36 | |
Billions of years later, the gravity of the sun and the moon together | 0:11:41 | 0:11:45 | |
act as a sort of counterweight, stabilising our tilt. | 0:11:45 | 0:11:51 | |
It's extraordinary to think that the moon is both evidence | 0:11:51 | 0:11:55 | |
of what caused Earth's 23 degree tilt | 0:11:55 | 0:11:59 | |
and the celestial object that helps maintain it. | 0:11:59 | 0:12:03 | |
Without this stabilising effect, the planet would wobble in space. | 0:12:03 | 0:12:10 | |
There would be no seasons, and the weather would be chaotic. | 0:12:10 | 0:12:15 | |
Spring triggers a seasonal transformation on land. | 0:12:24 | 0:12:29 | |
But the rising temperatures also transform our weather, | 0:12:29 | 0:12:34 | |
in some places with dramatic effect. | 0:12:34 | 0:12:38 | |
A tornado is the most volatile of these seasonal weather events. | 0:12:44 | 0:12:50 | |
They occur most frequently in the spring | 0:12:50 | 0:12:52 | |
and especially in the Midwest of America - | 0:12:52 | 0:12:56 | |
a region known as Tornado Alley. | 0:12:56 | 0:12:59 | |
MAN: 'Did you see that? The whole house came apart! | 0:13:00 | 0:13:04 | |
'Oh, my God!' | 0:13:04 | 0:13:06 | |
But despite its violence, | 0:13:14 | 0:13:16 | |
at the core of a tornado is a very simple process. | 0:13:16 | 0:13:20 | |
This goes on like a backpack. | 0:13:26 | 0:13:27 | |
'To experience it, I'm taking to the air, | 0:13:27 | 0:13:31 | |
'over the Midwestern state of Colorado.' | 0:13:31 | 0:13:34 | |
One, two, three, go. Run! | 0:13:34 | 0:13:38 | |
Paragliding pilots like Honza Rejmanek, | 0:13:40 | 0:13:42 | |
love this time of year. | 0:13:42 | 0:13:45 | |
Spring provides the perfect conditions for soaring... | 0:13:54 | 0:13:57 | |
..because the increasing temperatures generate thermals. | 0:14:05 | 0:14:09 | |
So right now we are in a thermal. | 0:14:13 | 0:14:15 | |
These are basically almost like invisible smokestacks of rising air. | 0:14:15 | 0:14:19 | |
Right now we've found one, I'm going to take a turn in it | 0:14:19 | 0:14:22 | |
and circle around and try to gain height. | 0:14:22 | 0:14:25 | |
'Thermals form when the sun warms the ground, | 0:14:25 | 0:14:30 | |
'and the ground, in turn, warms the air above it.' | 0:14:30 | 0:14:33 | |
What I'm experiencing | 0:14:33 | 0:14:36 | |
is one of the most fundamental principles of atmospheric physics - | 0:14:36 | 0:14:39 | |
warmer air rises. | 0:14:39 | 0:14:41 | |
'When air warms, it expands and becomes less dense. | 0:14:44 | 0:14:48 | |
'So this air has a lower atmospheric pressure | 0:14:48 | 0:14:51 | |
'than the cooler air that surrounds it.' | 0:14:51 | 0:14:55 | |
So it floats upwards, forming this rising thermal column. | 0:14:55 | 0:14:59 | |
The atmosphere tries to even out differences | 0:15:04 | 0:15:07 | |
in air temperature and pressure, | 0:15:07 | 0:15:09 | |
attempting to return to equilibrium. | 0:15:09 | 0:15:13 | |
So the rising thermal will mix with the cooler air above. | 0:15:13 | 0:15:17 | |
This basic process of moving towards equilibrium | 0:15:19 | 0:15:23 | |
lies at the heart of every significant weather event | 0:15:23 | 0:15:26 | |
on the planet. | 0:15:26 | 0:15:28 | |
'But in the springtime air over Tornado Alley, | 0:15:40 | 0:15:44 | |
'particularly powerful storms can develop. | 0:15:44 | 0:15:47 | |
'This is due to the unusual conditions here | 0:15:47 | 0:15:50 | |
'that intensify this basic atmospheric process.' | 0:15:50 | 0:15:53 | |
There's a stable layer of dry air that acts as a barrier | 0:15:57 | 0:16:00 | |
between the warm air down below and the cooler air higher up. | 0:16:00 | 0:16:04 | |
So the warm air is trapped, | 0:16:04 | 0:16:06 | |
and what's more, the ground keeps heating it as the day goes on. | 0:16:06 | 0:16:09 | |
WIND WHISTLES | 0:16:09 | 0:16:12 | |
THUNDERCLAP | 0:16:12 | 0:16:14 | |
The thermals get more and more powerful until, by late afternoon, | 0:16:14 | 0:16:19 | |
they finally punch through the barrier layer at colossal speed. | 0:16:19 | 0:16:24 | |
These rapid updraughts of less dense, lower pressure air | 0:16:24 | 0:16:28 | |
are so strong that they generate huge thunderstorms. | 0:16:28 | 0:16:33 | |
THUNDER RUMBLES | 0:16:33 | 0:16:36 | |
It's from these thunderstorms that, in certain conditions, | 0:16:38 | 0:16:42 | |
tornadoes can emerge. | 0:16:42 | 0:16:45 | |
'I'm going to investigate how this happens...' | 0:16:53 | 0:16:55 | |
Not as bad as north of us. | 0:16:55 | 0:16:58 | |
...with the help of atmospheric scientist, Josh Wurman. | 0:17:00 | 0:17:04 | |
I don't know what to make of these stringy little features. | 0:17:04 | 0:17:07 | |
The first step in our quest for a tornado | 0:17:08 | 0:17:12 | |
is locating a promising storm. | 0:17:12 | 0:17:14 | |
After a couple of days on the road, we manage to intercept | 0:17:14 | 0:17:17 | |
one moving north through Colorado. | 0:17:17 | 0:17:21 | |
So what's happening behind me is the storm is building | 0:17:23 | 0:17:26 | |
and in the middle of that storm over there, | 0:17:26 | 0:17:28 | |
there's an updraught with low pressure at the centre of it. | 0:17:28 | 0:17:31 | |
And all the air around the outside has higher pressure, | 0:17:31 | 0:17:34 | |
and that high pressure is pushing air into the centre | 0:17:34 | 0:17:37 | |
and up into the storm, and that's what building the storm. | 0:17:37 | 0:17:40 | |
The atmosphere tries to even out | 0:17:45 | 0:17:47 | |
the extreme differences in temperature | 0:17:47 | 0:17:49 | |
that have been generated. | 0:17:49 | 0:17:51 | |
So the air movements at the core of the storm | 0:17:51 | 0:17:53 | |
become exceptionally powerful. | 0:17:53 | 0:17:55 | |
'Hail is one characteristic product of this atmospheric violence.' | 0:17:58 | 0:18:02 | |
'The hail formed when an updraught cooled rapidly, | 0:18:06 | 0:18:09 | |
'so that water condensed out of the air, and turned immediately to ice.' | 0:18:09 | 0:18:15 | |
SHOUTING: This is what was carried from the south, | 0:18:15 | 0:18:19 | |
and it was pushed up into the storm | 0:18:19 | 0:18:22 | |
and it gave the storm its energy. | 0:18:22 | 0:18:25 | |
And now it's falling back down on me! | 0:18:25 | 0:18:27 | |
GIGGLES | 0:18:27 | 0:18:30 | |
Wow! | 0:18:30 | 0:18:31 | |
CAMERAMAN: That's it. Let's get inside. This is too hard now. | 0:18:31 | 0:18:36 | |
And even though this is chaotic and messy, | 0:18:36 | 0:18:39 | |
what this is, is a demonstration that the atmosphere is an unstable place, | 0:18:39 | 0:18:44 | |
and there are all these differences in temperatures and pressures. | 0:18:44 | 0:18:48 | |
And this is what happens when the atmosphere moves around | 0:18:48 | 0:18:52 | |
to even everything out, and make it all the same. | 0:18:52 | 0:18:57 | |
When tornadoes do form, they are often preceded by hail. | 0:18:57 | 0:19:01 | |
But this time, there's no twister. | 0:19:04 | 0:19:07 | |
So we're back on the road, | 0:19:09 | 0:19:11 | |
still trying to see a storm spawn a tornado. | 0:19:11 | 0:19:14 | |
Josh's specialist radar detects one | 0:19:17 | 0:19:19 | |
which shows a revealing swirl of clouds. | 0:19:19 | 0:19:23 | |
JOSH: Going out ahead, this big dark area's the core. | 0:19:25 | 0:19:27 | |
So we're basically going to penetrate through the core | 0:19:27 | 0:19:30 | |
and see what's interesting. | 0:19:30 | 0:19:33 | |
Tornadoes form when powerful rotating cylinders of air | 0:19:34 | 0:19:38 | |
within the storm | 0:19:38 | 0:19:40 | |
get caught by an updraught and are knocked on their side | 0:19:40 | 0:19:43 | |
by a powerful atmospheric wind called the jet stream. | 0:19:43 | 0:19:47 | |
Right now, we're kind of in the centre of the coiled part. | 0:19:49 | 0:19:54 | |
When that column of rotating air touches the ground, | 0:19:54 | 0:19:59 | |
a tornado is born. | 0:19:59 | 0:20:01 | |
At the tornado's core is an area of intense low pressure, | 0:20:12 | 0:20:16 | |
which draws high pressure air towards it. | 0:20:16 | 0:20:21 | |
The dust and debris picked up by the tornado | 0:20:21 | 0:20:24 | |
reveal the swirling pattern of winds. | 0:20:24 | 0:20:27 | |
Just 15 minutes after it first touched down, the tornado dissipates. | 0:20:46 | 0:20:52 | |
There's still so much that we don't understand about storms. | 0:20:54 | 0:20:57 | |
We don't understand when they're going to produce hail, | 0:20:57 | 0:21:00 | |
when they're going to produce rain, | 0:21:00 | 0:21:02 | |
when they are going to produce tornadoes. | 0:21:02 | 0:21:04 | |
But what we do understand is that a storm like this | 0:21:04 | 0:21:08 | |
is a manifestation of something happening round us all the time. | 0:21:08 | 0:21:13 | |
Our planet's atmosphere is a mosaic of warmer and cooler air masses, | 0:21:16 | 0:21:21 | |
constantly in motion. | 0:21:21 | 0:21:23 | |
The air is rising, falling and swirling around | 0:21:25 | 0:21:30 | |
as it seeks to balance differences in temperature and pressure. | 0:21:30 | 0:21:35 | |
During April and May, | 0:21:35 | 0:21:37 | |
the effect of the Earth's tilt is to enhance those differences. | 0:21:37 | 0:21:42 | |
So all over the northern hemisphere, | 0:21:44 | 0:21:47 | |
spring is the season for volatile storms. | 0:21:47 | 0:21:50 | |
Tornadoes are only one consequence. | 0:21:50 | 0:21:55 | |
The heavy and sudden downpours from storms can result in flash floods, | 0:21:57 | 0:22:02 | |
like the one that hit the town of Barranquilla in Colombia | 0:22:02 | 0:22:06 | |
in May 2011. | 0:22:06 | 0:22:09 | |
These occur when the rain inundates densely saturated ground. | 0:22:10 | 0:22:16 | |
The water isn't fully absorbed, but instead flows rapidly downhill | 0:22:16 | 0:22:21 | |
in a near-instantaneous torrent. | 0:22:21 | 0:22:24 | |
Thunderstorms can also give birth to an unexpected phenomenon... | 0:22:26 | 0:22:30 | |
..massive dust storms called haboobs. | 0:22:34 | 0:22:38 | |
This one blew into Phoenix, Arizona in 2011. | 0:22:38 | 0:22:41 | |
Haboobs are produced in normally arid regions, | 0:22:46 | 0:22:49 | |
when the leading edge of a storm collapses, | 0:22:49 | 0:22:52 | |
generating a super-fast downdraught | 0:22:52 | 0:22:55 | |
that kicks up a wall of dust and sand in front of it. | 0:22:55 | 0:22:59 | |
As May turns to June, more solar energy | 0:23:12 | 0:23:15 | |
is reaching the northern hemisphere, | 0:23:15 | 0:23:18 | |
and it drives the biggest single weather event on the planet. | 0:23:18 | 0:23:22 | |
An event centred on the Indian subcontinent. | 0:23:25 | 0:23:28 | |
TRAFFIC HUMS | 0:23:32 | 0:23:34 | |
CAR HORNS TOOT | 0:23:34 | 0:23:36 | |
This is the city of Udaipur in Rajasthan. | 0:23:36 | 0:23:40 | |
It's in the northwestern corner of India. | 0:23:44 | 0:23:48 | |
Since March, temperatures here | 0:23:52 | 0:23:55 | |
have been steadily rising as the Earth's tilt | 0:23:55 | 0:23:58 | |
has warmed the northern hemisphere. | 0:23:58 | 0:24:00 | |
But by June, everything is on the brink of an exhilarating change. | 0:24:00 | 0:24:06 | |
I'm here at the time of an epic weather event of huge importance | 0:24:13 | 0:24:18 | |
not just to Rajasthan but to the whole subcontinent | 0:24:18 | 0:24:21 | |
and the over billion people who live here. | 0:24:21 | 0:24:25 | |
'There's a wonderful place to appreciate the event's significance, | 0:24:27 | 0:24:33 | |
'on one of the hills that overlook the city, | 0:24:33 | 0:24:37 | |
'here, at this cliff-top palace.' | 0:24:37 | 0:24:41 | |
It was built at the end of the 19th century | 0:24:41 | 0:24:43 | |
by the 72nd Maharana of Udaipur | 0:24:43 | 0:24:47 | |
and it's known as Sajjan Garh. | 0:24:47 | 0:24:51 | |
He built this palace to get a pure, unadulterated view of the sky | 0:25:00 | 0:25:05 | |
and the clouds that start to build at this time of year. | 0:25:05 | 0:25:09 | |
Sajjan Garh is the monsoon palace. | 0:25:09 | 0:25:13 | |
When the rains do eventually arrive, | 0:25:17 | 0:25:20 | |
they'll be an essential relief from the heat of the Indian summer. | 0:25:20 | 0:25:25 | |
But what's intriguing is that the monsoon is actually a consequence | 0:25:25 | 0:25:29 | |
of the rising seasonal temperatures that precede it. | 0:25:29 | 0:25:34 | |
To reveal why this is, we need to travel 2,000 kilometres... | 0:25:34 | 0:25:39 | |
..south. | 0:25:39 | 0:25:41 | |
I'm in the coastal state where the monsoon first arrives in India - | 0:25:50 | 0:25:54 | |
Kerala. | 0:25:54 | 0:25:56 | |
The key to understanding the monsoon is here, on the beach. | 0:26:00 | 0:26:05 | |
The monsoon is powered by a simple, | 0:26:05 | 0:26:07 | |
but incredibly significant difference - | 0:26:07 | 0:26:10 | |
the difference between land and sea. | 0:26:10 | 0:26:14 | |
And in particular, the differing ways in which they respond to the sun. | 0:26:14 | 0:26:20 | |
Take this sand as an example. | 0:26:24 | 0:26:26 | |
The sun's energy is heating all of this surface, | 0:26:26 | 0:26:29 | |
but if I dig down just a little way... | 0:26:29 | 0:26:33 | |
..the sand underneath is quite cool, and that's quite familiar, | 0:26:33 | 0:26:36 | |
we see that on sunny beaches all the time. | 0:26:36 | 0:26:38 | |
And here, where it gets really hot, | 0:26:38 | 0:26:40 | |
the surface can reach 40 degrees Celsius. | 0:26:40 | 0:26:44 | |
Just 15 centimetres down into the sand, | 0:26:44 | 0:26:46 | |
it can be only 7 degrees Celsius. | 0:26:46 | 0:26:48 | |
So, all the sun's energy is going into a really thin surface layer, | 0:26:48 | 0:26:51 | |
and that layer heats up really, really, quickly. | 0:26:51 | 0:26:56 | |
The sun is also beating down on the ocean, | 0:26:56 | 0:26:58 | |
and that responds very, very differently. | 0:26:58 | 0:27:01 | |
This water is much warmer than the sea at home | 0:27:08 | 0:27:11 | |
but it's much cooler than the beach, | 0:27:11 | 0:27:13 | |
and the reason for that | 0:27:13 | 0:27:15 | |
is that the ocean takes much more of the sun's energy to heat it up. | 0:27:15 | 0:27:18 | |
So a kilogram of water will take three times as much energy | 0:27:18 | 0:27:22 | |
as a kilogram of sand to heat by one degree. | 0:27:22 | 0:27:24 | |
The ocean is also relatively cool because to heat the surface | 0:27:26 | 0:27:29 | |
you have to heat much more than just a thin layer. | 0:27:29 | 0:27:32 | |
What happens is that winds that blow across the surface of the ocean | 0:27:32 | 0:27:36 | |
generate turbulence which mixes that top layer. | 0:27:36 | 0:27:38 | |
So as soon as some water's been heated at the top, | 0:27:38 | 0:27:41 | |
it gets mixed down below. | 0:27:41 | 0:27:43 | |
'This means that, unlike the land, the ocean warms up only very slowly, | 0:27:46 | 0:27:50 | |
'as the sun's energy is absorbed. | 0:27:50 | 0:27:53 | |
'So as we enter summer, the land heats up quickly, | 0:27:55 | 0:27:58 | |
'while the ocean lags further and further behind.' | 0:27:58 | 0:28:01 | |
This increasing temperature difference is critical, | 0:28:03 | 0:28:06 | |
because both land and sea heat the air above them. | 0:28:06 | 0:28:10 | |
As the sun has baked the Indian subcontinent, | 0:28:13 | 0:28:16 | |
the land has warmed the air above it. | 0:28:16 | 0:28:19 | |
The warmer air is less dense, so it rises. | 0:28:19 | 0:28:23 | |
This draws in the cooler air from the ocean. | 0:28:23 | 0:28:26 | |
Because of India's particular geography, | 0:28:26 | 0:28:30 | |
this process is magnified. | 0:28:30 | 0:28:33 | |
It's a triangular peninsula, with wide, hot plains | 0:28:33 | 0:28:36 | |
and, crucially, a very long coastline. | 0:28:36 | 0:28:40 | |
This combination sets up a powerful | 0:28:40 | 0:28:42 | |
and sustained movement of cooler ocean air - | 0:28:42 | 0:28:46 | |
the monsoon wind. | 0:28:46 | 0:28:48 | |
Of course when most of us think of a monsoon | 0:28:51 | 0:28:53 | |
we think not of seasonal winds, but of rain. | 0:28:53 | 0:28:55 | |
'By setting up a time-lapse camera, | 0:28:58 | 0:28:59 | |
'I'm hoping to watch the rain clouds forming.' | 0:28:59 | 0:29:02 | |
THUNDERCLAP | 0:29:08 | 0:29:11 | |
Wow! | 0:29:13 | 0:29:16 | |
There is an enormous process on the go here. | 0:29:25 | 0:29:27 | |
When the sun shines down on the ocean surface, | 0:29:27 | 0:29:29 | |
some of the water at the surface will evaporate, | 0:29:29 | 0:29:32 | |
so water and energy are carried up into the atmosphere. | 0:29:32 | 0:29:35 | |
And as the monsoon winds come inland | 0:29:35 | 0:29:37 | |
and they carry that water vapour with them, | 0:29:37 | 0:29:39 | |
the heated land makes that moist air rise, | 0:29:39 | 0:29:43 | |
goes up into the clouds and there droplets condense - | 0:29:43 | 0:29:46 | |
the water condenses out, becomes visible, we see clouds. | 0:29:46 | 0:29:49 | |
When those droplets join together to form droplets which are large enough, | 0:29:49 | 0:29:53 | |
we get rain like this. | 0:29:53 | 0:29:55 | |
And it's really raining hard now! | 0:29:55 | 0:29:58 | |
None of this would be happening if it wasn't for the Earth's tilt. | 0:30:01 | 0:30:04 | |
It's the seasonal heating is what widens the gap in temperature | 0:30:06 | 0:30:09 | |
between the land and the sea, and this drives everything. | 0:30:09 | 0:30:12 | |
And this massive system of rain and wind rushes inland | 0:30:12 | 0:30:16 | |
and that's the monsoon. | 0:30:16 | 0:30:18 | |
80% of all India's rains arrive in this seasonal deluge. | 0:30:22 | 0:30:27 | |
It's not just the volume of the monsoon rains which is impressive. | 0:30:29 | 0:30:33 | |
It's the distance they travel. | 0:30:33 | 0:30:35 | |
As summer progresses in India, | 0:30:38 | 0:30:40 | |
the difference in temperature between land and ocean actually increases. | 0:30:40 | 0:30:45 | |
This makes the whole monsoon system more powerful, | 0:30:45 | 0:30:51 | |
drawing this moisture-laden air further and further inland. | 0:30:51 | 0:30:55 | |
From when the monsoon first arrives on the Kerala coast | 0:30:59 | 0:31:03 | |
around June the 1st, | 0:31:03 | 0:31:04 | |
it spreads more than 2,000 kilometres | 0:31:04 | 0:31:07 | |
until it eventually reaches the far north of the country. | 0:31:07 | 0:31:12 | |
It's now early September. | 0:31:21 | 0:31:24 | |
Although the summer is almost at an end, | 0:31:24 | 0:31:26 | |
in the northern hemisphere, it has a sting in its tail. | 0:31:26 | 0:31:29 | |
Because this is hurricane season. | 0:31:30 | 0:31:32 | |
The development of a hurricane is a wonderful example | 0:31:37 | 0:31:40 | |
of how the Earth's spin controls the weather. | 0:31:40 | 0:31:43 | |
I'm hoping to see one in action. | 0:31:44 | 0:31:47 | |
Tropical Storm Nate. Now, that one looks like it's got potential. | 0:31:47 | 0:31:50 | |
It's trapped in the Gulf, due to grow into a hurricane by tomorrow | 0:31:50 | 0:31:54 | |
and it looks as though it's almost certain to get to the Mexican coast. | 0:31:54 | 0:31:57 | |
24 hours later, I'm in eastern Mexico, | 0:32:03 | 0:32:07 | |
heading towards the Gulf of Mexico and the oncoming storm. | 0:32:07 | 0:32:11 | |
The winds are building up and the normal sunny skies | 0:32:17 | 0:32:20 | |
are replaced with cloud and rain. | 0:32:20 | 0:32:26 | |
At this time of year, the Gulf of Mexico | 0:32:26 | 0:32:29 | |
has the perfect ingredients to make a hurricane. | 0:32:29 | 0:32:33 | |
The sea is relatively shallow and close to the equator, | 0:32:33 | 0:32:37 | |
so the water gets particularly hot. | 0:32:37 | 0:32:40 | |
This water is warm, really warm | 0:32:40 | 0:32:43 | |
and the reason for that is that the ocean out there | 0:32:43 | 0:32:46 | |
has been absorbing the sun's energy, storing it up. | 0:32:46 | 0:32:48 | |
And now, it's that energy which can build tropical storms. | 0:32:48 | 0:32:52 | |
The way the storm is built is that the warm ocean | 0:32:58 | 0:33:01 | |
heats the air above it. | 0:33:01 | 0:33:02 | |
And once the air is warm, it expands and rises. | 0:33:02 | 0:33:05 | |
THUNDER RUMBLING | 0:33:05 | 0:33:08 | |
As the warm air rises, the pressure drops, | 0:33:08 | 0:33:11 | |
sucking in even more moist air, creating powerful winds. | 0:33:11 | 0:33:15 | |
But there's one final ingredient needed to create a hurricane. | 0:33:20 | 0:33:24 | |
It needs to start turning. | 0:33:25 | 0:33:27 | |
And that rotation comes from the spin of the Earth | 0:33:27 | 0:33:32 | |
through a phenomenon known as the Coriolis effect. | 0:33:32 | 0:33:37 | |
Now, let's say this is our planet, the northern hemisphere | 0:33:37 | 0:33:41 | |
and that's the North Pole. | 0:33:41 | 0:33:43 | |
Now, this planet isn't spinning, | 0:33:43 | 0:33:45 | |
so when I throw a ball in a straight line...it travels in a straight line. | 0:33:45 | 0:33:50 | |
But we live on a rotating world. | 0:33:52 | 0:33:55 | |
So, let's take our planet and make it spin, | 0:33:57 | 0:34:01 | |
round anticlockwise, like in the northern hemisphere. | 0:34:01 | 0:34:04 | |
So, now I'm on a spinning planet, things look quite different. | 0:34:09 | 0:34:12 | |
When I try and throw a ball in a straight line, | 0:34:12 | 0:34:15 | |
it bends around to the right. | 0:34:15 | 0:34:17 | |
From my point of view, this ball is always curving to the right, | 0:34:24 | 0:34:28 | |
even though I'm trying really hard to throw it in a straight line. | 0:34:28 | 0:34:31 | |
Now, the reason that this matters | 0:34:34 | 0:34:37 | |
is that this ball represents winds on Earth | 0:34:37 | 0:34:40 | |
and when the wind blows in the northern hemisphere, | 0:34:40 | 0:34:44 | |
the wind is also moved to the right. | 0:34:44 | 0:34:46 | |
In the southern hemisphere, the effect is reversed | 0:34:47 | 0:34:50 | |
and the winds bend to the left. | 0:34:50 | 0:34:52 | |
And that is all the Coriolis effect is. | 0:34:53 | 0:34:57 | |
A hurricane shows the Coriolis effect in action. | 0:35:03 | 0:35:07 | |
Winds are drawn inwards towards the low pressure | 0:35:08 | 0:35:12 | |
at the centre of the hurricane. | 0:35:12 | 0:35:14 | |
But as they head towards the centre, | 0:35:15 | 0:35:18 | |
the Coriolis effect makes them turn to the right. | 0:35:18 | 0:35:21 | |
This creates the hurricane's characteristic | 0:35:24 | 0:35:28 | |
circular swirl of wind. | 0:35:28 | 0:35:31 | |
It also means that the wind never reaches the centre of the storm. | 0:35:31 | 0:35:35 | |
So the eye of the hurricane remains calm. | 0:35:36 | 0:35:40 | |
Out at sea, Nate has the characteristic rotating, | 0:35:47 | 0:35:50 | |
swirling clouds of a hurricane. | 0:35:50 | 0:35:52 | |
but frustratingly, Nate begins to lose power. | 0:35:54 | 0:35:58 | |
Before it can make landfall, the winds die away. | 0:36:00 | 0:36:03 | |
Instead, the 2011 hurricane season | 0:36:10 | 0:36:14 | |
became famous for a different storm. | 0:36:14 | 0:36:17 | |
Hurricane Irene. | 0:36:17 | 0:36:20 | |
Unusually for a hurricane, | 0:36:20 | 0:36:22 | |
it travelled far enough up the east coast of the USA | 0:36:22 | 0:36:25 | |
to flood parts of New York city. | 0:36:25 | 0:36:27 | |
It caused billions of dollars worth of damage. | 0:36:29 | 0:36:34 | |
And all this because our planet spins. | 0:36:34 | 0:36:37 | |
In January, the northern hemisphere is locked in winter. | 0:36:43 | 0:36:47 | |
And yet there is a paradox about our winter, | 0:36:47 | 0:36:52 | |
because in January, winter is still getting colder, | 0:36:52 | 0:36:56 | |
even though the northern hemisphere is receiving more energy from the sun. | 0:36:56 | 0:37:00 | |
I've come to Northern Canada, | 0:37:10 | 0:37:12 | |
to the best - or perhaps the worst - place to explore this paradox. | 0:37:12 | 0:37:17 | |
Whoo! | 0:37:20 | 0:37:23 | |
Cor! This... | 0:37:23 | 0:37:27 | |
..is Yellowknife. | 0:37:27 | 0:37:30 | |
It has the dubious distinction | 0:37:30 | 0:37:33 | |
of being the coldest city | 0:37:33 | 0:37:36 | |
in the whole of North America. | 0:37:36 | 0:37:39 | |
Today is January the 19th. | 0:37:41 | 0:37:44 | |
On average, this is the coldest day of the year across the northern hemisphere. | 0:37:44 | 0:37:49 | |
It's minus 35 degrees Celsius, which certainly qualifies as cold to me. | 0:37:49 | 0:37:55 | |
When you breathe, it hurts. | 0:37:57 | 0:38:01 | |
It kind of gets you at the back of the throat. | 0:38:01 | 0:38:04 | |
Your nose feels like it's permanently frozen solid. | 0:38:04 | 0:38:07 | |
And despite the fact that I've got the feathers of about 25 geese | 0:38:07 | 0:38:11 | |
stuffed into this jacket, and more thermal underwear | 0:38:11 | 0:38:14 | |
than I thought possible to wear at exactly the same time, | 0:38:14 | 0:38:17 | |
I still feel cold. | 0:38:17 | 0:38:20 | |
In these conditions, even familiar things behave in unfamiliar ways. | 0:38:20 | 0:38:26 | |
You can take a lovely, hot, steaming cup of coffee, | 0:38:26 | 0:38:29 | |
throw it in the air, and the steam from that coffee will freeze instantly. | 0:38:29 | 0:38:35 | |
Well, you've got to give it a go, haven't you? | 0:38:35 | 0:38:37 | |
Right... | 0:38:39 | 0:38:41 | |
Here goes. | 0:38:41 | 0:38:42 | |
Wow! | 0:38:46 | 0:38:47 | |
That is amazing! | 0:38:48 | 0:38:51 | |
There's something curious about the way winter peaks towards the end of January. | 0:38:58 | 0:39:04 | |
The winter solstice falls on December the 21st | 0:39:04 | 0:39:07 | |
and this marks the day when the northern hemisphere | 0:39:07 | 0:39:10 | |
receives the least amount of solar energy from the sun. | 0:39:10 | 0:39:14 | |
So you might expect the December solstice to be the coldest day of the year. | 0:39:14 | 0:39:20 | |
But it's not. | 0:39:20 | 0:39:21 | |
On average, temperatures on the 19th of January are colder | 0:39:22 | 0:39:26 | |
than they are in mid-December. | 0:39:26 | 0:39:29 | |
But, you say, the days are getting longer. | 0:39:29 | 0:39:32 | |
The northern hemisphere is getting more sun. | 0:39:32 | 0:39:35 | |
It should be warming up. | 0:39:35 | 0:39:38 | |
In Yellowknife, there are people | 0:39:41 | 0:39:44 | |
whose livelihoods depend on the way winter's peak is delayed. | 0:39:44 | 0:39:48 | |
In the driving seat is Blair Weatherby. | 0:39:49 | 0:39:52 | |
His family have been driving through the bitter cold of this region | 0:39:52 | 0:39:55 | |
for three generations. | 0:39:55 | 0:39:57 | |
He's not an ordinary trucker. He's an ice road trucker. | 0:39:57 | 0:40:03 | |
And this is his highway. | 0:40:03 | 0:40:05 | |
In the summer, what happens here? | 0:40:24 | 0:40:27 | |
We'd be in a boat! | 0:40:27 | 0:40:28 | |
That's because we're not driving on land, but on a frozen lake. | 0:40:30 | 0:40:35 | |
So really to appreciate Yellowknife's splendid isolation, you have to look at a map. | 0:40:40 | 0:40:44 | |
And here it is, right on Great Slave Lake. | 0:40:44 | 0:40:49 | |
At this time of year, of course, it freezes. | 0:40:49 | 0:40:52 | |
So what time of year can you start driving on the lake, | 0:40:52 | 0:40:57 | |
as opposed to boating on the lake? | 0:40:57 | 0:40:59 | |
The season starts towards the end of January. | 0:40:59 | 0:41:03 | |
It's about 30 inches thick at this point. It just keeps getting thicker and thicker. | 0:41:03 | 0:41:09 | |
So whilst the northern hemisphere's coldest day is the 19th of January, | 0:41:12 | 0:41:16 | |
here in Yellowknife, it's still bitterly cold for many weeks to come. | 0:41:16 | 0:41:20 | |
For the truckers, this delayed winter means their work season | 0:41:20 | 0:41:25 | |
runs from late January well into March. | 0:41:25 | 0:41:28 | |
So why is the worst of winter delayed so long | 0:41:34 | 0:41:38 | |
after the solstice on December the 21st? | 0:41:38 | 0:41:41 | |
It's all about the balance | 0:41:42 | 0:41:44 | |
between the heat coming in and the heat going out. | 0:41:44 | 0:41:48 | |
Throughout early winter, the northern hemisphere | 0:41:50 | 0:41:53 | |
receives declining amounts of the sun's energy, | 0:41:53 | 0:41:57 | |
so it starts to cool down. | 0:41:57 | 0:42:00 | |
But there's a lag in this cooling, | 0:42:00 | 0:42:02 | |
because the Earth's surface loses heat relatively slowly. | 0:42:02 | 0:42:06 | |
So well into January, the Earth's surface is still losing heat, | 0:42:06 | 0:42:11 | |
even though solar energy is slowly increasing. | 0:42:11 | 0:42:16 | |
It isn't until around the 19th of January that a tipping point is reached. | 0:42:16 | 0:42:22 | |
From this day onwards, the increase in solar radiation | 0:42:22 | 0:42:26 | |
will overwhelm the effects of the heat loss | 0:42:26 | 0:42:29 | |
and the northern hemisphere will begin to warm up. | 0:42:29 | 0:42:32 | |
But it'll still be a few more weeks yet | 0:42:32 | 0:42:35 | |
before the ice here is too thin to support the weight of the trucks. | 0:42:35 | 0:42:41 | |
We've seen how the Earth's journey through space is critical for life | 0:42:46 | 0:42:51 | |
and how the Earth's angle of tilt defines our seasons. | 0:42:51 | 0:42:56 | |
But you only really understand just how important our orbit is for our planet | 0:42:56 | 0:43:01 | |
when you look into the Earth's past. | 0:43:01 | 0:43:04 | |
There's evidence in the most unexpected places. | 0:43:07 | 0:43:11 | |
A few miles out there is one of the most spectacular wonders of the world, | 0:43:13 | 0:43:17 | |
but I can't see it from here because it's underwater. | 0:43:17 | 0:43:20 | |
I'm in Belize in Central America | 0:43:20 | 0:43:22 | |
and what I'm going to see is known as the Blue Hole. | 0:43:22 | 0:43:26 | |
It's not often that nature produces something | 0:43:34 | 0:43:37 | |
as beautifully symmetrical as this. | 0:43:37 | 0:43:39 | |
It's almost a perfect circle. | 0:43:39 | 0:43:41 | |
But it's more than just a stunning piece of natural architecture, | 0:43:45 | 0:43:49 | |
because deep down there are clues | 0:43:49 | 0:43:51 | |
to some of the most dramatic events in Earth's history. | 0:43:51 | 0:43:54 | |
The bottom here is 120 metres down, | 0:44:06 | 0:44:09 | |
and I'm just dropping into the abyss. | 0:44:09 | 0:44:13 | |
Finally, I've reached my goal. | 0:44:20 | 0:44:23 | |
So down here at 40 metres... | 0:44:28 | 0:44:31 | |
..it's really eerie. | 0:44:32 | 0:44:34 | |
Gloomy. | 0:44:34 | 0:44:35 | |
And this is what I've come to see. | 0:44:37 | 0:44:40 | |
And they're stalactites. | 0:44:41 | 0:44:42 | |
But there's only one way I know of for stalactites to form. | 0:44:48 | 0:44:53 | |
And it isn't down here, in 40 metres of water, | 0:44:53 | 0:44:57 | |
with sharks swimming about nearby. | 0:44:57 | 0:44:59 | |
Stalactites are created when mineral-rich water drips from the roof of a cave, | 0:45:04 | 0:45:10 | |
over hundreds or even thousands of years, | 0:45:10 | 0:45:13 | |
leaving behind mineral deposits. | 0:45:13 | 0:45:16 | |
In other words, they didn't form in the ocean. | 0:45:16 | 0:45:20 | |
That means that when these grew, | 0:45:24 | 0:45:28 | |
the sea level was much, much lower than it is today. | 0:45:28 | 0:45:32 | |
Scientists have precisely dated stalactites from the Blue Hole | 0:45:35 | 0:45:40 | |
and, by comparing these and other sea level indicators from around the world, | 0:45:40 | 0:45:45 | |
they've built up a picture of changing sea levels | 0:45:45 | 0:45:49 | |
dating back hundreds of thousands of years. | 0:45:49 | 0:45:52 | |
It reveals a striking pattern. | 0:45:53 | 0:45:56 | |
Sea levels across the world have risen and fallen over time. | 0:45:56 | 0:46:01 | |
20,00 years ago, the entire surface of the world's oceans | 0:46:08 | 0:46:12 | |
was 120 metres below where it is today. | 0:46:12 | 0:46:16 | |
And that means if I was standing here 20,000 years ago, | 0:46:16 | 0:46:21 | |
all of this, including the Blue Hole cave system, would be dry land. | 0:46:21 | 0:46:25 | |
So where did the ocean go? | 0:46:28 | 0:46:30 | |
The answer is that it was on land. | 0:46:32 | 0:46:35 | |
But it wasn't liquid water, it was ice, | 0:46:35 | 0:46:38 | |
because 20,000 years ago, our planet was in the middle of an ice age. | 0:46:38 | 0:46:42 | |
The Earth has experienced regular ice ages | 0:46:45 | 0:46:48 | |
in a cycle going back several million years. | 0:46:48 | 0:46:52 | |
These dramatic changes to the state of our planet | 0:46:54 | 0:46:58 | |
are triggered by small changes in the Earth's orbit. | 0:46:58 | 0:47:02 | |
I've travelled back to Britain | 0:47:10 | 0:47:12 | |
to uncover the relationship between the Earth's orbit and an ice age. | 0:47:12 | 0:47:16 | |
Snowdonia's peaks and valleys were carved out in the last ice age. | 0:47:19 | 0:47:24 | |
It's in mountainous locations like this that an ice age would have begun | 0:47:26 | 0:47:31 | |
as snow gradually built up. | 0:47:31 | 0:47:34 | |
When we think of ice ages, we think of extreme cold during the winter. | 0:47:40 | 0:47:44 | |
But, counterintuitively, | 0:47:44 | 0:47:46 | |
it's summer temperatures which are important in starting ice ages. | 0:47:46 | 0:47:50 | |
And the reason for that is, now, ice will build up here during the winter, | 0:47:52 | 0:47:56 | |
but it will all melt away in the summer. | 0:47:56 | 0:47:58 | |
But if the summer is a little bit cooler, a layer of ice will be left behind. | 0:47:58 | 0:48:03 | |
And a series of cool summers | 0:48:03 | 0:48:04 | |
will leave layer after layer, one on top of the other, building up. | 0:48:04 | 0:48:08 | |
And here, the ice could have been hundreds of metres high. | 0:48:08 | 0:48:11 | |
Ice ages always start in the northern hemisphere | 0:48:13 | 0:48:16 | |
because there's so much more land surface on which ice can build up. | 0:48:16 | 0:48:20 | |
So the question is, what causes cooler summers in the northern hemisphere? | 0:48:24 | 0:48:29 | |
The answer comes from small changes in the Earth's orbit, | 0:48:30 | 0:48:34 | |
caused by the gravitational pull of other planets. | 0:48:34 | 0:48:38 | |
Our orbit isn't exactly the same every time. | 0:48:40 | 0:48:43 | |
Aspects of it change just slightly, in cycles lasting thousands of years. | 0:48:43 | 0:48:48 | |
And when all of those cycles reach their most extreme point | 0:48:48 | 0:48:52 | |
all at the same time, | 0:48:52 | 0:48:54 | |
that can change our summer temperatures just enough to tip us into an ice age. | 0:48:54 | 0:48:59 | |
There are three cycles to do with the Earth's orbit | 0:49:01 | 0:49:04 | |
that must all coincide to trigger an ice age. | 0:49:04 | 0:49:07 | |
The first of these cyclical changes | 0:49:10 | 0:49:12 | |
affects the time of year when perihelion occurs. | 0:49:12 | 0:49:15 | |
This is the day when the Earth is closest to the sun. | 0:49:15 | 0:49:19 | |
Today, perihelion is in January, | 0:49:21 | 0:49:25 | |
but over thousands of years, the date of perihelion changes. | 0:49:25 | 0:49:29 | |
When perihelion occurs in the northern hemisphere summer, | 0:49:29 | 0:49:33 | |
it makes summers particularly hot. | 0:49:33 | 0:49:36 | |
But when it occurs in winter, as it does today, | 0:49:37 | 0:49:40 | |
then northern hemisphere summers are cooler. | 0:49:40 | 0:49:44 | |
So at the moment, the perihelion cycle is at the right point to generate an ice age. | 0:49:45 | 0:49:52 | |
But two other cycles are not in an ice age phase. | 0:49:53 | 0:49:58 | |
The first is the angle of the Earth's tilt. | 0:49:58 | 0:50:00 | |
The Earth's tilt is currently at an angle to the vertical of 23.4 degrees. | 0:50:05 | 0:50:10 | |
But that angle changes between 22 and 24.5 degrees. | 0:50:10 | 0:50:16 | |
It's only when the angle is at its shallowest - 22 degrees - | 0:50:16 | 0:50:21 | |
that the seasons become less extreme and the summers cooler. | 0:50:21 | 0:50:24 | |
Today, the angle of tilt is too great for an ice age. | 0:50:27 | 0:50:31 | |
The final cycle affecting an ice age is the shape of the Earth's orbit. | 0:50:35 | 0:50:40 | |
The Earth's orbit is an ellipse, | 0:50:42 | 0:50:44 | |
but over time, it becomes slightly more, and then slightly less, elliptical. | 0:50:44 | 0:50:49 | |
When the orbit is at its most elliptical, the result is lower summer temperatures. | 0:50:49 | 0:50:54 | |
At the moment, the Earth is midway through this cycle, | 0:50:57 | 0:51:01 | |
so again, it's not in an ice age phase. | 0:51:01 | 0:51:04 | |
It's only when all three of these changes to the Earth's cycle line up together | 0:51:09 | 0:51:14 | |
that they produce the really cool summers | 0:51:14 | 0:51:17 | |
in the northern hemisphere that result in ice ages. | 0:51:17 | 0:51:20 | |
It'll be around 60,000 years before the cycles line up again | 0:51:27 | 0:51:31 | |
and the next ice age starts. | 0:51:31 | 0:51:34 | |
In our journey around the sun, it's now the beginning of March | 0:51:38 | 0:51:42 | |
and the shackles of winter are being loosened in Britain | 0:51:42 | 0:51:46 | |
as we move into spring. | 0:51:46 | 0:51:49 | |
The land starts greening as longer days bring more energy from the sun. | 0:51:49 | 0:51:55 | |
But in some parts of the world, | 0:51:55 | 0:51:57 | |
the effect of that warming has yet to be felt. | 0:51:57 | 0:52:00 | |
I've come to Greenland, where there's definitely not much sign of spring yet. | 0:52:08 | 0:52:13 | |
This is Kulusuk. It's a tiny settlement of just 355 people | 0:52:15 | 0:52:21 | |
perched on the edge of an island in eastern Greenland. | 0:52:21 | 0:52:24 | |
To the north of here is the Arctic Circle and the Greenland ice cap. | 0:52:24 | 0:52:28 | |
Kulusuk is surrounded by the Arctic Ocean. | 0:52:32 | 0:52:36 | |
At this time of year, it's frozen, covered in a thick layer of sea ice. | 0:52:36 | 0:52:42 | |
Each year, the extent of the sea ice is different. | 0:52:47 | 0:52:50 | |
To see how far it reaches this year, | 0:52:50 | 0:52:53 | |
I need to travel right to the edge of the sea ice. | 0:52:53 | 0:52:57 | |
-Want me to bring this? -Yeah. | 0:53:03 | 0:53:05 | |
'I meet up with my guide, local hunter Gio Utuaq. | 0:53:06 | 0:53:12 | |
'His hunting grounds lie right at the edge of the sea ice. | 0:53:12 | 0:53:16 | |
She's so keen! | 0:53:17 | 0:53:20 | |
How far do we have to go to get to the hunting grounds? | 0:53:21 | 0:53:26 | |
20, maybe 25 kilometres. | 0:53:26 | 0:53:29 | |
After two hours, we reach a huge expanse of sea ice. | 0:53:42 | 0:53:48 | |
It's impossible to comprehend that the snow we're travelling across sits on ice, | 0:53:49 | 0:53:54 | |
which sits on the ocean. | 0:53:54 | 0:53:56 | |
We're travelling across a frozen sea. And look at this! | 0:53:56 | 0:53:59 | |
This is an iceberg actually trapped within the sea ice. | 0:53:59 | 0:54:03 | |
It's the most astonishing landscape, or seascape or ice-scape... | 0:54:03 | 0:54:09 | |
What do you call it?! ..that I've ever seen! | 0:54:09 | 0:54:12 | |
It's like another world. | 0:54:12 | 0:54:14 | |
And then, surprisingly quickly, the edge of the ice comes into view | 0:54:28 | 0:54:32 | |
and I can see the Arctic Ocean. | 0:54:32 | 0:54:36 | |
For obvious reasons, we make the last stretch of the journey on foot. | 0:54:39 | 0:54:44 | |
-Are you sure? -SHE CHUCKLES | 0:54:47 | 0:54:50 | |
There is something | 0:54:50 | 0:54:54 | |
very disconcerting | 0:54:54 | 0:54:57 | |
about walking on sea ice | 0:54:57 | 0:55:00 | |
when the open sea is so close. | 0:55:00 | 0:55:04 | |
Yeah, it looks pretty solid. How thick is the ice? | 0:55:06 | 0:55:10 | |
Like this thick? | 0:55:10 | 0:55:12 | |
It seems strange to be walking across a frozen sea here in Greenland | 0:55:16 | 0:55:21 | |
when back at home, the daffodils are beginning to come up. | 0:55:21 | 0:55:25 | |
But what's even stranger | 0:55:25 | 0:55:26 | |
is that measurements of the sea ice over the last 50 years | 0:55:26 | 0:55:30 | |
show that it only reaches its full extent now, in early March. | 0:55:30 | 0:55:36 | |
So clearly there's a lag between the arrival of the warmth of the sun | 0:55:36 | 0:55:40 | |
and the melting of the ice. But why? | 0:55:40 | 0:55:43 | |
It comes down to the properties of water. | 0:55:45 | 0:55:48 | |
We've already seen that, well into January, | 0:55:48 | 0:55:52 | |
land continues to lose more heat than it gains. | 0:55:52 | 0:55:55 | |
Because water radiates heat even more effectively than land, | 0:55:55 | 0:56:00 | |
the oceans take even longer to start warming up. | 0:56:00 | 0:56:04 | |
So although the land has been warming since January the 19th, | 0:56:04 | 0:56:08 | |
the sea is still losing heat and the ice continues to grow. | 0:56:08 | 0:56:14 | |
Greenland sea ice is at its maximum extent at this time of year, in March. | 0:56:14 | 0:56:18 | |
But over the next few weeks, the tilt of the Earth towards the sun as it orbits it | 0:56:18 | 0:56:23 | |
will allow the northern hemisphere to get an increasing amount of solar energy. | 0:56:23 | 0:56:27 | |
The days will get longer and warmer | 0:56:27 | 0:56:30 | |
and the sea ice will begin to break up and recede. | 0:56:30 | 0:56:33 | |
Then the hunting season will be over. | 0:56:33 | 0:56:35 | |
The existence of the sea ice here in Greenland | 0:56:45 | 0:56:49 | |
is testament to the complex response our planet has to the sun, | 0:56:49 | 0:56:54 | |
in whose orbit we travel. | 0:56:54 | 0:56:56 | |
But it's a very delicate balance | 0:56:56 | 0:56:59 | |
and no-one is more acutely aware of that than the people who live here. | 0:56:59 | 0:57:04 | |
Gio tells me that this year, | 0:57:07 | 0:57:09 | |
there was less ice than in previous years. | 0:57:09 | 0:57:12 | |
It's part of a trend over the whole of the Arctic. | 0:57:12 | 0:57:16 | |
The area covered by sea ice has shrunk significantly in the last 20 years. | 0:57:16 | 0:57:22 | |
A series of warm winters | 0:57:22 | 0:57:23 | |
have meant that the seas haven't cooled down as much as normal | 0:57:23 | 0:57:27 | |
so not as much ice has been able to form. | 0:57:27 | 0:57:32 | |
And there are many scientists who argue | 0:57:32 | 0:57:36 | |
that the cause of the warmer winters is us. | 0:57:36 | 0:57:39 | |
Global warming can feel like a myth when, back in the UK, | 0:57:41 | 0:57:47 | |
we've endured a string of very cold winters. | 0:57:47 | 0:57:51 | |
But here on the front line, it's a reality. | 0:57:51 | 0:57:56 | |
Most predictions suggest that the Arctic will continue to warm rapidly | 0:58:01 | 0:58:05 | |
over the course of this century. | 0:58:05 | 0:58:08 | |
It could be that we may well prove capable of generating the kind of climate change | 0:58:09 | 0:58:16 | |
that in the past has been created by changes in the Earth's orbit. | 0:58:16 | 0:58:20 | |
Subtitles by Red Bee Media Ltd | 0:58:51 | 0:58:54 |